I've bought a broken scope haven't I ? - Page 1
Forum section: electronics / testgear

author: aneng
author_rank: Regular Contributor
timestamp: November 03, 2015, 07:29:16 pm
content: Hi Everyone !This is my first post and the start of my venture into electronics repair. Forgive the appallingly short introduction but I need to ask a question quickly ! I've just bought my first ever scope, a Trio 2100, 100MHz analogue. It's just arrived and whilst I admit I don't really know my way around a scope properly yet, I think I've been sold a dud.Basically, no matter what I do, I can't get it to display any waveform that extends higher or lower than the first line on the graticule...That trace is with the probe clipped to the CAL terminal (which is supposed to be 0.3V pp @ 1KHz). The scope is set to 5mV / division and 0.5ms sweep, DC coupling, Ch1 only. I tried connecting a wall-wart to it to get some DC there to get the trace up higher, but it doesn't budge higher than 1 division, even set to 5mV / division and the wall-wart putting 3V onto the probe. Also, the Y-axis trace position dial barely moves the trace at all. I tried channel 2 and it behaves just the same. Lastly, are scope beams supposed to be sharper than that ? - That is the most focussed I can get it, and that's with the focus knob turned fully clockwise.I've been sold a dud haven't I ?

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2015, 07:33:31 pm
content: Check the beam finder button isn't jammed.

author: aneng
author_rank: Regular Contributor
timestamp: November 03, 2015, 07:35:20 pm
content: The beam finder works OK - or at least how I expect it's supposed to.... I press it, the button lights up and I get a bright dot appearing on the screen, swooping in from one side.

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2015, 07:45:15 pm
content: So its got vertical deflection problems at or after the beamswitch. Now you need the schematic.

author: nctnico
author_rank: Super Contributor
timestamp: November 03, 2015, 07:49:52 pm
content: Aren't the 50 Ohm termination buttons pressed in? Then again the vertical position knobs should allow the trace to be moved all the way across the screen.

author: commie
author_rank: Frequent Contributor
timestamp: November 03, 2015, 07:50:22 pm
content: I feel for you mate, you have done nothing wrong either, with what you have said it could possibly be one the Y-amps supply rails gone low, any difference with the lower channel?. Did you get any sort of guarantee with it or was it the dreaded 'sold as seen' scenario.?

author: aneng
author_rank: Regular Contributor
timestamp: November 03, 2015, 08:03:23 pm
content: It's off eBay. It was advertised as 'fully working', so I'm covered. I just wanted to be certain that it really is faulty and it's not just me being a noob and not knowing how to drive the thing.Is there anything I can do to prove it's faulty, or have I already established that ?

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2015, 08:14:11 pm
content: You've proved its faulty, but I'd shoot a 1 minute video showing that the vertical position controls don't work properly, in case of dispute.

author: nctnico
author_rank: Super Contributor
timestamp: November 03, 2015, 08:17:44 pm
content: Set the input coupling selectors (AC/GND/DC) to GND and see if you can move the trace using the vertical control knobs all the way across the screen. If that doesn't work then it is safe to say it's broken. And don't be ashamed: I have got burned by buying a defective scope a couple of times.

author: aneng
author_rank: Regular Contributor
timestamp: November 03, 2015, 09:00:05 pm
content: Do you mean the 'Position' knob at the very top left of the scope ?

author: commie
author_rank: Frequent Contributor
timestamp: November 03, 2015, 09:02:55 pm
content: Quote from: aneng on November 03, 2015, 09:00:05 pmYes,yes and yes.The trace should sweep up and down with the knob.

author: aneng
author_rank: Regular Contributor
timestamp: November 03, 2015, 09:10:16 pm
content: It moves the beam total of 1.5 of the small divisions vertically over its whole range of movement... i.e. about 4mm.Input coupling set to ground. It does exactly the same thing on channel 2.

author: commie
author_rank: Frequent Contributor
timestamp: November 03, 2015, 09:23:44 pm
content: Quote from: aneng on November 03, 2015, 09:10:16 pmYeah, it sounds as though one of the Y amp supply rails has gone down, definitely faulty though.

author: aneng
author_rank: Regular Contributor
timestamp: November 03, 2015, 09:26:37 pm
content: Right. Thanks very much for that. He's having it back. I quite like the scope from an aesthetic point of view, but as I bought it to start my journey in electronics repair, I don't think a good starting point would be to try fixing a scope ! - especially with nothing more than a DMM !Thanks for all the replies folks.

author: commongrounder
author_rank: Frequent Contributor
timestamp: November 03, 2015, 09:31:40 pm
content: Just before you send it back, a long shot. Did you work the 5X gain switches on the position controls (pull the knobs out to engage)? If those are dirty/corroded, the Vertical gain could suffer. Just a thought.

author: KJDS
author_rank: Super Contributor
timestamp: November 03, 2015, 09:35:36 pm
content: I've still got a few of these lefthttps://www.eevblog.com/forum/buysellwanted/fs-uk-eu-philips-pm3217-oscilloscopes-analog-dual-channel-50mhz/msg781677/#msg781677

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2015, 09:36:33 pm
content: Incidentally the ibrightness appears to be far too high, which could be why the spot is blooming and appears as poorly focussed. Does the grey A intensity control (below left from the CRT) actually work? If you cant turn the intensity down, or if the focus is stii poor after turning it down, that's another fault to report.If you are setting up an analog scope, adjust the intensity (brightness) towards the lower end of the acceptable contrast range *BEFORE* twiddling focus or astigmatism controls. If its badly out of focus, you may need to further reduce the intensity after bringing it into focus.

author: aneng
author_rank: Regular Contributor
timestamp: November 03, 2015, 09:46:28 pm
content: Quote from: commongrounder on November 03, 2015, 09:31:40 pmYeah, I tried that. They did almost nothing - the trace moved a little bit but hardly anything. Same with the x1 & x10 switch on the probe. That's another thing... I'm p**sed off because I bought new probes specially as the scope didn't have any. OK, they were cheapy ones but still... Grrr. !

author: lowimpedance
author_rank: Super Contributor
timestamp: November 03, 2015, 10:32:16 pm
content: These scopes are a real pig to get into for service, not well layed out for ease of access to 'modules'. So yeah send it back.The new probes will most likely come in handy with what ever you replace it with.

author: aneng
author_rank: Regular Contributor
timestamp: November 07, 2015, 06:07:49 pm
content: I've decided to buy it off the seller anyway. It originally cost me 60GBP (128$AU / 90$US) but I've agreed to pay him 20GBP (43$AU / 30$US) for the faulty scope as I love the way it looks (!) I'm hoping I can repair it, but first I'm going to need another scope ! - and - more importantly, a service manual.I've managed to get my hands on the service manual for the CS-2070, which is the 70MHz version and looks identical visually to my CS-2100 with the exception of one coupling selector for the B channel trigger. I don't know whether the internals will be similar enough for this to be useful though. I have been searching extensively for the correct service manual and have found several sites that all look a bit dodgy to me, that all claim to have it. Being a suspicious type, I did some Googling to try to establish the integrity of these sites and all I could find were people moaning that they had not received their manuals etc, so I'm steering clear of them. I know I would be protected, allegedly, by the 'Paypal guarantee' but I suspect that may be fraught with problems trying to claim, or some clause in the small print that said 'goods delivered electronically are not covered' etc !!This leads me to my next question... Where do you guys get your manuals from ? Can anyone recommend particular sites ?Thanks in advance !

author: Ian.M
author_rank: Super Contributor
timestamp: November 07, 2015, 06:48:34 pm
content: Open it up CAREFULLY (the tube EHT will bite!) and compare the boards with the manual you've got, paying particular attention to the PSU and vertical deflection sections. You'll soon see if the boards are similar enough for it to be usable. You can probably fix it without another working scope.First check the power supply for missing voltage rails. Don't worry at the moment about +/-10% differences as that could be the different model. If you find one or more missing rails, dont just fix the obvious fault, look for another fault on the boards they feed that could have overloaded them.Once you ave all the rails up, find the beamswitch and test the voltage after it going to the deflection amplifier. With the scope in single channel mode and the input grounded, it should respond to the vertical deflection control with a swing greater than the p-p amplitude of any test signal the manual gives for that section. With the timebase set to a very slow sweep and the channel selection on ALT, it should flip between two voltage levels set by the two vertical defection controls. Once you've found a good vertical deflection signal, its just a matter of tracing it through the vertical amp all the way to the CRT.Get it basically working and try it on some known signal sources e.g, a CMOS 555 running from a stable supply, or a sinewave signal from a PC soundcard buffered and amplified by an OPAMP, with the level checked on a good multimeter. Once you are satisfied its basically working, you can use it to check its own power rails for excess ripple to see if it needs new caps in the PSU.DO *NOT* twiddle any presets, unless you have the full service manual with complete alignment and calibration procedure + all the jigs and test equipment required.

author: aneng
author_rank: Regular Contributor
timestamp: November 07, 2015, 07:25:11 pm
content: Wow ! - Thanks for the help mate, much appreciated. However, you're gonna have to be a bit gentler with me on the instructions ! I bought the scope to start my venture into component-level repair, so this is a bit of a sudden learning curve. I've repaired stuff at component level before, usually with a helping hand from someone on a forum :-) I know my way around the various components but, for example, I don't know what the 'beamswitch' is or where I could find the deflection amp on the schematic (unless it was labelled !).Talking of schematics.... here's the service manual ;-) ...https://www.dropbox.com/s/ikw5aa81hvlm5ut/trio_kenwood_cs-2070_70mhz_oscilloscope.pdf?dl=0I'll be back once I've had the lid off over the weekend !

author: Ian.M
author_rank: Super Contributor
timestamp: November 07, 2015, 07:49:01 pm
content: The beamswitch is where the two Y channels combine to get a single signal to feed to the vertical amp. The vertical deflection amp is everything that steps up that signal to drive the tube's vertical deflection plates. There's probably a delay line between the beamswitch and the vertical amp as that allows the trigger signal tap-off from the Y input channels to be a little earlier than the displayed waveform so the trigger point isn't lost off thel eft edge of the screen. Find the block diagram and all should become clear. Then you've got to look at the detailed schematics and try to correlate them with the boards in front of you. If the component designators differ (because its the wrong model) you will have a hard time of it without an experienced analog 'scope man (or other high grade eGeek) beside you.

author: Ian.M
author_rank: Super Contributor
timestamp: November 07, 2015, 08:30:20 pm
content: The block diagram is on the eighth page (2-4) with the delay line top right near the CRT. Block level circuit description is one page earlier. After checking the PSU, work forwards and backwards from the delay line, (a fancy coil of cable, 2/3 of the way back on the left hand side of the chassis).

author: NF6X
author_rank: Unknown
timestamp: November 07, 2015, 08:43:18 pm
content: While aneng's scope probably has a different issue, I thought I'd share my similar tale of getting a lovely analog scope with a deflection problem.A while back, I bought an AN/USM-488 scope on eBay. This is a militarized version of the Tektronix 2235. In this case, I think the militarization just consisted of changing a label or two to show the US military nomenclature. Just like aneng, I bought it as much for its looks as its features. My specific application for it was to use it as a modulation monitor for my great, bigT-368C/URT radio transmitter. An analog scope is the right thing to use for that application, the 2235 is a good size to sit on top of the transmitter next to the outboard MD-239A/GR FSK modulator box, and the military markings are icing on the cake.When it arrived, I was disappointed to discover that one side of the horizontal drive was not working. I could have returned the scope, but I really wanted one with the military markings on it. So, I decided to take it apart before contacting the seller, and I'm glad that I did. In this scope, the deflection plate connections are made to individual pins sticking out of the sides of the glass envelope, rather than through the multi-pin connector on the back of the electron gun. The connections are made with individual wires that have female contacts installed on the ends. Once I dug far enough into the scope to see the bottom of the CRT, I discovered that one of the two horizontal deflection wires had simply fallen off the pin during shipping. I plugged it back in, and the scope began working perfectly!Most repairs aren't that easy, but sometimes you get lucky. Good luck to you, aneng!

author: aneng
author_rank: Regular Contributor
timestamp: November 09, 2015, 05:07:00 pm
content: Right then... I've been poking about in the darkest recesses and I've taken lots of measurements, but... I'm not really certain where I should be looking. Here's what I've got so far:-The power supply outputs are pretty close to the values on the schematic;i.e.:Schematic (=) Actual Voltage--------------------------------10 = -10.11+10 = +9.96+5.2 = +4.97+20 = +19.73+55 = +54.65+120 = +120.7-12 = -12.16+12 = +12.64+7 = +6.79+130 = +129.2+55 = +54.7+24 = +23.53The input to the delay line (P9, Pin1 on the Vertical Preamp Unit) varies between about +1.07 (Ch1 Position knob fully ccw) to +0.55 (fully clockwise). P9, Pin3, the other half of the delay line input varies between +0.06 to +0.55 (fully ccw to fully cw).I don't know what those voltages are supposed to be though. There is a reference to a 0.55V on the schematic, but it doesn't say (or I've not read !) what position the knob is supposed to be in for those readings.The Y+ and Y- connections to the CRT read as follows: Y+ = 52.8 (ccw) 54.7 (cw), Y- = 54.7 (ccw) 54.4 (cw).I've checked the vertical position pot and that's OK, and I've started checking voltages on the Vertical Preamp Unit around where the vertical position pot joins the board. So far, everything is within 10% of the schematic voltage, but it would be easier if I had a better idea of where to poke next !The schematic I have for the CS-2070 seems (so far) to be identical to my CS-2100. There are, however, a couple of extra components that look like they've been added as an afterthought.... Whether that is as a result of a repair or some post-production-line changes I don't know.

author: aneng
author_rank: Regular Contributor
timestamp: November 09, 2015, 05:44:22 pm
content: A-HA !!! Something has changed !!I thought I'd have a poke around the Vertical Output Amp Unit and measured the voltages (figure 7-7 on the schematic). They are supposed to be 38.7 on either side, but they were 54 - ish on both. After measuring the Y- line, I noticed the trace on the screen had vanished. I re-measured the Y+ line and it was now 25 - ish volts, whereas it had been double that a second previously. Then I noticed that if I twiddled the vertical position knob on CH1, I could tell from the brightness changes that the line was moving a hell of a lot more than previously, even though it is now off the bottom of the display and you can only just see it with the knob turned fully up.THEN - I noticed evidence of a previous repair at this point:- (see below Q9 - the resistors and the soldering)So.... I'm in the right place at last I think. I'll take the board out and check the solder joints next.

author: aneng
author_rank: Regular Contributor
timestamp: November 09, 2015, 06:03:57 pm
content: Ugh !!

author: NF6X
author_rank: Unknown
timestamp: November 09, 2015, 06:06:10 pm
content: Looks like you're about to get some good experience with component-level-repair repair...

author: Ian.M
author_rank: Super Contributor
timestamp: November 09, 2015, 06:14:01 pm
content: Section 7-1 (page 90) is annotated with the conditions the voltages and waveforms were measured with. Set vert. position controls as near as you can to their middle.If you can apply the 1KHz 1V pk-pk test signal and have a good multimeter, you should simply be able to measure the RMS voltage where most of the waveforms are shown, with a 1uF non-polarised cap with an adequate working voltage for the circuit, in series with the multimeter lead. As long as the waveform is still sinusoidal, the pk-pk voltage will be 2*sqrt(2)*Vrms (approx 2.8 times bigger).The vertical preamp is on page 98.The first thingI'd check that all its supply voltages are actually getting to the board. It looks like it gets +/-10V on P4 for its analog circuits and +5V on P5 for the logic. Don't forget to check all the 0V connections as well!Two more posts just in - it looks like you may be narrowing it down a bit now.

author: aneng
author_rank: Regular Contributor
timestamp: November 09, 2015, 08:21:02 pm
content: Getting there... After I touched-up all the joints on the board in the pics above, CH2 now works - vertical control works, waveform from the CAL point is shown below:-I'm not sure that is displaying correctly though - maybe someone could comment ? - That is the CAL waveform, which is supposed to be a 0.3V p-p square wave at 1kHz. The scope is set to 0.1V/Div, 0.2mS/Div Horizontal, Probe is on 1X. Also, is a square wave supposed to appear like that ? I can't see the rise or fall lines and it looks more like a 'dovetail joint' than a square wave.I managed to sort out the focus though !After I'd resoldered the board, CH2 sprang to life as I said, but CH1 was just the same - barely any response from the position control, though a little bit more movement than before. I had the scope on its side at this point, and then laid it down flat and CH1's position control started working. However.... CH1 will not display the CAL waveform using the same probe, so something is up there.I don't know if this next bit is a fault or me just not knowing how to drive the thing, but the trigger will not lock once I move the Vertical attenuator higher than 20mV/Div - it just becomes a pair of solid lines -unlessI pull the 'Fix' knob in the centre of the Slope Level dial. Is this a fault ?Many thanks once again for all the help !!

author: NF6X
author_rank: Unknown
timestamp: November 09, 2015, 08:31:04 pm
content: On an analog scope, it's normal for rapid edges to be dim or invisible. They will probably appear if you turn up the brightness. You'll frequently need to play with the brightness to adjust the trade-off between seeing rapid edges vs. avoiding blooming from an over-bright trace. That's just part of the charm of analog scopes.Regarding that "dovetail" effect, try putting the scope into X-Y mode with grounded inputs to get a stationary dot. If it's a wide blur instead of a tiny dot, then the focus and/or astigmatism adjustments are probably off.

author: aneng
author_rank: Regular Contributor
timestamp: November 09, 2015, 08:53:35 pm
content: That was it - I'd adjusted the PSU focus trimpot too far and when I followed your instructions, I got the dot rounded instead of oval and the dovetail effect has gone. The focus knob on the front is still at maximum to get it sharp though - it's right on the stop.

author: Ian.M
author_rank: Super Contributor
timestamp: November 09, 2015, 08:54:46 pm
content: I suspect you still have too much brightness smearing the ends of the top and bottom of the squarewave. If its a god sharp squarewave, the rising and falling edge are always going to be between really dim and invisible on an analog scope. When the beam is being moved fast vertically, there are just too few electrons hitting the phosphor due to how far they've been spread out by the vertical movement and the very short time it happened in. Its like trying to water the yard by standing in the middle of the lawn and whipping the hoze round in a very fast circle ONCE.It also looks like there's ripple affecting the horizontal timebase. Turn down the brightness, ground the input, and select the slowest sweep speed. You should see a moving spot, if you see a short horizontal line the timebase and/or X output (deflection) amp have issues.Triggering should be a lot better than that. I would expect it to trigger OK on a 1 square pk-pk waveform and probably not to drop out down to half a square. There may be too much DC offset rom the Y amp for the slope control to handle. If that's the case, you will find the slope knob turned right to one of its limits at the lowest signal level you can trigger on. However you have AC trigger coupling selected and that *SHOULD* block any DC offset, so more likely the timebase has issues.

author: aneng
author_rank: Regular Contributor
timestamp: November 10, 2015, 12:14:44 am
content: I did as you suggested Ian, and I get a slow moving spot ! As for the triggering, that was down to me not knowing what I'm doing... I didn't realise you had to turn the slope control to get it to lock ! Turning the slope control until it does lock means I can now get the vertical attenuator up to 1, sometimes 2 volts per division before it becomes too much for it to lock. I'm not sure what that really means !, but I think it's better than you initially thought :-)There is another issue, the input coupling switch for CH2 is physically damaged and will need replacing - it needs a bit of wiggling to get it to work on the AC setting, but I can sort that out no problem.So... to sum up where I am at the moment;I'm unsure if the scope is reading correctly, as I calculated the CAL signal should trace over more divisions than it is.I don't know why CH1's vertical control suddenly started working after I turned the scope the right way up - but it does suggest a flaky connection somewhere.CH1 doesn't respond to any input yet.The focus control needs to be on fully clockwise for the trace to actually be in focus.

author: Ian.M
author_rank: Super Contributor
timestamp: November 10, 2015, 01:51:47 am
content: Slow moving spot is good. Slow moving short vertical or horizontal bar is bad, and would most likely indicate bad caps in the PSU.Time to provide it with a sine wave signal, (unless you have an $expensive$ meter that's good enough to actually measure the amplitude of squarewaves correctly). Once you have one channel working, with a known relationship between input level and displaied amplitude, you can use that to debug the other channel's waveforms!I'd leave the focus problem for the moment. See page 140, "Power Blanking Unit". What's probably happened is that one or more of the resistors in the focus divider chain have cooked themselves to a higher value, and you need to check them cold when the scope has been off overnight as any residual voltage is not only dangerous (through there shouldn't be more than a few hundred volts there during operation), but gives misleading results measuring in-circuit on high ohms ranges even if they've dropped to tiny fraction of their operating levels. The meter needs to read up to 50M to check the multiplier bleeder resistors R73,74 and 20M to check the chain R70-R72 + the Focus pot. Replacements *MUST* have an adequate voltage rating for the circuit, and you will need a meter with >100M input resistance on its 1KV DC range if checking voltages round this section!

author: aneng
author_rank: Regular Contributor
timestamp: November 10, 2015, 08:20:31 am
content: Great stuff !! I'm enjoying thisFeed it with a sine wave ? - I'm going to need a function generator for this I presume ? I suppose I'll need one anyway for future things, so I'm looking at one of these cheap jobs off ebay;http://www.ebay.co.uk/itm/DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-Digital-/400971779496?hash=item5d5bc7cda8:g:rVMAAOSwT6pVwRTQMeantime, can I feed it with any sinewave ? (say from a soundcard output) or is it critical to know the exact amplitude, freq and DC offset ?

author: KJDS
author_rank: Super Contributor
timestamp: November 10, 2015, 08:37:44 am
content: A soundcard will be fine, you're only interested in comparing channel A vs B for now.

author: Ian.M
author_rank: Super Contributor
timestamp: November 10, 2015, 09:34:27 am
content: You *DO* need to know the amplitude of what you are feeding it so you can check if CH2 is anywhere near its specified performance. However if you pop the sound card line output into a x10 inverting OPAMP circuit, using 1% or better resistors, and check with the scope that it isn't clipping, you can get the level up to where you can make a reasonably accurate voltage measurement with an ordinary DMM. Then use the scope's switched input attenuator to get the level you need further into the Y amp. Don't worry about DC offset, just use a couple of uF of non-electrolytic coupling cap at the soundcard output and leave the scope input on AC coupling, which gives an effective offset of zero. Soundcards are usually pretty good for frequency accuracy, though they may be a bit glitchy if there's a lot of apps running on the PC. Try:http://www.zeitnitz.eu/scope_enwhich has a signal generator as well.

author: TerraHertz
author_rank: Super Contributor
timestamp: November 10, 2015, 12:21:28 pm
content: Glad you like the scope's looks. I have a Trio CS2110, and it's my favorite also.But it took me a long time to find any kind of service manual. What I eventually found was a photocopy, and it's not very good. It seems the original is almost all multi-page foldouts.Anyway, maybe it's closer to yours than the 2070 pdf manual you have. How are you finding the similarity?And btw, where did you find that one?https://www.dropbox.com/s/ikw5aa81hvlm5ut/trio_kenwood_cs-2070_70mhz_oscilloscope.pdf?dl=0I'd like to check if they have a CS2110 manual too.

author: MarkL
author_rank: Unknown
timestamp: November 10, 2015, 05:38:31 pm
content: B&K Precision resold the same 100MHz quad-trace scope under their name as model 1590A.You could also look at their service manual to see if it matches yours more closely:http://elektrotanya.com/bk-precision_1590a_parts_sch.pdf/download.htmlI had one years ago. Great analog scope with a really bright and sharp screen.

author: aneng
author_rank: Regular Contributor
timestamp: November 10, 2015, 10:22:01 pm
content: Quote from: TerraHertz on November 10, 2015, 12:21:28 pmHi TerraHertz, I couldn't find any service manual for the 2110, so I started Googling for other models that were close - that's how I found the 2070 manual here:-http://elektrotanya.com/trio_kenwood_cs-2070_70mhz_oscilloscope.pdf/download.htmlThey don't have the 2100 though.Since then, I've found a website that has the service manual for the 2110A, which looks as if it's a newer revision of my 2110. They want $6.99 US for it and I haven't bought it yet as the manual for the 2070 is a 100% match so far... Here's a screenshot of the 2110A manual they sent me (I queried if it was the Instruction Manual or the Service Manual)..Their website is here if you want to look:http://www.emanualshop.com/index.phpQuote from: MarkL on November 10, 2015, 05:38:31 pmHi Mark, Thanks for that link. I've had a quick look at the document but I think I'll need to be inside my scope to do any sensible cross-checking, and I'd better not attempt that now as I'm 80% of the way though a bottle of Pinot Noir ! Red wine and CRT HT voltages are not a happy combo !

author: TerraHertz
author_rank: Super Contributor
timestamp: November 11, 2015, 04:22:15 am
content: Quote from: MarkL on November 10, 2015, 05:38:31 pmFor the record here, I checked that 1590a pdf against the photocopied manual for my CS-2110 and the schematics are very close but not the same. Maybe it's the same as the 2100 or 2100A?Also turns out I'd downloaded the 1590a pdf before, back in 2013. But didn't have the paper copy of the correct manual then so couldn't tell it matched, without disassembling my scope (which doesn't have any problems, so I'd rather not.)

author: aneng
author_rank: Regular Contributor
timestamp: November 11, 2015, 08:43:41 am
content: Well, the repairs are on hold until the weekend now. I've ordered a function generator kit which should be here Friday. After I've built that, in the immortal words of Mr Schwarzenegger, I'll be back.

author: MatthewEveritt
author_rank: Unknown
timestamp: November 11, 2015, 01:33:59 pm
content: Is it just me or are the UNCAL lamps lit in the latest picture? Easy enough to move the control while moving the thing around, and that would possibly explain the low reading from the CAL point.

author: aneng
author_rank: Regular Contributor
timestamp: November 15, 2015, 07:25:41 pm
content: Right... I've built the Function Generator, turned it on and blew it up. (I had the wrong glasses on and mistook my DMM's DC symbol for a negative sign, and wired up the power connector back to front !!). A quick visit to Maplins later with replacements for one exploded electrolytic and a 5V regulator and it's up and running. I turned the scope on and straight away noticed it's reverted back to its faulty behaviour - Y-Pos knobs move the beam no more than 1cm and my newly-generated sine wave signal is a distorted square - as are square waves and triangles.One step forward, two steps back.Tomorrow I'm going to reexamine the vertical amp board, clean off all the accumulated flux and possibly bypass a couple of dodgy-looking tracks with wires.Annoyingly, I'm in a situation now where I can't be certain that the Function Generator is operating correctly after its pyrotechnics yesterday, but given that the Y-Pos knobs are ineffective once again, the scope is definitely playing up.

author: aneng
author_rank: Regular Contributor
timestamp: November 16, 2015, 11:40:31 pm
content: OK, so I've been taking some measurements from the Vertical Output Amp board (the board I resoldered last time and got CH2 working (temporarily), and also has evidence of a previous repair), and there is something definitely wrong here, but due to my lack of knowledge I can't pin down where the fault may be. I'm hoping someone here may be able to point me towards the next step !Basically, if you look at the schematic below....All the voltage readings are roughly correct (give or take 5%), but... the collectors of Q9 and Q10 are reading about 55V, not 38.7V as specified in the schematic. The collectors of Q13 and Q14 are about 19.2V, which is just 0.5V above so I'm ignoring that difference, the bases of Q9 & Q10 are 19.6V.I don't know if I'm barking up the wrong tree but I can't work out why the collectors of Q9 and Q10 are so high.Any takers ? (please !)

author: rqsall
author_rank: Regular Contributor
timestamp: November 17, 2015, 11:31:47 am
content: I'm not an expert by any means, but....Well, the collectors are at the supply voltage. That would imply that no current is flowing through Q9 and Q10. My bet is that they failed open. It seems they're heatsinked from the schematic, so they could be under some stress. Alternatively R30 or R31 have failed open.It's however very possible I am mistaken. I would also measure the voltage between R41 and R47, it should be 55 volts, if it isn't, then the supply voltage is too high and you need to look at the power supply..... I think.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2015, 12:05:41 pm
content: The high emitter voltages indicate they are *NOT* open. To be certain, measure Vbe for each of them. I think you will wind it to be around 0.6V and they are close to cutoff.Whats going on in the circuit? Q7 & Q13 area Darlington pairin cascode with Q9. Similarly Q8 & Q14 areanother Darlingtonin cascode with Q10. The twoDarlingtonsparalleled transistorsform a quasi-long tailed pair (its individual emitter degeneration resistors are relatively large compared to its tail resistor). As the fault is common to both Y plate channels, it must be in a common feed or tail resistor or supply. => check R29.The bases of theDarlingtonspairs of paralleled transistorsare driven by two more cascodes Q1,Q3, and Q2,Q4, with Q1 and Q2 forming another long tailed pair. Insufficient base voltage at theDarlingtonsparalleled transistorswould also cause the observed problem. That would require excessive current in Q1,Q2, so check round the tail current source Q11. It would also be worth checking for gross shifts in the DC levels at the P10 input connector and the bases of Q3,Q4.Edit: Whoops! Many thanks, Mark for the correction.

author: MarkL
author_rank: Unknown
timestamp: November 17, 2015, 12:27:03 pm
content: Quote from: Ian.M on November 17, 2015, 12:05:41 pmLook closer. Q7 & Q13 are in parallel, as is Q8 & Q14.They should be matched and then thermally coupled, but it's still not a great design. You would usually add a small emitter resistor to each to have better current sharing between the two.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2015, 01:47:09 pm
content: Thanks for spotting that. WTF were the original designers thinking?However it doesn't substantially affect the analysis of the circuit and I *think* my suggestions where to check for trouble are still sound.

author: rqsall
author_rank: Regular Contributor
timestamp: November 17, 2015, 01:49:08 pm
content: Quote from: Ian.M on November 17, 2015, 12:05:41 pmBut he's saying the collector voltages are high (point 5 and point 6). He says nothing about the emitter voltages. Doesn't rule out Q9 and Q10 being in cutoff though, I missed thatbut I would think "open" is still a possibility.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2015, 02:03:09 pm
content: Quote from: aneng on November 16, 2015, 11:40:31 pmQuote from: rqsall on November 17, 2015, 01:49:08 pmI've just read aneng's voltages more carefully. There is only 0.4V Vbe for Q9,Q10, so they are definitely either both cut-off of both shorted b-e. If they were open either the emitter voltages would be *MUCH* lower or if just c was open, the Vbe drop wold be around 0.7V (1 silicon diode drop). Also, as it has stayed fairly well balanced, exactly the same damage would have had to happen to both Q9,Q10, which is highly unlikely. Furthermore reduced amplitude (and probably heavily clipped) signals are still making it to the Y deflection plates, which again woukd be impossible if they were O/C.I still suspect a bias fault or a damaged supply trace on this board.

author: rqsall
author_rank: Regular Contributor
timestamp: November 17, 2015, 02:33:01 pm
content: Quote from: Ian.M on November 17, 2015, 02:03:09 pmAfter reading more carefully myself, I agree... I think. Curious what the voltage drop over R30 and R31 is though, Vbe+ VR3= 0.4, not Vbeby itself, so Vbeis even less than 0.4V if there is voltage drop over R30/R31.

author: c4757p
author_rank: Super Contributor
timestamp: November 17, 2015, 02:47:02 pm
content: Quote from: Ian.M on November 17, 2015, 01:47:09 pmThose paralleled transistors are handling 65mAtotalat about 7V - almost low enough to do with justone. Considering they are operating under identical conditions, I suspect they will share current just fine. It's possible they were matched, but I doubt they even really need to be.On one hand, some ballast resistors would have helped share power. On the other, look at the frequency compensation network between them - that would have got rather more complicated throwing ballast resistors into the mix, if you want a good flat frequency response. It's a tradeoff, and personally I think they went the right way with it.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2015, 03:03:54 pm
content: Quote from: rqsall on November 17, 2015, 02:33:01 pmA typical silicon small signal diode at room temperature will pass a few tens of uA at 0.4V Vf. Even if a 2SC1164 b-e junction passes an order of magnitude more current, there will still only be a few millivolts across R30/31.It looks increasingly like the quasi-long tailed pair has lost its tail supply

author: rqsall
author_rank: Regular Contributor
timestamp: November 17, 2015, 03:36:45 pm
content: Thanks for clearing that up. I was wondering if there even was base-to-emitter current at all at Vbeof 0.4. I guess that is where my confusion comes from, didn't figure there'd be any current and the 19.6V and 19.2V would not be directly related.

author: MarkL
author_rank: Unknown
timestamp: November 17, 2015, 04:31:05 pm
content: Quote from: c4757p on November 17, 2015, 02:47:02 pmIt is a trade-off, so who knows what they were thinking. But I agree one would be better. Maybe they had piles of 2SC2644 and it was an easy fix to a design issue.If you look at the actual photos, there is no Q13 or Q14 (just noticed this). It doesn't really matter for the troubleshooting task at hand since they were in parallel, but it does bring up the question of what else might be different. An amount of verification against the schematic is probably in order. Or at least a visual comparison with the parts placement diagram (pg. 7-6) to detect if there's anything else extra or missing.

author: MarkL
author_rank: Unknown
timestamp: November 17, 2015, 05:14:56 pm
content: Quote from: aneng on November 16, 2015, 11:40:31 pmThe problem with give or take 5% at these higher voltages and the cascode configuration as Ian explained, is that it can make the difference between a transistor being on or off.Would it be possible to measure the voltages on the schematic, and then overlay them on the schematic? Then we could see expected vs. measured from input to output plus the supplies. (And yes, recognizing we don't have a completely accurate schematic...)

author: aneng
author_rank: Regular Contributor
timestamp: November 17, 2015, 09:54:51 pm
content: Firstly, a big Thanks to everyone who is helping me here. It's proving quite an adventure and I'm learning a fair bit as I go along.Here is the schematic with the actual voltages overlaid in red. I also measured the Vbe of Q9 & Q10 and my meter is showing it as AC (!) and 0.1V for both transistors. My meter sometimes gets confused and shows me AC, but I pull the probe away and reconnect and it sorts itself out, but not in this case - it sticks on 0.1VAC.Looking at the mess around the bottom of both Q9 & Q10, I'd say they've been replaced (or certainly removed and refitted) in the past.I've got a horrible feeling that Q9 & Q10 are shot... and they are 2SC1164's. I've had a quick Google and the only place I can find them is on eBay - at £13.50 EACH !!!($20US !).I disconnected the base and emitter leads from Q9 and tested it as best I can with a DMM in diode mode. This is what I get;(incidentally, the connections between the emitter and R30 and the base and R32 were nothing more than a solder bridge - no wire !)+-be = 0.694Veb = OLbc = 0.686Vcb = OLec = OLce = 1.3, 1.8, OL (this flashes up 1.3 and 1.8 very briefly before going to OL)

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2015, 10:46:08 pm
content: Check R29.The upper transistor in a cascode is fairly non-critical as long as its got adequate ratings and good enough HF performance. I would expect any of these to work OK in place of Q9,Q10. Watch the pinouts though!A horribly long Farnell UK parametric search

author: aneng
author_rank: Regular Contributor
timestamp: November 17, 2015, 10:57:04 pm
content: R29 is bang on 82 ohms.Would you say Q9 and Q10 are faulty then ? (I'm just following instructions on some website on how to test transistors with a DMM !).

author: Macman
author_rank: Regular Contributor
timestamp: November 17, 2015, 10:57:04 pm
content: Checking equivalents; 2SC1164 equivalent is BFW47 that has an equivalent of 2N3553 which you can find at more reasonable prices.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2015, 11:35:05 pm
content: Quote from: aneng on November 17, 2015, 10:57:04 pmDid you check all the connections round it?QuotePossibly. Try connecting a temporary 370R 1W resistor to ground from one of their emitters. The collector voltage should then be much closer to 38.7V and the trace should go off-screen or at least right to the top or bottom. If it doesn't do that, those particular transistors are FUBARed. You can scab in just about any NPN you have with >=45V Vceo, >=200mA Ic, >40 Hfe and 500mW or better dissipation for testing, but you'll need properly rated substitutes for long term reliability and to get the proper bandwidth

author: aneng
author_rank: Regular Contributor
timestamp: November 17, 2015, 11:45:12 pm
content: Quote from: Ian.M on November 17, 2015, 11:35:05 pmWell, they look ok.. I did touch-up every joint at the very beginning of this project so I'm assuming they are ok. I haven't pulled a leg off every component nearby to check it with a meter though. I'm not sure the board will stand a great deal of abuse - it wasn't in the best of states to begin with.QuoteI'll have to pay a visit to Maplins first. I don't have any stock of used bits - yet (!).Those transistors are fairly beefy - screwed into a heatsink etc. I'm guessing any replacement would need to be in the same style case to fit the heatsink ?

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2015, 12:23:10 am
content: Good luck finding parts at Maplins nowdays. The stocking policy for non-shopfront items seems to be 2 each of most components per store, and any store with easy parking is almost always out of stock of popular components. Use their webshite to check store stock before visiting!Three ordinary 120R resistors in series would do for testing but would probably get quite hot if you left them connected for a long time.Yes the transistors *should* be heatsinked. Worst case they could dissipate just over 1W, However trying to get the same case style is likely to be expensive, so you'll need an offcut of 1/4" aluminium sheet or bar you can cut small blocks from and drill a hole in for a light push fit on the new transistor, and two holes for the fixing screws. Use heatsink grease!

author: MarkL
author_rank: Unknown
timestamp: November 18, 2015, 12:34:33 am
content: Interesting. The diode test would indicate Q9 is ok, although something bizarre could be wrong. I wouldn't be running out to buy a new one yet.I'm interested why there's an imbalance between (3) and (4): 12.4V vs. 13V.What is the voltage on the emitter of Q8 and on the collector of Q2?

author: aneng
author_rank: Regular Contributor
timestamp: November 18, 2015, 01:52:44 am
content: I've decided to order from CPC in the end. Not sure of the difference between them "CPC Farnell" and the "Element 14 Farnell" site you linked to. Either way, I managed to find a couple of transistors that met the rough spec you gave, they are BC639RL1G. If nothing else they will help with diagnosing anything else that's wrong ! I also ordered some 1W 360 Ohm resistors and a few other bits and bobs. I couldn't bring myself to run the gauntlet of acne and apathy at Maplin again. The last time I was there it was like pulling teeth trying to communicate with them.@MarkL: The imbalance is just down to the position of the Y Position knob on the selected channel. If I move it, those voltages swing up and down (i.e. one up, one down etc). At least I think it does... I'm too tired to think straight now :-)I've put the lid back on it for tonight, I'll get the voltages you enquired about tomorrow. Thanks again !

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2015, 02:24:40 am
content: AAgh!! I meant 390R resostor, but anything over350R would be fine. How I worked it out: consider the total tail current in Q7,Q8. The emitters should both be at 12V when the circuit is balanced, which means you can treat R27,R28 as effectively in parallel (for DC working point analysis only). That's 56/2+82 = 110 ohms with 12V across it, giving 109mA, Normally its split evenly between Q7,Q8, but at the extreme limit of the deflection it coulod swing all one way or the other. Currently due to whatever fault exists, very little if any current is flowing in Q9,Q10. Therefore drawing 55mA from the emitter of either is extremely unlikely to do any damage 20V/55ma is near enough 360R. Drawing 55mA from the emitter should cause that transistor's collector voltage to drop by about 16.5V if the transistor is good. 390R would give only a 15V drop. I suspect the transistors are OK, as I cant think of a failure mode that would kill both without shorting them.

author: aneng
author_rank: Regular Contributor
timestamp: November 18, 2015, 08:48:04 am
content: What's bugging me is how CH2 sprang to life, albeit for a day or so only, when I poked about with this last week. The Y-Position control was working fine, and the voltages on the Y+ and Y- pins reflected the schematic (or near enough). Then, when I turned it on a day later to see the fruits of my labours on my newly-built function generator, it had reverted back to being faulty. I'm suspicious because I can't see how a 'faulty' component could spring back to life temporarily. I know intermittent faults exist (they drive me mad in my day job as a network engineer), but I can't see a blown transistor repairing itself somehow.Looking at the physical aspects of the board, there has definitely been some shoddy work done around Q9 & Q10 in the past; a couple of PCB hole pads gone around their lead bases, a couple of which are connected into the circuit with nothing more than a stretched-out dollop of solder. There is excessive heatsink paste all over the transistors and on the heatsink itself where it screws to the main chassis. Someone has obviously been poking around here before.As I'm very much a novice with all of this, I don't really understand some of the things that have been mentioned in this thread, e.g. some of the exchanges between yourself and other members earlier had me like a rabbit in the headlights ! I know what individual components do (mostly !) but looking at that schematic, you see a pattern; I see a pile of individual componentsI'll get there though !... eventually !Anyway - I have been trying to work out how on earth Q9 &Q10 could have 55V on their collectors, and it's just dawned on me - it's coming up from pin 1 of connector P22 on the bottom left of the schematic isn't it ?. So.... what Idon'tgrasp now is how can the Y+ and Y- lines could have 38.7V on them under normal operation if there is 55V coming in, albeit through that cluster of resistors and inductors around C9. Conversely, I don't get how there could be 55V across them now, as that is going through that cluster of resistors !As you can tell, there are some big holes in my theory at the moment - but, I thought I'd mention it as sometimes even an idiot can say the right things !

author: NF6X
author_rank: Unknown
timestamp: November 18, 2015, 08:50:46 am
content: Are you sure it's not a dodgy switch contact or potentiometer in the vertical circuit? Those can cause all sorts of intermittent headaches in older equipment.

author: aneng
author_rank: Regular Contributor
timestamp: November 18, 2015, 10:06:39 am
content: I'm beginning to get disillusioned with this now. Whilst waiting for something more to do with the board we've been working on, I thought I'd do a bit of probing around the bigger Vertical Preamp board as there is still something wrong with CH1 - i.e. sweeping CH2's vertical position knob causes changes on the P10 input to the smaller dodgy board we've been messing with, but sweeping CH1's knob does nothing. I went poking about, measuring voltages starting at the P7 & P8 connectors on the preamp board and the surrounding circuitry, and some of them are a bit weird, e.g. the supposed +5.3V around R22 & R23 (and their CH2 equivalents) arenegative, and now, the CH2 Invert button no longer illuminates when it did before.I'm thinking maybe I'm just flogging a dead horse and I ought to chuck it in the bin and go back to scouring ebay for a scope.

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2015, 10:38:10 am
content: @aneng,The Y plate lines have a zero signal trace centered nominal operation point of 38.7V, because there is meant to be just under 55mA flowing through the two series 150R resistors feeding them. The 22uH inductors short out the 470R resistors at low frequencies so can be ignored when calculating the DC working point.The 55mA current is controlled by the emitter current of Q9,10, as Ic=HFE/(HFE+1)*Ie, which for HFE>50 is near enough the same 55mA, neglecting the small base current.The 55mA is eventually controlled by Q7,8, the resistance between their emitters and ground (R27,28,29) and their base voltages which set their emitter voltages and thus the current. Their base voltages appear to be near enough correct, (especially if you adjust the Y position control to make them equal) so you don't have to chase this particular cause & effect chain back any further.If you measure all the voltages at each pin of all the transistors on the part of the diagram you've shown + measure all the supply voltages, I'm sure we can narrow it down to only a few components and tracks. The most likely cause is a hairline crack in a track where it joins a pad.The preamp board problems are *PROBABLY* not related, except in that they will most likely be due to the general condition of the boards and previous owner hack repair work. Getting Ch2 Inv working again shouldn't add too much to the tedious but not impossible job of refurbing that board. I'd concentrate on getting the vertical output amp right - as long as the preamp is working well enough to be able to provide the output board with the correct input at some setting of the front panel controls.By all means go back to scouring Ebay, but don't bin this one. Its often the case that having a working scope makes it vastly easier to fix a dead one. Persevere with it and you'll learn a lot. Either you will decide eventually its BER, and with luck get most of your money back parting it out, or you'll get it going and can then decide to either EBAY it or keep it (a good choice if your replacement either has unobtanium ICs / hybrid modules, or is an older digital scope).

author: aneng
author_rank: Regular Contributor
timestamp: November 18, 2015, 10:47:51 am
content: Thanks for the boost IanI was beginning to feel overwhelmed a bit. I would like to keep the scope. I like the look of it (yeah, I know that's not a great reason for a bit of scientific kit!). I'll put the kettle on, have a brew, then go back to getting that list of voltages for you. Thanks again for taking the time to help me with this - it is hugely appreciated.

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2015, 11:06:01 am
content: If you start with the supply rails (get close to where they are used, not just where they come onto the board) + the transistors to the right of waveform test points 3 and 4, if we get lucky, you may not need to go over the rest in such detail.Please also annotate the diagram by crossing out any parts that aren't fitted, and mark the value fitted for any resistors where that doesn't match the schematic.When you probe for a voltage, and its not as expected, its often worth checking more than one solder joint on what you *think* is one track - that's how you find cracked tracks.

author: TerraHertz
author_rank: Super Contributor
timestamp: November 18, 2015, 02:05:08 pm
content: Quote from: aneng on November 17, 2015, 09:54:51 pm(Man, that dropbox site does horrible things to page load times in my browser. Fixed.)The 54.6V Collector voltages on Q9 & 10 say there is no collector sink current into those transistors, since that's the supply voltage.And yet the voltages around Q7, 8, 13, 14 suggest there is current being drawn from the emitters of Q9 & 10.(Ha ha, notice the out of sequence numbers of Q13 & 14 indicates they are an afterthought.)If there's b-e current in Q9 & 10, but no C current then they are shot. OR... they are somehow wired wrong. Reversed E & C? Could the person who hacked up the board have done that?Another point: This whole chain is supposed to be balanced differential. But by test points 3 & 4, the voltages are unbalanced. Can you put your meter on DC V across TP 3 & 4, and then adjust VR1 (emitter balance of Q1 & 2) to see if you can get zero on the meter? (Ie TP 3 & 4 the same voltage.)Moving VR1 and seeing the TP 3 to 4 meter volts going between neg and pos voltages about equally, would show the chain up to there is working.Once you have TP 3 & 4 equal, you can better tell if there's maybe something weird with Q7, 8, 13, 14.I certainly wouldn't give up on this scope and bin it. More of an interesting learning experience, isn't it?Though, pity you don't have a working scope to help fix this one. Putting an AC signal into this amp chain and following it through with a scope would find the problem very quickly.Edit: Btw, re price of 2SC1164http://www.aliexpress.com/item/1PCS-2SC1164-R-2SC1164R-2SC1164-TO-99/32540056793.html?http://www.aliexpress.com/item/2SC1164/1871392372.html?spm=2114.01020208.3.10.Ijmuqj&ws_ab_test=searchweb201556_7,searchweb201644_5,searchweb201560_9spm=2114.01020208.3.2.kgP7o5&ws_ab_test=searchweb201556_7,searchweb201644_5,searchweb201560_9

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2015, 05:48:37 pm
content: Just the board layout to go with the schematic so we can better steer you to places to test etc.

author: MarkL
author_rank: Unknown
timestamp: November 18, 2015, 10:46:39 pm
content: I'm having trouble finding where the current is coming from to get 12.5V on the emitter of Q7.If it's all coming out of Q7's emitter, through R27 and R29, it would be 90mA (=12.5V/(56R+82R)).And if the current through R29 is being shared with Q8, Q8's emitter is around 12.3V, so R29 would be shared (approx) equally. That would make Q7's emitter current around 57mA.So, the current flowing through Q7's emitter is somewhere between 57mA and 90mA. Where's it coming from?It's not coming through Q7's base because there's no voltage drop across R18 (22R) + Q7 Vbe.And it's not coming through Q7's collector, connected to Q9's emitter, because there's not enough voltage drop on R32 (22R) through Q9's base (20V rail - 19.7V = 0.3V), or R40+R41 (300R) through Q9's collector (55V rail - 54.6V = 0.4V).The mirrored side with Q8 and Q10 has a similar story.It's not adding up for me. Maybe I'm overlooking something?I think something must be open somewhere, maybe in the Q7/Q8 resistor network. At least that's where I'd look first.Aneng: I know you checked R29 early on and it was fine. Can you measure Q8's emitter, and also the R27/R28/R29 node? This will confirm the above reasoning, or make me totally wrong. It's certainly not the first time.You could also measure Q7 and Q8's bases and collectors to verify some the above voltage drop assertions.Since you're measuring on an active circuit, also make sure the trace separation knob is at minimum so that it's not injecting any AC into the circuit while you're trying to measure DC values (it shouldn't be doing anything anyway when not in delayed mode - just a precaution). And I assume you only have one channel selected. If you have both (or more) channels on, it will also cause an interfering AC signal at the input as it bounces back and forth between Ch1 and Ch2.

author: TerraHertz
author_rank: Super Contributor
timestamp: November 18, 2015, 11:25:26 pm
content: Quote from: MarkL on November 18, 2015, 10:46:39 pmYeah, same. Where's the Vbe drop that Q7 should show?QuoteAh ha! I keep forgetting he has only a multimeter, and so can't see if there are active signals. The presence of which could explain all the weird readings that don't make sense.Maybe, to eliminate that source of confusion, disconnect P10 (the input) and apply a fixed 0.5V DC signal to pins 1 & 3. Same as the circuit test voltages say.Then do the VR1 balancing to verify similar +/- range of adjustment between TP 3 & 4, then set zero difference there.Then re-measure all the voltages through the latter stages of the amp. (And this time, include the actual voltages on the 20V and 55V supply rails. Also don't drop trailing digits off the measurements.)

author: MarkL
author_rank: Unknown
timestamp: November 19, 2015, 12:56:02 am
content: Quote from: TerraHertz on November 18, 2015, 11:25:26 pmIt's nowhere. Q7 B to E could be shorted within +/-0.1 measurement accuracy, or take it "as is" with a slight reverse bias. Either way, no current to be found there. It's like it's floating. Wierd, isn't it.As you pointed out, this is a balanced differential design. I'd expect all the transistors in the amplification path to be in the active region.R29 was already tested as ok, but maybe there's a break somewhere along the way from the R27/R28/R29 node through R29 to ground. I think that fits the readings, if you assume the input is being driven a little off-balance. If there's no path to ground, there's no current through Q7 or Q8 bases, and the 12.5V on Q7 emitter is actually coming from the emitter of Q8 after a (very low current) Vbe drop from 13V.Credit to Ian who identified this a couple of pages back, but I think only R29 got checked. The requested voltage measurement at the R27/R28/R29 node would say for sure.

author: MarkL
author_rank: Unknown
timestamp: November 19, 2015, 01:55:22 am
content: I'm looking more closely at the board photos. Below is a close up of one end of R29 on the top of the board.The copper around the resistor lead on top is ground, and the lead is clearly going into the hole. There's no signs of cracks, but it's not the best picture of that area.On the bottom, the lead goes to a single isolated pad where it is soldered. The soldering looks fine.However, note there's no blob of solder on the top connecting the lead to the copper.A possible cause is that the plating process failed on this hole, and this end of R29 goes nowhere except to an isolated pad. And perhaps it touches the edges of the hole sometimes and causes things to work until heat or other movement causes it to lose contact again.Or, that's how the theory goes, anyway.Up to you now, aneng. Let us know.

author: Ian.M
author_rank: Super Contributor
timestamp: November 19, 2015, 03:49:51 am
content: +1 to Mark's suggestion that its probably the joints on that isolated ground pad at the bottom of R29 on the track layout. You can see from the nearby C9 pad that component side, the ground pads have the usual cross shaped thermal relief from the ground plane, but due to the proximity of Q8 and its emitter track, it looks like the R29 pad is only grounded on two sides, so if it isn't simply a bad joint there, it could be cracked tracks at the edge of the pad. The other possibility is a crack at the edge of its top pad, where it joins the boomerang shaped track to R27,28.The other possibility, of intermittent b-e shorts in Q9 *AND* Q10 is, I think fairly unlikely (though possible if the transistors are of the right age to have developed internal tin whiskers), and will be eliminated when you try the test resistor to ground from their emitters, unless they continue to draw no collector current. An identical intermittent collector bondout failure in both transistors is *EXTREMELY* unlikely.

author: aneng
author_rank: Regular Contributor
timestamp: November 19, 2015, 09:58:11 am
content: Wow !! Thankyou all so much for the help here. It's so kind of you to give your time.I've been holding off until today as I'm expecting a delivery of the parts I ordered from CPC, which includes some decent fine solder and some good desoldering braid. It's been bugging me that we're trying to locate a fault on a board that looks like it's been soldered with plumbing solder and a blowtorch ! Once the braid arrives, I'm going to clean everything up around Q9 & 10, get all that heatsink crap off the board, clean up the pads, get the excess flux off and resolder the 2W resistor network (R29 and friends) and get Q9 & 10 connected up as best I can, using component leads to replace the missing tracks (instead of solder bridges at present).Once I've cleaned up the mess and I'm confident that it's as good, physically, as I can manage, then I'll get in there with the meter.In the interim, here are some photos of what we're dealing with !

author: Ian.M
author_rank: Super Contributor
timestamp: November 19, 2015, 10:23:26 am
content: Don't just 'shotgun' it. Check each component's connections to the furthest extents of the tracks it goes to before removing it otherwise you wont know for sure if its fixed.

author: Deathwish
author_rank: Unknown
timestamp: November 19, 2015, 10:44:02 am
content: Late to the party but that last pic seems to show dry joint on R29 and Q8

author: Ian.M
author_rank: Super Contributor
timestamp: November 19, 2015, 11:16:54 am
content: Yep. We've been saying the odds are >90% its in the R29 area. The *FIRST* thing I'd try would be checking continuity between that leg and the ground plane while gently disturbing the resistor, then I'd get that leg and top-side pad as clean as possible in-situ with a fiberglass pencil aqnd resolder it.

author: aneng
author_rank: Regular Contributor
timestamp: November 19, 2015, 11:14:13 pm
content: Funny you should say that Ian ! I did exactly that before reading your post, and there was no connection between R29 and the ground plane - until I moved the resistor ! However.... don't get too excited just yet, as there's a 50:50 chance that that was the leg I lifted to measure R29 with the meter (I can't remember which leg I lifted) - so it could just be a bad joint that I created putting it back rather than discovering the original fault.However.... I've got my new solder and my Multicore/Loctite NC-AB braid (wow... I'm going to get through a lot of this aren't I !! I was previously trying to use the stuff that Maplin's sell, boy.. what a difference !!), and I've removed Q9 & Q10, cleaned up all the flux and heatsink paste and wicked up the big blobs around the 2W resistor network etc. Just as well I did.... some of those pads barely have any copper on them, and of those that do, they are covered in a layer of crud - between the base of the solder and the pad itself.I'm going to stop for tonight (11pm here and my neck is hurting from leaning over), but I'll be back on it in the morning and getting a razor blade and scraping the green insulation (there has to be a proper word for it !) off the tracks near the dodgy pads so I can resolder them onto good copper. A couple of the shorter tracks just lifted off with the solder, so I need to sort that out too.

author: Ian.M
author_rank: Super Contributor
timestamp: November 20, 2015, 12:29:25 am
content: You'll want an X-Acto style knife with a small scoring or curved blade for similar for scraping off the green solder-mask. I wouldn't even try with razor blade! If you don't have anything suitable, make it - take a medium or large crappy all metal jeweller's screwdriver, preferably one the tip snapped (because hard/brittle is GOOD here) and grind flats each side of the tip for the last 1/4" or so tapering to about its original tip thickness. Then, grind a knife edge at a 45 deg angle across the tip. Rock it slightly to get a very small curve on the edge, and make sure the bevel angle is about 25 to 30 deg. Finally, hone the edge with a drop of oil on 400 grit wet & dry paper backed by a sheet of glass. It wont hold an edge as well as a good scalpel blade, but there's nothing much better for track scraping.Also you'll want a fiberglass abrasive pencil. Most places that sell car paint in small quantities, will stock them. You cant beat it for localised cleaning back to bright copper or tinned surface, either of scraped tracks or crusty pads. Get two, and some spare brushes, as you'll want one set short for scrubbing, and one set long can be used to brush molten solder off a pad with less risk of lifting it than trying to get it really clean with desolder braid. Beware of the debris - the small broken fragments of glass fibre are a severe skin irritant.You may need to do some quite significant board repairs ti get this good again. those totally missing traces are problematic, because there is absolutely nothing to anchor the component leads to. Don't bend over leads and bodge it. You need to make up a solid wire (or better copper foil) jumper with the ends formed to an eyelet matching the hole size, (or if foil, punched or drilled to size) and if it goes any distance, epoxy glued in place. Self-adhesive copper foil may work, but the glue is often less heat resistant than good epoxy. If epoxying, tin the pad end and the end to be soldered to the remaining trace before gluing. Seehttp://www.circuitrework.com/guides/guides.shtmlfor methodology and advanced techniques. Most of what you need is in sect, 4.Copper foil can be found in small quantities on Ebay and in some craft shops - its used by stained glass crafters amongst others.

author: aneng
author_rank: Regular Contributor
timestamp: November 20, 2015, 11:09:22 pm
content: Thanks for that Ian. That's a load of good info. I already have a fibreglass pencil, craft knife and a knackered jewellers screwdriver ! I got some copper sheet today - 0.13mm thick, which is significantly thicker than the PCB traces, but after experimenting tonight, ideal for replacing the missing tracks - it's thick enough (just !) for me to shape it with a wheel on the Dremel and thin enough for it to cut easily with scissors. I'll cut out the required shapes for the missing pads and tracks and epoxy them to the board, then drill the holes through. This is as much an exercise for me in PCB reworking as it is a learning curve in electronics, so I'm determined to do a good job. There was no chance of me bodging it... I'm way too OCD for thatI'll tackle the traces tomorrow, but I won't be able to do any more work after that until next weekend. I don't have the drill bits small enough (they're coming by Wednesday) and I'm working away all week doing WiFi surveys. I'll post some pics of my new pads and traces and then, hopefully, once I'm back on Friday, I can refit Q9 & Q10, resolder the 2W resistor network and check the continuity of the tracks, and plug it in.Who knows.... it may just work ?!

author: Ian.M
author_rank: Super Contributor
timestamp: November 20, 2015, 11:26:50 pm
content: Copper doesn't epoxy as well as some other metals. As you have thicker foil, you can sand the surface you will be gluingimmediatelybefore doing so, which will vastly improve adhesion. You should probably prick the foil at the hole locations before gluing so you can drill from the track side to minimise the risk of ripping the new track off. I suggest practice with some scrap veroboard in place of the PCB until you are confident in your abilities and to see how well it holds up to soldering.

author: aneng
author_rank: Regular Contributor
timestamp: November 20, 2015, 11:30:48 pm
content: You know, it never even occurred to me that I could use a different metal! I could have saved a couple of quid if I'd used steel. Never mind - it'll look nicer :-)

author: Ian.M
author_rank: Super Contributor
timestamp: November 20, 2015, 11:38:19 pm
content: Steel is a right PITA to solder or drill, Tin and Lead melt too easily, Aluminium is virtually unsolderable and difficult to glue. Neglecting precious metals, that only really leaves copper or brass and brass is too stiff to bend sideways if you need to 'torture' a track into place because you didn't quite cut it accurately enough.

author: sidspop
author_rank: Contributor
timestamp: November 20, 2015, 11:44:16 pm
content: i may be wrong, but the diagram appears to show a ferrite bead on R31 and R32? which aren't there. Not sure if relevant.Try your local Poundland type places for trimming knives, ACTO etc. I bought a set from one of those discount type places for a few quid.

author: Ian.M
author_rank: Super Contributor
timestamp: November 21, 2015, 12:19:42 am
content: R31 certainly isn't meant to have one. The diagram shows ferrites on the upper cascode transistor base leads (to prevent RF oscillation), and they are most likely the other side, where we cant see them due to the heatsink However I cant see any on Q3,4 either so they may not be fitted on this model.

author: sidspop
author_rank: Contributor
timestamp: November 21, 2015, 12:27:52 am
content: Sorry, I meant r32 and r33. It's been a long day and brain and eyes are tired

author: aneng
author_rank: Regular Contributor
timestamp: November 21, 2015, 07:54:53 am
content: There are ferrite beads on Q9 and Q10's base and emitter leads (I think.... but it's definitely two of the four wires on each). Incidentally, I'm presuming the fourth lead is a ground, or not connected internally ? It appears to be going to the ground plane on the board.

author: aneng
author_rank: Regular Contributor
timestamp: November 28, 2015, 10:10:37 pm
content: Right, back on it ! This is proving to be far more difficult than I envisaged. I replaced the tracks with copper sheet and epoxied them down. Got a 0.8mm drill and made the holes, then soldered the first transistor in, then realised I'd left the ferrite beads off, so had to desolder it. Unfortunately, the epoxy melted on two tracks during this procedure so I had to re-glue them down. I'm now a bit puzzled about the reassembly of the transistors and heatsink etc.The tracks going to the transistor leads are on theupperside of the board, underneath the heatsink / bracket. Looking at the service manual, it looks like they were soldered to single pads on the lower side of the board with through-plated holes connecting to the upper side tracks. The through-plating is long gone, as are the pads on the lower side of the board, which means the only place I can solder the leads is on the upper side of the board - i.e. get the iron inbetween the heatsink and the board. It's do-able, if a little tricky. Actually, thinking about it, I can't see how I'm going to be able to solder the leads with the ferrite beads on at all... The beads cover the leads completely from the transistor bottom down to the board.. I'll never get an iron in there, there's no gap!! Arghhh !! Also, about the ferrite beads... I measured their resistance, and it's somewhere about 7Meg. Given that they are touching each other and the heatsink (ground) is this a problem ? - or is resistance that high of no consequence ? (They're on the base & collector leads).My new soldering station came today - a Hakko FX-888D. Wow.... what a difference from using my old iron !! Incredible !!

author: vk6zgo
author_rank: Super Contributor
timestamp: November 29, 2015, 06:32:45 am
content: It's too late now,as you have already cut the traces,but a good trick is to steal tracks off a dead board.I don't mean the same type of board---------you will find useful shapes on most random "through hole" boards.You carefully overheat the track so it just starts to lift off,then slowly ease it off with a blade.Cirkitin the UK used to sell kits of "standard" tracks,but more importantly,they supplied "eyelets".These were thin copper rivets,designed to replace damaged "through hole" plated holes.They have the double advantage that they not only restore the original hole's performance,but also anchor the tracks,so reducing reliance on adhesive.That source has disappeared,but the "eyelets" are available on eBay.They are a bit expensive in commercial quantities,as they always were,but not so much in the numbers you would use.

author: Unknown
author_rank: Unknown
timestamp: November 29, 2015, 07:12:04 am
content: Quote from: vk6zgo on November 29, 2015, 06:32:45 amA bit of an aside, but I've used thin thin-walled hard copper tube to make DIY vias/PTH eyelets. The stuff is available in sizes from about 0.8mm OD (0.6mm ID) upwards from jewellers' suppliers, hobby stores, etc., along with tools for flaring / pressing.

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 07:14:39 am
content: I can see now why the previous repairs were done in the way they were - there was no access to solder the base and collector leads to the upper traces due to the heatsink and ferrite beads, which is why they bodged it and made solder bridges from the leads where they came through the board, onto the resistors directly.Hmmm... I think I may have to resort to some creative bodgery myself now, as I doubt my skills are good enough to try replacing through holes.Sent from my Moto G using Tapatalk

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2015, 07:22:18 am
content: +1 for eyelets. Its the only way to properly fix plated through holes with topside tracks with no access. However, it this case you don't need them as you can simply fit a new bottom-side track from each transistor lead to its associated resistor. You may need to trim back the pointy bits of ground plane for clearance thoughYes, epoxy does soften and let go when hot. If you don't overheat it and are careful not to shear the joint, it will recover most of its strength as it cools and in some cases will be even stronger (when moderate heat is applied after it has set hard but before its fully cured).On the ferrite beads resistance issue, don't worry about the emitter and base beads touching each other as there's only 0.7V between them, and as this is the final output stage and is in common base so is low impedance, I doubt a few megs leakage to ground will make any difference whatsoever, though it wouldn't hurt to put a small piece of Kapton tape on the heatsink.

author: SeanB
author_rank: Super Contributor
timestamp: November 29, 2015, 07:24:36 am
content: Next time just drill the hole out a little larger, and place and solder a thin strand or three of wire from the top side to the bottom, without filling the hole with solder so you can get the lead through. Then place the lead and solder from below. Another method is to drill a 0.8mm hole next to the original into the top trace and place a single 0.8mm copper wire through there, soldering on the top. Underneath then wrap the wire end around the component lead, no pad required, and solder. Works well, and if you need extra strength simply use a drop of superglue after soldering and cleaning the flux off to hold the leads in the holes.

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 07:33:22 am
content: Genius! I like that copper wire trick!, particularly as there aren't any pads left on the bottom of the board for these leads.I can manage that, but I doubt I'd be able to secure eyelets and also get them connected properly to home-made tracks on both sides AND get the leads through and soldered in with it all melting and coming apart! The home-made tracks are hard enough on their own!Sent from my Moto G using Tapatalk

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2015, 08:12:36 am
content: Quote from: aneng on November 29, 2015, 07:33:22 amThe whole point of the eyelets is they rivet through from one side of the board to the other. As such, they will significantly aid retention of tracks and individual pads. See:However there are a PITA to get in small quantities with the tooling you need to set them, so if you can avoid them for this repair, do.You are teaching yourself the skill-set for high quality PCB repair. You will make mistakes that cost you time and even money, but with a little bit of luck will have a working scope at the end of it + PCB repair skills as good as or better than those of many techs working in electronics servicing, which alone is worth far more than the fully restored scope would be.

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 10:23:50 am
content: Whilst looking for eyelets on the net (out of curiosity rather than any plan to use them here) I came across this document:-http://www.intertronics.co.uk/resource/guide1.pdfIt seems to be a single doc version of the website you linked me to earlier, and has some detailed instructions on all kinds of PCB repair procedures. I don't know if it's of any use to anyone else but the link is above.Meantime, I've ordered some 0.8mm enamelled copper wire but I'm not going to wait for that to arrive for this repair. I'll just use component leads to attempt that method SeanB suggested, though I can't see me being able to wrap the wire end around the transistor leads, seeing as they will be roughly the same diameter wire. I'll try a single loop around it if I can though. At least I'll be able to mount the transistors and have it all pretty secure physically. I've learned a lot so far, and will hopefully use this knowledge in the future. Right now, I'm in danger of getting hung-up on getting 'the perfect repair' on this particular board when all I truly need is a repair that will last - and get me a working scope ! (Yeah, that's the sound of my standards slipping !).

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2015, 10:47:43 am
content: Pre-forming copper wire ends is quite easy if you have a stiff steel pin to form it round.I usually use small diameter jeweller's screwdrivers. For even smaller loops, clamp a steel needle of an appropriate diameter in a pin vice, or drill a hole in a block of wood and superglue it in. Start with a wire longer than you need so you can wrap it completely round to get a single turn you can pull tight, cut off the excess, then add a rightangle bend to get the loop inline with the wire, then flatten it into a single plane.Its good not to loose sight of the desired end result, and IIRC, the Y inputs/preamp board is also going to need some work.Do the bits that will be a PITA to rework properly, then semi-bodge the rest with tinned copper wire with formed end eyes, simply soldered, not glued. If it offends you later you can always go back and re-do/tidy it up.

author: Deathwish
author_rank: Unknown
timestamp: November 29, 2015, 10:52:35 am
content: could be interesting , wonder who sells the rivet gun.http://www.ebay.co.uk/itm/0-5mm-0-8mm-1-0mm-Miniature-Brass-Rivets-x50-Pocher-Tamiya-Model-Upgrade-/141476696032

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2015, 12:20:49 pm
content: I suspect those are solid dome head rivetsFor miniature stuff I'd want a dolly in a close fitting drilled hole on a block of wood to position the board over to hold the eyelet or rivet pressed into place, and a set of forming punches. For larger diameter eyelets, its possible to use a ball bearing as an initial forming tool to flare the end, but peening the end over properly without a forming punch is a PITA and is probably impossible at the sizes commonly used for PCB work.Of course, you *COULD* rivet though the board with solid copper wire then drill through the rivet for the component leg after its set and soldered, but copper is so gummy to drill that you'd need to peck with a precision drill press to get that done without snapping or welding the bit.

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 07:24:38 pm
content: Right then... Finally getting somewhere !! I tried my best to stick to the plan of keeping it all neat and tidy, adding wire as suggested, but in the end it was becoming obvious that the most sensible solution was to recreate the repair that had been done previously. I would have been drilling holes and adding wires etc when the components I'm ultimately trying to connect had leads less than 3mm from each other. So, I'm ashamed to admit I bodged it, though I did add little wire links instead of just using solder to bridge the gap as previously !No... you're not getting a picture - it's not pretty ! - but.... it's solid and... it works. Sorta.I say 'sorta' because I still have a dead channel 1. However - channel 2's trace now goes up and down with the Y-Pos knob and it looks rather pretty !The more observant (or just those with eyes!) will spot the Function Generator claiming to be outputting a 1kHz sine wave at 5V p-p and the scope displaying... well... a dog's dinner. I was rather despondent when I saw that, and switching waveforms to square, triangle, ramp etc, just slightly changed the display - I still got a wonky square-ish wave. I then wondered whether there was a problem with my Function Generator, so just to be sure, I downloaded a signal generator app for my phone and connected that up - a 440Hz sine wave (of undetermined amplitude) displays as below:SOOOOO..... It appears that when I connected the power back-to-front to my newly built Function Generator and an electrolytic exploded, it must have caused more damage than just the cap and the 5V regulator that I subsequently replaced. That's another repair project to tackle !So, where do we go from here ? Without a working function generator, how can I calibrate Ch2 of the scope ?The 0.3V p-p @ 1kHz CAL point on the front of the scope shows as below, but... bear in mind the scope is set to 50mV/Div vertically and 0.5ms/Div Sweep. It doesn't look right to me... unless I'm being a numpty, surely two and a half divisions at 50mV/Div equals 125mV peak to peak ? Shouldn't it be displaying six divisions (300mV/50mV=6) ?

author: donmr
author_rank: Regular Contributor
timestamp: November 29, 2015, 07:35:30 pm
content: I'd start by looking at DC signals. You can measure some DC source with a DVM as an initial reference, then connect that to the scope input and track the levels through the scope circuits. Once it is displaying DC levels accurately then look at pulses and sine waves.

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2015, 07:59:00 pm
content: I assume the red variable volts/div knobs are fully clockwide in their 'cal' positions.It could be a problem with the beamswitch if in addition to not switching, its dragging down the CH2 preamp output.Back in post#38 on page 2 of this topic, I suggested using a x10 OPAMP circuit to amplify the output of a PC (or phone) signal generator enough for the amplitude to be measured with a DMM.

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 08:04:59 pm
content: Hi Ian, yeah I remember you saying about doing that but to be truthful, I don't know how to build what you are describing, so I bought the function generator kit!Sent from my Moto G using Tapatalk

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 08:19:22 pm
content: If it's just a case of amplifying a signal, I've got plenty of kit to do that... Mixer, guitar amp etc. And yes, the scope dials were all on calibrated settings.

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2015, 08:53:25 pm
content: Its just a case of amplifying a signal in a predictable manner. If you use an OPAMP, you have a reasonable probability of being able to maintain the same gain from DC to some tens of KHz. If you use an audio amp, then the results are far less predictable, you will only be able to calibrate it for sinewaves and will need to re-measure the amplitude every time you change the frequencyHere's the page from Forrest Mims' OPAMP book:I'd use something more modern than a 741 though. Just about any 8 pin DIP sinngle OPAMP that will work with +/-12V supplies would do. If you don't have a suitable PSU, use two 9V batteries. Make R1 and R3 10K and R2 100K. The precise value of R3 doesn't matter much with a modern OPAMP. It was only the old bipolar ones that required you to follow the formula given to equalise the offset due to the input bias current.

author: MarkL
author_rank: Unknown
timestamp: November 29, 2015, 09:04:26 pm
content: Glad you got the PCB repaired! Honestly, it's what I would have done also given the age of the scope and the previous repairs, but I wasn't going to interrupt and say not to do a professional job.At the moment we don't know if it's the cal output or the vertical channel.Expanding on what donmr said, I would keep it simple and try to measure some DC voltages.Find a 1.5V battery and verify the actual voltage with your DMM. Assuming it's around 1.5V, Set the scope on 50mV per division and see if you get 3 divisions of deflection if you're using a 10x probe. If you're using a 1x probe, use 0.5V per division.If you don't get the required deflection, let's make sure it's not the probe. Unplug the probe and set the scope to 0.5V per division. Hold the "-" of the battery to the ground post on the front of the scope. Then, using a short jumper wire held on the "+" side of the battery, *carefully* touch the inside contact of the Ch2 BNC connector and observe the deflection. It should be 3 divisions.If not, there's plenty of places that are suspect, including the vertical amp section you just repaired.It would also be worthwhile to try the above with the beam in the upper part of the screen and again in the lower part of the screen to make sure the results are not different (asymmetrical). Asymmetry can happen when one half of a balanced amplifier is damaged.If ok, note the deflection that you're getting, even if it's wrong. It will at least give you a rough verification of the output level of your phone app output and/or your questionable function gen.EDIT: Actually, the sine wave looks fine, so there's not a symmetry problem. You can skip that test.

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 10:19:16 pm
content: First quick DC tests; I measured a fresh 9V battery with my DMM at 9.52V. Connected it to CH2 on the scope, set to DC coupling, 1V/Div and it registered only 7.8V. Initially, it didn't register at all ! - until I fiddled with the coupling selector and found a point halfway between DC and GND where it registered. There is definitely something wrong with that switch - it feels gritty compared to the one on CH1. I'm going to read the service manual now to work out how to get the fascia off so I can get at the switch and either repair it, or swap it for the one on CH1.

author: aneng
author_rank: Regular Contributor
timestamp: November 29, 2015, 11:55:37 pm
content: Right... that's the switch repaired ! Things are heading in the right direction thankfully ! (I think !)OK, so my 9.52V battery is still showing as 7.8V on the scope, but the 0.3V CAL signal is now registering at 250mV, which is better than before.However... have a look at the following pics... I'm not knowledgeable enough to know if they are correct or not. I thought that the GND position on the input coupling switch grounds the input ? - if so... why am I getting a square wave from the CAL signal ?Input is the 0.3V/1KHz CAL signal, the scope is set to 0.1V/Div vertically.Coupling set to AC:Coupling set to DC:Coupling set to GND:Is this right ??Edit: I've just measured the PSU for the Function Generator. On the DMM it registers 14.9 V (it's a 15V PSU), on the scope it registers 12V, so there is definitely something wrong here.

author: MarkL
author_rank: Unknown
timestamp: November 30, 2015, 01:08:29 am
content: It appears that Ch2 is reading consistently about 18% low. Since Ch1 is not working, we don't know if it's something specific to Ch2 or something common to both, so I wouldn't attempt any calibration adjustments yet.18% is a pretty big drift from nominal, so either something is still wrong or someone has messed with the adjustments.The AC and DC coupling traces are correct (albeit low by 18%), but switching to ground should show a flat trace. I'd take a close look at the AC/GND/DC switch and examine the contacts when it's in the GND position. If you can identify the center pole of the selector switch, you should be able to measure a short to ground when in the GND position (at least according to the schematic for the BK 1590A).It's certainly possible your model is different than the 1590A, in which case try to trace what the switch is supposed to do in GND mode.EDIT: Never mind about the 1590A. I didn't see the schematic for the Trio input switch. It's on the interconnect diagram. Repeated below for your convenience. It operates the same way, so the above still applies.

author: aneng
author_rank: Regular Contributor
timestamp: November 30, 2015, 01:04:09 pm
content: It's not jumping out at me what is supposed to connect where with switch diagrams - they never seem to make sense to me! I'm guessing that in AC, the signal goes through R7, C2 and R8 in that order. In GND, it grounds the switch end of R8 ? (and leaves the BNC connector floating ?) and in DC, it goes through R7, R4 and R8 ?

author: MarkL
author_rank: Unknown
timestamp: November 30, 2015, 03:10:23 pm
content: Multi-position switches can be drawn in a number of ways, so it can be confusing.As you've surmised on the diagram, the arrow pointer moves vertically on the left three positions and stays anchored on the right (the common).You're correct about the signal path for AC and GND. For DC it goes through R7 and R8. R4 is there to make sure C2 is not left floating and it dissipates any charge that might be left on C2.The problem you're seeing in GND mode is the signal through R7, C2, R4,R22R8 and into the vertical attenuator. The input impedance of the vertical attenuator input is 1M. This signal path gives you a zero-centered display as in AC mode (since it's through the capacitor), and half the amplitude since you now have a voltage divider made up of R4 and the vertical attenuator.So, there's something wrong with the path from the switch common to ground when in GND mode. There should be no signal reaching the vertical attenuator.See if you can spot a bad contact, solder joint, etc., but at the moment I wouldn't invest a huge amount of time in fixing this issue. You can always remove the probe from the circuit to set the ground reference level, and you can come back and fix it later. I would give higher priority to getting Ch1 working.EDIT: R8, not R22 (value is 22).

author: vk6zgo
author_rank: Super Contributor
timestamp: November 30, 2015, 03:18:04 pm
content: It looks a bit funny,the way it is drawn,but it is the way draughtsman like.If you redraw it,showing the moving contact centrally located,you will see that with Ch2 in the AC pos,the connection is via R7,C2,fixed contact moving contact,R8,to pin (2) of the attenuator.In the DC position,connection is R7,fixed contact,moving contact,R8,to pin (2) of the attenuator.In the GND pos,switch end of R8 is grounded.R4 is provided to discharge C2.

author: aneng
author_rank: Regular Contributor
timestamp: November 30, 2015, 03:52:25 pm
content: Thanks for the explanations guys - it's all become clear now. Ian, I bought the op-amp and resistors to make the amplifier you suggested. I'll breadboard it later if I've got any energy left ! ...I'm currently halfway through replacing the exhaust on my wife's car, outside, in the rain, blowing a gale. I'm waiting for a break in the rain to continue trying to remove some rusted bolts, but it's getting dark now... so I may end up under the car for a while. From one extreme to another eh ! - soldering minute copper pads under a magnifying glass one minute, then beating the hell out of a rusty bolt with a lump hammer and inventing new swear words the nextI know which one I prefer !

author: aneng
author_rank: Regular Contributor
timestamp: November 30, 2015, 10:39:20 pm
content: OK, I've had another look at the AC-GND-DC switch on Ch2 and I definitely didn't forget to solder the ground lead back from the switch when I 'fixed' it last night. Nosireebob... not me. Well.. maybe a bit.So.... that's the switch properly fixed this time, and indeed, the ground position results in a nice flat trace now !I've been poking about checking voltages to see if I can narrow it down a bit as to why CH1 doesn't respond and why CH2 is reading low.I've found something that may or may not be of help.. Where the connectors for each channel leave the Vertical Attenuator Unit board and join the Vertical Preamp Unit board (P3 and P4 on both schematics (pages 95 & 99 of the service manual for the CS-2070 (which so far is identical to my CS-2100!);https://www.dropbox.com/s/ikw5aa81hvlm5ut/trio_kenwood_cs-2070_70mhz_oscilloscope.pdf?dl=0), according to the schematic, the pins on each connector P3 and P4 should be (1-6 in order) Out-, Out+, +10V, -10V, NC, GND.On CH2 (P4), with the scope setup as per the manual (most knobs centred etc), and set identically for both channels, I'm getting -1.3V approx on both the Out- and Out+ pins. (The +10/-10 V pins are ok on both channels),howeverthe same Out- and Out+ pins on the CH1 connector (P3) are reading-9.7V.So - either the fault is somewhere around IC12 on the Vertical Attenuator Unit or the erroneous voltages are coming in from the Vertical Preamp Unit. I was going to try to separate them and read the connectors on each board to see, but I'm not sure whether that could be dangerous to the scope.

author: aneng
author_rank: Regular Contributor
timestamp: December 01, 2015, 12:21:17 am
content: I can't get this damn Op-Amp thing to workI bought a TL082CN and the resistors, wired it up as per the diagram and the datasheet and I just get nonsense out of the output. I've got the signal from my phone app going into pin 2 (the inverting input) via a 10K resistor, the grounds of the input and output going into pin 3 (the non-inverting input) via a 10K resistor. Pin 2 is connected to Pin 1 (output) via a 100K resistor. +12V is connected to Pin 8, 0V to Pin 4. I just get a freaky mush on the scope.The retailer's pin-out diagram is wrong, I'm pretty sure..http://www.maplin.co.uk/p/tl082cn-ra71nThe manufacturer's datasheet shows it as a dual op-amp with different outputs and VCC pinshttp://www.st.com/web/en/resource/technical/document/datasheet/CD00000492.pdf.I'll stop fiddling now until someone tells me what to do next otherwise I'll end up going around in circles !

author: Andy Watson
author_rank: Super Contributor
timestamp: December 01, 2015, 12:44:43 am
content: Quote from: aneng on December 01, 2015, 12:21:17 amAre you saying that "ground" and "0V" are one and the same? If so, that could be the problem - the chip is not rail to rail, it will only work if its inputs are about 3V (or more) above -Vcc, pin 4.QuoteYep! Good ole Maplin. I'm sure they employ several people just to screw-up their website

author: aneng
author_rank: Regular Contributor
timestamp: December 01, 2015, 01:14:09 am
content: No... I have signal ground and -VCC as different connections. Signal ground is shared between input and output only. -VCC goes nowhere but pin 4.You say it won't work unless the input is at least 3V? (0V+3). The input is from the earphone socket of my phone... That won't reach 3V will it ?

author: Andy Watson
author_rank: Super Contributor
timestamp: December 01, 2015, 01:20:18 am
content: Quote from: aneng on December 01, 2015, 01:14:09 am+3V relative to pin 4.

author: Ian.M
author_rank: Super Contributor
timestamp: December 01, 2015, 02:09:40 am
content: For the OPAMP, the positive and negative supplies must be referenced to ground, and *SHOULD*. be symmetrical about it You either need a dual output supply, or a fully floating single output one + a circuit to split it and get a mid-rail 'ground'. You can do that with the spare half of the OPAMP (and yes, the Maplin catalog has been FUBARed since before the millennium, so use the manufacturer's pinout!), wiring -in to out of that half and connecting two 10K resistors to +in, one to each supply rail, then use out as ground for the rest of the circuit. This only works properly with a floating supply because if the -ve supply is grounded, connecting any signal ground (e.g. scope ground clip) to the rail-splitter output 'ground' will short out the OPAMP.

author: aneng
author_rank: Regular Contributor
timestamp: December 01, 2015, 07:40:47 am
content: Right, I see. OK, two 9V batteries it is then! I'll tackle that this evening, presuming I'm not still underneath the wife's car, swearing profusely!

author: aneng
author_rank: Regular Contributor
timestamp: December 01, 2015, 08:08:37 am
content: Just another quick post to reiterate my thanks and appreciation for the help here. I certainly wouldn't have learned as much as I have if it wasn't for you guys (esp Ian.M). This sounds a bit like an Oscar speech doesn't it! I'll save that till the end when I'm holding a working scope at the podium :-)

author: MarkL
author_rank: Unknown
timestamp: December 01, 2015, 03:56:06 pm
content: Quote from: aneng on November 30, 2015, 10:39:20 pmOne thing you could do is try swapping the cables plugged into P3 and P4 to see if the problem moves to the other channel. That will at least get us to the right board.

author: aneng
author_rank: Regular Contributor
timestamp: December 01, 2015, 04:43:47 pm
content: Alas, that's not possible. They aren't cables - they're push-together headers on the boards. One board mounts on top of the other, piggy-back style and P3 and P4 are not moveable.

author: MarkL
author_rank: Unknown
timestamp: December 01, 2015, 06:41:17 pm
content: Quote from: aneng on December 01, 2015, 04:43:47 pmDoh! So much for the easy way.Can you get access to the pins on IC12? Pins 8 and 9 are OUT+ and OUT- and it would be worth verifying they are also at -9.7V. (I'm rather suspect of solder connections in this scope now.) Also check pin 14 +10V supply, and pin 3 -10V supply.What is the voltage on pin IC12 pin 15? How does that compare to IC13 pin 15? That's the input.Are there any markings on IC12? The parts listing says it's an ATM-4010, but I'm having trouble finding a datasheet for it. It's some kind of high speed amplifier.

author: aneng
author_rank: Regular Contributor
timestamp: December 01, 2015, 10:37:17 pm
content: Here's what we're talking about... IC12 is the black square IC in the top of the picture.The only way I'm getting access to the pins of that IC is by powering it up with both piggy-backed boards unscrewed from the chassis. Given the significant potential for a quick game of 'shorty-shorty-zap-bang' I don't think it would be wise for me to attempt it !I'm curious why you want me to test on the pins of that IC. Surely if the Out+ and - on P3 are reading -9.7V then the connected pins on P3 must be the same ?Edit: I'm still not getting anywhere with this bloody Opamp circuit ! I've got two 9V batteries connected up as +9V - 0V - -9V. +9V to VCC, -9V to -VCC. Is the 0V supposed to be connected to signal ground or not ? I haven't connected it and I'm not seeing any amplification at all - it's like it's just passing the same signal through it at 1:1.

author: MarkL
author_rank: Unknown
timestamp: December 01, 2015, 11:40:14 pm
content: Ok, that's going to be a bit of a challenge.I was interested in the voltages around IC12 since I'm a little distrustful of the connectors and solder joints in this scope. I wanted to verify directly on the chip that's using them. And on the IC12/IC13 input, it would be useful to know if there's a problem with the JFET input stage that's preceding the amp.Let's try from the other board. Can you measure the Q1 base (should be -6.25V), and then the Q1 base to emitter (should be 0.7V)? Q1 is biasing the input stage consisting of IC1a and IC1b. It's probably ok, which is why I wanted to start on the attenuator board.Also, an exact reading on the +10V and -10V supply will be useful. Everything in the input stages is using them.

author: MarkL
author_rank: Unknown
timestamp: December 01, 2015, 11:52:29 pm
content: Looking a little more closely at the picture, it looks like you have a shot at getting to the pins on Q1 or R28 on the attenuator board.The node that's between Q1b and R28 is connected to the amp input (IC12, pin 15).If you're concerned about shorting something out with your probe, you could try slipping a piece of insulation over the exposed tip and just leave a tiny bit sticking out. If it's really tight, a solid piece of wire can also be a probe, again with the slightest amount of copper sticking out. In some cases a piece of wire is better since you can bend it to get around leads and other obstacles.If you can get to it, try to also measure the input on the other amp (IC13) in the same way.

author: Ian.M
author_rank: Super Contributor
timestamp: December 02, 2015, 03:29:50 am
content: To get access to boards like that, you often have to make up extender cables. If the boards have a lot of connectors, that becomes a total P.I.T.A.Otherwise, its possible to tack-solder thin insulated wire (e.g. Kynar wire wrapping wire) to the points you want to test and bring it out to something like a 0.1" pitch pin header (to give you something to safely anchor the free wire ends to so they don't short out) so you can probe them with minimal risk of letting the magic smoke out.Re the OPAMP: Yes 0V is also signal ground. I had to simulate it to convince myself that without that connection it would pass the signal unamplified and non-inverted, but it did, so probably that would be enough to get it working.You should probably take a look at the rest of Forrest Mims' Engineer's Mini-notebook: Op-ampshttps://archive.org/details/Forrest_Mims-Engineers_Mini-Notebook_Op_Amp_Ic_Circuits_Radio_Shack_Electronics

author: aneng
author_rank: Regular Contributor
timestamp: December 02, 2015, 11:14:56 am
content: Will do. However, it'll have to wait until Friday now as I'm working away for a couple of days. I'll get back with the readings then. Thanks !

author: aneng
author_rank: Regular Contributor
timestamp: December 02, 2015, 02:39:20 pm
content: I just popped back to say I'd had an idea of removing the board and soldering wires to the inaccessible points and reassembling and found you'd already suggested it ! I do get there in the end, just not as quickly as you guys ;-)Thanks for the link Ian, I'll definitely check it out.

author: Ian.M
author_rank: Super Contributor
timestamp: December 05, 2015, 11:01:30 pm
content: The fact that the gain as determined by measuring amplitudes with the scope is not x10, means either you've still got a problem with the OPAMP circuit (e.g. incorrect resistor values) or there is a serious problem with the Y2 input attenuator. If you used the correct component values, it certainly shouldn't have a gain of over x127 (13.8V/108mV)Post a good closeup photo of the OPAMP circuit please.

author: aneng
author_rank: Regular Contributor
timestamp: December 05, 2015, 11:08:28 pm
content: Er... You may have noticed I quickly deleted my post ! I had the input going in on the wrong side of R1 ! - hence the insane gain.I've just corrected it and it's all working nicely with a perfect x10 gain.Presuming the next step was going to be measuring the output from the opamp with a DMM, here are the figures...Input (measured on the scope, which we know is reading low)= 1.15V p-p (1kHz Sine)Output (measured on the scope) = 11.5V p-pOutput (measured on DMM) = 4.75V AC.I'll solder up those extensions to the IC and get some more voltages now...

author: Ian.M
author_rank: Super Contributor
timestamp: December 05, 2015, 11:25:43 pm
content: 4.75V RMS *2*sqrt(2) is 13.44V pk-pk ==> your scope is reading 15% low. (ASS-U-MEing the frequency was within the specified input range of your multimeter's AC volts range and that your multimeter is accurate to better than three sig. figures)

author: MarkL
author_rank: Unknown
timestamp: December 06, 2015, 12:41:58 am
content: Quote from: Ian.M on December 05, 2015, 11:25:43 pmIt's consistent with the DC readings which were reading about 18% low.

author: aneng
author_rank: Regular Contributor
timestamp: December 06, 2015, 12:43:35 am
content: I've managed to get to the board with IC12 and 13 on (Vertical Attenuator Unit). That wasn't easy!I'm too tired this evening to attempt soldering on extension leads to the chips, but I have spotted that there are some minor variations between the schematic and the board I have. If it's required, I'll try to draw a diagram illustrating what, and where they are. However, I have spotted a thing that looks like a long resistor that is not on the schematic, but goes in the line between the output of pin 8 of IC12 ( the 'OUT+' connection) and the header for P3. I have absolutely no idea what it is ! - any suggestions ? :...It's the long brown thing with the single black band and long leads, running down the side if IC12.There also appears to be evidence of some previous work around IC12 and 13 in the past, judging by the presence of flux...

author: MarkL
author_rank: Unknown
timestamp: December 06, 2015, 12:49:22 am
content: That's a zero-ohm jumper. One black band = 0R.https://en.wikipedia.org/wiki/Zero-ohm_link

author: aneng
author_rank: Regular Contributor
timestamp: December 06, 2015, 08:34:48 am
content: Well well, I never knew about those ! Cheers Mark. One other thing (until I get the voltages later) - what's that extra bit connecting Pin 1 to ground ? It's not on the schematic, but it appears to consist of two diodes in series with the cathodes facing each other, followed by a resistor. What would that do ?

author: aneng
author_rank: Regular Contributor
timestamp: December 06, 2015, 02:31:55 pm
content: Quote from: MarkL on December 01, 2015, 06:41:17 pmRight then... readings time ! I made up an extension by chopping the last foot off an ethernet cable. That gave me 8 wires terminating in an RJ45 plug which worked nicely.With the scope set as per the settings listed on p90 of the manual;IC12(i.e. Channel 1)Pin 8 (OUT+) = -9.73VPin 9 (OUT-) = -9.65VPin 14 (+10V) = +9.77VPin 3 (-10V) = -9.94VPin 15 (INPUT) = -1.375VIC13(i.e. Channel 2)Pin 15 (INPUT) = +0.449VBoth channels controls are set identically. There is obviously something going on with the input as Ch1 is minus 1.375V and Ch2 is plus 0.449V.

author: MarkL
author_rank: Unknown
timestamp: December 06, 2015, 09:52:39 pm
content: Quote from: aneng on December 06, 2015, 08:34:48 amOne diode is probably a switching diode, and the other is probably a zener. If that's right, the pair would be trying to limit the voltage excursions to zener_voltage+0.7V. And the resistor is added to limit current once the voltage limit is reached.I think pin 1 is likely still ground, so this extra cricuit is limiting the input pin 15, perhaps to prevent overload and/or to improve recovery time for overly large signals. Sure looks like an after-thought to the design.And yes, I also see some variations from your layout and the schematic. Going off-road.More in a bit on the measurements...

author: MarkL
author_rank: Unknown
timestamp: December 06, 2015, 11:01:44 pm
content: Quote from: aneng on December 06, 2015, 02:31:55 pmThe discrepancy in voltage is certainly interesting. It doesn't exonerate IC12, however, since something wrong inside IC12 could be pulling down the input pin 15.Let's assume IC12 is ok and let's see what the dual JFET Q1 is supposed to be doing.To start, we have the diode network of D13 + D2 and D12 + D1. D13 and D12 are both 12V zeners.D12 cathode is connected to +10, with a pulldown to -10V via R15. That would make D12 anode -2V (approx). D13 anode is connected to -10V with a pullup to +10 via R16. D13 anode is therefore +2V. The various caps provide filtering. So, this circuit generates symmetric -2V and +2V reference points. (Hint: Those are some voltages to check.)Q1a gate is the input to the first stage, and is connected to the reference points by diodes D1 and D2. Anything more positive than D13 anode + 0.7V (= +2.7V) will cause current to flow through D1, thereby clamping the input voltage. Conversely anything less D13 - 0.7V (= -2.7V) will be clamped through D2.So, the input voltage to Q1a gate is limited to +/-2.7V (approx). R18 47R doesn't matter because the input current to Q1a is on the order of a few 10's of picoamps. Your reading of -1.375V is within range, but check that the -2V and +2V reference points are ok.JFET Q1a is configured as a voltage follower. The source (the bottom) will be a couple of volts *higher* than the gate (JFETs operate reverse biased). Q1b is providing the right amount of current so that the source will be pulled back down to 0V when Q1a gate is also at 0V.How does Q1b know how much current to provide? Well, that's why Q1a and Q1b are a matched pair. Q1b's drop from gate to source and then through R29 100R is equal to the same drop on Q1a and R28 100R.So, that should tell you some other voltages to check. See if Q1a gate to source (Vgs) is the same as Q1b Vgs. And if you want to keep me honest, you can check Q2a and Q2b Vgs also.I know soldering test wires to probe voltages is a pain. From your photos, it looks like you can get to Q2's leads from the top. Maybe Q1 if the potentiometer isn't too much in the way.Here's another thought. A dynamic test on IC12's input is also possible. While watching IC13 pin 15 with your DMM, try the 9V battery DC input test on Ch2. Keep the trace on the screen. Note the two DMM readings between 0V and 9V. Then move the DMM to IC12 pin 15 and input into Ch1 and compare results. The absolute voltages may be different, but the difference between 0V and 9V input should be the same on both channels.Perhaps do this test first before the above JFET tests since you already have those wires soldered and accessible.

author: aneng
author_rank: Regular Contributor
timestamp: December 06, 2015, 11:15:53 pm
content: I found a site selling the service manual for the CS-2100A and bought that in the hope that it would be more accurate than the CS-2070 manual I have. Unfortunately.... it's not ! Certainly on the section of this board anyway, it's identical to the CS-2070, schematic-wise. The PCB layout seems slightly different though - but again, it doesn't match my board ! I've since found another site that claims to have the service manual for the CS-2100 (i.e. mine) - and although I've paid, the download link hasn't appeared yet. They reckon it can take up to 24h... so undoubtedly this time tomorrow I'll be emailing them saying "Where's my *&%ing manual" and shortly after asking Paypal for a refund (cynical old sod that I am !).Meantime, here is a couple of pics of the relevant board... IC12 (Ch1) is at the bottom right - it's the two rows of 8 pins....and the flip side. Notice the big blobs of solder joining the ground lands/traces either side midway down. That doesn't look original.I've already removed my extension wires, so it's no problem to do any test. I must confess, your last post was a bit advanced for me at this stage in my learning (the explanation of the circuit) - my knowledge isn't developed enough yet to really understand. However... what test do you think is the best next ? - the 9V battery one ?Incidentally, here is the service manual for the CS-2100A :https://www.dropbox.com/s/a9dgag80gldz56s/Trio%20CS-2100A%20Service%20Manual.pdf?dl=0

author: MarkL
author_rank: Unknown
timestamp: December 07, 2015, 12:26:59 am
content: Quote from: aneng on December 06, 2015, 11:15:53 pmSorry, got carried away on the explanation. I'll keep it shorter unless detail is needed.Yes, I think the 9V test would be best to do next. Comparing a working circuit (Ch2) point by point to an identical non-working circuit (Ch1) is a great debugging technique. You've already found a static voltage difference. Adding an input stimulus of an on/off 9V signal (essentially a manually controlled square wave) allows comparison under dynamic conditions.It's not known if the input pin 15 is stuck at -1.375V, or if it moves with the right magnitude to track the Ch1 input. If it's stuck, we need to dig into Q1. If it moves and is consistent with Ch2, we'll need to start looking after IC12 to see if something is holding its outputs low.Yeah, looks like the new download manual is still not right. It's also possible that your actual scope circuit was after one or more engineering revisions and the final schematic was never published.

author: MarkL
author_rank: Unknown
timestamp: December 07, 2015, 02:38:42 pm
content: Thanks for posting the full board photos.I'm noticing that those zero-ohm jumpers are actually the IC12 and IC13 outputs, OUT+ and OUT- on pins 8 and 9. At least it looks that way to me; you can verify up close.If so, getting back to my initial suggestion of "swapping the P3 and P4 cables", you could still do the equivalent by swapping the amplifier outputs. You could unsolder all four zero-ohm jumpers, and then solder in temporary jumpers cross-connecting IC12's OUT+ and OUT- to where IC13 OUT+ and OUT- was connected, and visa-versa.The only other pins that wouldn't get swapped are +/-10V power, but you've already confirmed those are ok.Just a thought to isolate the issue to this board or the vertical preamp board. But try the 9V test first.

author: aneng
author_rank: Regular Contributor
timestamp: December 07, 2015, 06:27:15 pm
content: Hi Mark,Thanks for your help again... Once again, I've been sent to work away, so I won't be back home until Thursday evening now. HOWEVER !! - Look what I've managed to get !!! -https://www.dropbox.com/s/y2s3u2tqknquwwz/CS2100_SM_TRIO.pdf?dl=0The real-deal-genuine andcorrectService Manual for my exact scope !! I've checked the traces on the board we're working on in the new manual and they match my board !! ....and there I was being a cynical old sod expecting to get ripped off, and they came through with the prize !!I'll be ready to tackle the scope as soon as I'm back home. I must say - I'm loving this ! My day job of computer network engineer has elements of fault-finding, hunting down the culprit that's causing the trouble, and it's that part of the job I like most. This is like a concentrated version of the same thing.I'm just hoping it's not IC12 that's faulty, as the chances of getting a replacement for something that doesn't even appear on a Google search are nil I reckon.

author: Ian.M
author_rank: Super Contributor
timestamp: December 07, 2015, 06:49:56 pm
content: If it is the IC, you put the word out that you want a scrap scope of the right make and series or the board in question from one. Worst case, one channel could be got to work within spec.

author: MarkL
author_rank: Unknown
timestamp: December 07, 2015, 08:46:54 pm
content: Wow - I'm glad you found the right manual! The input stage with IC12 and IC13 are quite different. We surely would have been off into the weeds.What I said about the zero-ohm resistors is still correct, if you want to try it.And the 9V test might still provide some clues, but Q1 is now co-mingled with two pins on IC12, so interpreting results will be less straight-forward.There's also a new labeled voltage to check, +5Va on Ch1 and compare it with +5Vb on Ch2.It looks like pins 2 and 15 are inverting and non-inverting inputs. Comparing IC12/IC13 pin 2 on both channels could be useful too, if possible.I wish we had the ATM-4010 datasheet. I can't even find that, let alone a possible replacement. That's the Trio logo on top of the chip. It's probably a custom hybrid given the shape.Good luck on your trip.

author: aneng
author_rank: Regular Contributor
timestamp: December 11, 2015, 11:14:59 pm
content: OK... here we go again ! I swapped the outputs from IC12 and 13 by desoldering the chip ends of the two zero-ohm resistors and soldering a couple of insulated wires onto the boards at those points (i.e. in the holes left vacant by the lifting of the zero-ohm resistor ends), then basically soldering the other end of each wire onto it's opposite channel's zero-ohm resistor. Unfortunately, this produced nothing amazing - CH1 was unchanged - the trace stuck, unmoving, on the screen and CH2's trace was way off the bottom of the screen somewhere - couldn't see it at all.However.... measuring the drain voltages on Q1b and Q2b produced some interesting figures; Q2b's drain is at +7.00V and Q1b's drain is atminus0.9V.Q1b's Source (i.e. IC12, Pin 2) is also at -0.9V, while Q2b's source (IC13, Pin2) is +0.4VWhat are all those voltage references, such as "+5Vb", "+10Vb", "+5VL" etc ? Are they test points to be measured or are they places where supply voltages are fed in from somewhere ?

author: MarkL
author_rank: Unknown
timestamp: December 12, 2015, 12:17:05 am
content: Quote from: aneng on December 11, 2015, 11:14:59 pmPerhaps nothing amazing, but certainly useful! You've just proven the issue is on the attenuator board.And to confirm... Now you can select Ch1 on the scope and feed a signal into the Ch2 connector, and it works? You can use the CAL output or your sig gen if you want.QuoteThe drains, if both channels were working, should be about the same. The +7.00V is not too surprising. In a quick simulation I did based on your previous measurements, I came up with +6.65V for the drain. It's really not too critical unless it starts getting close to +10V, at which point we're going to lose dynamic range on the channel.The "+5Va" is a hint that the voltage should be around "+5V". It took me a while searching the schematic to figure out there was no "+5Va" and "+5Vb" supply. With the biasing arrangement, +5V is approximately what the designers expected and that's how they labeled that signal. And instead of making it an arrow as if it's going off somewhere, it wouldn't have killed them to just run the line over to the other drain. Rather confusing on multiple counts, I'd say.The -0.9V, however, is quite wrong.Our task now is to see if Q1 has sensible voltages and if we can conclude anything about its health.Can you measure the other 4 pins on Q1? (The other drain is not necessary since the drains are connected together, but feel free to check it anyway.) Q1a and Q1b are a balanced pair, so we need to know what the other half is doing too.The yellow arrow you've drawn is the drain. If you look underneath you can confirm it is connected to the other drain.EDIT: And I don't see where any of +10Va, -10Va, +10Vb, -10Vb goes on to be used elsewhere. Maybe I'm not looking hard enough.

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 12:28:52 am
content: Hi Mark ! I was editing my post above to include the source voltages on Q1b and Q2b whilst you were writing your reply. I'll get the readings off the other leads on Q1 now... 2 ticks...

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 12:40:49 am
content: Right...Q1a Source is -1.38V, Q1a Gate is -0.83V, Q1a Drain is -0.93VQ1b Source is -0.93V, Q1b Gate is -151mV, Q1b Drain is -0.93V (obviously !)Just for reference, the readings from Q2 are:-Q2a Source is +0.42V, Q2a Gate is 0V, Q2a Drain is +7VQ2b Source is +0.40V, Q2b Gate is 0V, Q2b Drain is again, +7V.

author: MarkL
author_rank: Unknown
timestamp: December 12, 2015, 12:58:48 am
content: Looking at it...

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 01:17:32 am
content: Take your time mate... It's 1:15am here and I'm off to bed! I'll resume this tomorrow. Incidentally, I've now got one of those little multifunction component testers so it's no problem for me to pull Q1 out and test it. Obviously it's not going to help diagnose IC12 though.

author: MarkL
author_rank: Unknown
timestamp: December 12, 2015, 03:24:33 am
content: I'm not seeing anything at the moment that would say Q1 is dead.Q1b's gate to source and gate to drain junctions are both forward biased, which is wrong for an operating JFET. But the junction voltage is 0.77V which at least says the junction is ok.On Q1a, the gate to source junction is also forward biased, but the junction voltage is much less at 0.55V, which also makes sense since the gate connection is much higher impedance.So, on both halves of Q1 the gate seems to be ok, which would be the typical failure.Q1b's gate current is about 6.8mA (151mV across R30). Computing the currents down through R28, R29, and VR1 to -10V we get about 6.0mA. So, there's about 800uA of current unaccounted for, perhaps it's flowing into IC12.If you look at Q2's drains, they are both at +7.00V and being supplied by R73 from the +10V supply. So current being supplied by the +10V is (+10V-7.00V)/470 = 6.38mA.So, where is the 6.38mA from the +10V supply on the Ch1 side? It seems nowhere, unless I missed something.Can you check that R31 has a resistance of 470 ohms? If you don't get 470, lift one side to be sure. And check that there's continuity from one lead of R31 to the +10V supply, and to Q1a drain on the other.That's my best guess at the moment. I'll continue to look at this.Feel free to try out your new device tester if it knows about JFETs.More things to do can include swapping Q1 and Q2, or even more desperate is swapping IC12 and IC13. I'm not quite at that point yet. I try to exhaust all measurement options before attacking with a soldering iron.EDIT: Nothing changed.

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 08:35:03 am
content: I didn't realise accuracy was so critical with the measurements... I've been dropping the least significant digit or rounding to the nearest mV. The 0V figures at Q2's gates are actually 0.154 mV (I think they're negative too.... can't remember now !). I'm not sure if any of this makes any difference to your calculations, but I'll give you the full meter display in future !I'll pop Q1 off and stick it in my tester later - just for the hell of it (plus I want to see how my tester handles it !).I'll check the other things you mentioned too. Breakfast first though !

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 08:59:36 am
content: Quote from: MarkL on December 12, 2015, 12:17:05 amSorry... I missed this in your post above... No, it doesn't work. With the (positive only) outputs of IC12 and 13 swapped at the zero-ohm resistors, I get no visible waveform on any channel. Ch1's trace was the same as it's always been - i.e. a flat line across the middle of the screen, with the Y-Pos knob managing to move it barely a fraction of an inch up and down. Ch2's trace was off the bottom of the screen - I could just see a glow to indicate is was down there - but, as it was off-screen, I have no idea what it was 'displaying'.The results of this test puzzled me as I expected the behaviour of Ch1 and Ch2 to either swap, or not swap. I wasn't expecting something new. I scratched my head for a bit then discovered the wacky voltages around Q1 and ran off in that direction !I can't be100%certain now of what channels were selected when I observed CH2 going off-screen etc, but I'm 80-90% sure that I would have referred to them as CH1 when the CH1 button was selected and so on. I wouldn't naturally have quoted you CH1's display behaviour with the CH2 button depressed. I can repeat the experiment if you wish, though I'll have to do it from the other ends of the zero-ohm resistors to avoid damaging the board. I did wonder why we weren't also swapping the '-OUT' connections too ?

author: MarkL
author_rank: Unknown
timestamp: December 12, 2015, 04:50:39 pm
content: Swapping just the OUT+ of Ch1 and Ch2 is going to create unpredictable results, as you've discovered.The reason is that OUT+ and OUT- are differential signals, created by the amplifiers IC12/13. The actual signal being conveyed is the difference between the two, i.e. (OUT+) - (OUT-). Another way to think about it is if you had an input signal that was going up, the output OUT+ would also go up, but OUT- would go down.Why use differential signaling? If you had noise, drift, or some other distortion that was affecting your circuit or transmission of your signal, in many cases it manifests itself equally in both OUT+ and OUT-. For example, a noise pulse might add 0.5V to each at the same time.In the very end when you want to recover the original signal, all you need to do is look at the difference and all that junk cancels out.So, back to the test. By NOT moving the OUT- as part of the test, you have essentially taken half of the bad channel and mixed it with half of the good channel, creating two bad channels. Mathematically, it's this:Displayed_Ch1 = Ch2_Out+ - Ch1_Out-Displayed_Ch2 = Ch1_Out+ - Ch2_Out-So, suffice to say you'd want to move each channel's OUT+ and OUT- as a pair when doing a swap test. Sorry if my previous explanation wasn't clear that all four signals need to be swapped.Since you're half way there, you might as well complete the swap test as a verification. I'm still extremely suspicious of R31 and its connections. There's just no positive bias voltages appearing in the Ch1 side and it should be coming from R31.On the measurements, if Q2's gate is indeed 0.154mV that would change my mind and I'd be wondering why Ch2 is working. When properly biased, a JFET gate would draw on the order of picoamps, so any resistor supplying it would should show zero voltage drop when measured with a DMM.Because you're trying to measure voltages close to 0V, make sure you're using a good ground reference for the negative of your DMM, such as the ground lead on a nearby resistor or cap. If you're just using a convenient metal chassis ground, you could be inadvertently measuring the drop in ground potential from one board to the next due to resistance of the interconnecting wires. (Another reason why differential signals are used - it's immune to that!)EDIT: Fix minor typo.

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 07:54:54 pm
content: OK, I've swapped over the OUT- and OUT+ of each channel to the other channel - but it's behaving in a way I can't get my head around !With the scope set to display CH1, and a signal going into the CH2 probe, I get behaviour similar to the original faulty CH1 (V-Pos control barely moves the trace (but it's the knob on CH1 I'm turning), Ch2 V-Attenuator dial is pretty ineffective [i.e. changing from 5mV/Div to 50mV/Div results in only a small change in displayed waveform amplitude, and then it only moves the top half of the wave, the bottoms stay put], x5 Gain does nothing.)With the scope set to display CH2, and a signal going into the CH1 probe, I get nothing but a flat trace.I have now returned the wiring back the way it was - and I'm in no hurry to repeat the experience; the copper traces on this board are pretty fragile and I couldn't get my desolder wick to give me clear holes to replace the component leads for some reason. Either way, I've lost another couple of pads and I'd rather not touch those zero-ohm resistors anymore !Another experience I don't want to repeat is removing and replacing Q1 - I almost lost a pad or two there as well. However... my tester gives the following information (identical for both Q1a and Q1b)...I have to go get dinner now, but I'll be back in an hour or so with voltages and readings from the network around R31.

author: MarkL
author_rank: Unknown
timestamp: December 12, 2015, 10:15:40 pm
content: Sorry the swap test was inconclusive. I don't usually encourage getting out the soldering iron unless we can identify a specific bad component, or if it's needed to make a decision on direction.In this case, IC12 is more or less a black box and I thought it would be worthwhile to at least determine if the issue was on this board. It's conceivable that the vertical preamp board was doing something horrible to IC12's outputs that was making its way back to IC12's inputs. We still don't really know.Is the trace functionality (or lack thereof) back the way it was?To go a little OT here, you might want to get yourself a solder sucker. I have one of these:http://www.edsyn.com/product/DHT/DS017.htmlThis model has been around forever and for a good reason. It works well. You can get them from Amazon and a million other retailers. Don't get a cheapie look-alike. A couple of people have reviewed this one that looks pretty nice:https://www.eevblog.com/forum/reviews/engineer-ss-02-solder-sucker-or-other-nice-desoldering-pumps/https://www.eevblog.com/forum/reviews/do-you-need-a-new-solder-sucker/Of course there's powered suckers too, which are more expensive. Dave reviewed one at one point. I have a powered one too but I actually prefer the Soldapullt because it's convenient and always ready to go.I find that solder wick does not remove as much as the sucker, which is perhaps why you're losing pads. Too much heat can also cause the pads to release. I normally use 300C - 320C, which some people may find a bit high, but works as long as you're fast about it. Sometimes it also helps to add solder when using a sucker or wick to help spread the heat and get more of the solder freely flowing. And for really stubborn wires adding solder, heating the joint, and *lightly* pulling will work, but that can really ruin a pad if you're not careful (obviously not a suitable technique for component packages with many pins).Back to testing, at least you've proven Q1 is ok, which is what the measurements had said also. I got 1.14V for the threshold (Vg on your tester) from the simulator. Gate thresholds can vary quite a bit, and in this transistor it's -1V to -5V. Ok there.Waiting for R31 measurements... (I hate to say it, but while Q1 was out would have been a really good time to measure it.)

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 11:12:45 pm
content: Readings first; The C9 end of R31 reads -0.946V, the other side of it, -0.492V. Interestingly, the non-ground end of C13 is +9.76V !WTF !! It's on the same line ! How... what... I'm taking the board out now to check the traces - I can't see from the print on the schematic.Something is definitely not right here !! ...Back in ten mins or so !

author: aneng
author_rank: Regular Contributor
timestamp: December 12, 2015, 11:55:00 pm
content: I think I may have found something....I haven't tested it yet... but it looks very suspicious !!THIS.......is C12.The dodgy looking connection on the right is where the +10V feed comes in from L1. It's a thru-plated hole that picks up a trace on the other side of the board to take that +10V to R31.I'm going to resolder the joint and I'll get back to you.....(sounds of very quiet drum roll in the background....)

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 12:24:53 am
content: ...Sounds of drumkit being put back in the box.Well.... we're getting there. That joint was definitely faulty. CH1 unfortunately still doesn't work properly, the V-Pos control moves the trace a little more than before, but it's still the same. :-( However....after cleaning C12 up and resoldering, I now have +9.71V on one side of R31 and +6.97V on the other..EDIT: WTF (again !) !!! - after measuring the voltages on R31 and the Drain of Q1b, I look back at the screen and CH1 has suddenly sprang to life !!! How the hell could that happen ??!!! It was definitely still faulty a minute ago.... CH1's V-Pos control now works properly and it appears to be behaving the same as CH2. I'm just going to feed it a sine wave, then do the 9V battery test. Be right back...

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 12:37:51 am
content: Wow... It appears to be working perfectly !!!!!!!I've given both channels the same sine wave, and with each channel set the same, I get exactly the same trace from each. Measuring the peak-to-peak waveform gives me 2.6 x 5V/Div = 13V. Measuring the same signal with my DMM gives me 5.35V AC.Does that pan out, or is it still all reading 15% lowHow the hell did it suddenly spring to life when I was merely checking voltages ? Perhaps I ought to resolder the joints around R31 just in case.

author: MarkL
author_rank: Unknown
timestamp: December 13, 2015, 12:57:08 am
content: Great - I'm glad it's working! And I'm glad it wasn't IC12!But I'm also not so glad we're not exactly sure where the problem was. All we know for certain is that +10V wasn't getting through R31 to bias Q1.Reflowing R31 is one thing to do.But before doing that, I would try poking at R31 and the other components in the area with a wooden or plastic stick while watching the screen. Try slightly tapping or gently pressing on the board with the stick in that area.If you don't find it, I guarantee intermittents will come back at a time when maximally annoying.And yes, it's about 16% low. To convert between the two:Vpkpk = Vrms * sqrt(2) * 2There's adjustments for that, if you're ready for that step...

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 01:03:59 am
content: That's still reading low. However, now its basically working, you can go through the trouble-shooting charts and see where the signal is dropping.I'd want to sort out a reasonably accurate known amplitude signal source before I fiddled with anything though. Its possible your meter is out of calibration on its low AC volts range(s).If you take a CMOS 555, run it from a regulated supply and configure it in bistable mode, you can measure its output high and low voltages, then reconfigure it as an astable oscillator from the same supply voltage and it will produce a square wave that swings between the two voltages previously measured.On the subject of intermittents: a pointy wooden dowel to prod and tap with will help you localise many faults. A hair-drier + a can of electronics grade freezer spray can also be helpful if you suspect the fault is heat related.

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 01:49:13 am
content: Before I read the two replies above, I waded in there with a fibreglass pencil and my soldering iron... I found several ceramic caps on Ch1 that were grounded at one end on the component side, and.... not a trace of solder. The pads were clean copper. It looks like they never saw solder at the factory !After checking and reflowing all the relevant components, I reassembled it all again and.... to my annoyance... Ch1 was back to misbehaving. I took a wooden stick and prodded C12 - which has my very best soldered joints on now - and when I next looked at the screen, it was displaying correctly. Despite prodding C12 and bending it a bit back and fore - I couldn't reproduce the fault. GRRRR !!!!!I have to admit Ian, I don't know how to do what you are describing. Is there a link somewhere that I can follow a schematic to build what you're describing ?That's me for tonight.... almost 2am !! Catch you tomorrow !! (and a big thanks again to Mark and Ian !!)

author: MarkL
author_rank: Unknown
timestamp: December 13, 2015, 01:59:04 am
content: Quote from: Ian.M on December 13, 2015, 01:03:59 amYou can also use the built-in cal output to do the equivalent. As described in the adjustments section, you can stop the cal oscillator by shorting TP2 on the Horizontal Mode board and then check that the cal output is 0.3V on the DMM on DCV (not ACV). If out of spec, it can be adjusted with VR1. Previous measurements on the scope showed the cal output was also low by the same percentage, so I'd expect it to be ok.Once adjusted or verified, unshort TP2 and you now have a square wave with a known amplitude of 0.3V pk-pk.If you're using probes, that is another source of measurement error. You should double check that inputting a signal directly to the BNC gives you the same results. If you don't have a BNC jumper cable, use a wire that you can hold to the cal output and (carefully!) touch the center pin of the input BNC.We haven't even mentioned Ch3 and Ch4 yet. Are those working? Are those 16% low too?What kind of DMM do you have? Are you confident of its accuracy?Just read the last post. Need to get to the bottom of that intermittent first...

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 02:18:45 am
content: Got another book for you.Forrest Mims' Engineer's Mini-notebook: 555 Timer IC Circuitshttps://archive.org/details/electronics_-_Forrest_Mims-engineers_mini-notebook_555_timer_circuits_radio_shaSee page 7 for the Basic Astable Circuit to get a squarewave output and modify the pin 2 connection to that of the Bouncefree Switch on the next page to check the output high and low levels at DC. You can leave R2 between pins 7 and 6 while you do that.However you need to know the original bipolar NE555 chip pulls an absolutely horrible 200mA spike from its supply as it switches, and its output levels aren't rail to rail. If you use a CMOS 555 compatible chip (See list), it will be much better behaved and give far far closer to a rail-to-rail output.

author: Brumby
author_rank: Unknown
timestamp: December 13, 2015, 03:32:19 am
content: Quote from: MarkL on December 13, 2015, 12:57:08 amYep. Murphy will ensure it.(Been watching from the sidelines)

author: MarkL
author_rank: Unknown
timestamp: December 13, 2015, 06:02:07 am
content: From your photo of the bottom of board, below is a close-up of the IC12 and Q1 area.According to the schematic, +5Va only goes to the two Q1 drains, one side of R31, and one side of C9. That's four components. However, if I have things in the right place, and please correct me if I got component placements wrong, there appears to be a fifth connection as shown.What is that?The PCB layout in the manual shows a zener D2 connected to that spot. The PCB layout is really hard to see, but I think there's some traces that don't quite match up with the photo. (The PCB layout is from the TOP, for anyone trying to figure out what you're looking at.)Is the schematic wrong? Is there a D2?Arghhh!!

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 09:19:06 am
content: Relax !! There is no D2. The schematic appears to be correct. There are solder pads and screen printing on the PCB for a D2, but there is no component there. There is also a similar 'missing' zener, D4, on the other channel at that point. I've noticed a few places dotted about the boards (both these and the output board we worked on previously) where components have obviously never been installed. I've seen that before on boards though, so I never paid it any attention as I presumed it would be for other models.See the shots below to reassure you that there's nothing on the other side of the board !D2 :...and it's mate D4 :

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 09:37:49 am
content: Going back to desoldering for a minute.. I do have a pump, but it's only a cheap thing..I've found that the nozzle is too big (and melts !), and I can't seem to keep the joint molten while positioning the pump over the top. Trying from the side sucks nothing up. Maybe I need a better one !Also - I've only recently got my first temp-controlled soldering station, a Hakko FX-888D, and so temperatures are still a mystery to me. I've been using it on the default setting of 350 C - and I'm using 60/40 solder. What should I be using for soldering ? - and is it different from desoldering ?I still have a lot to learn ! - as you can see

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 10:31:23 am
content: Genuine Hakko with genuine bits?If so, that's probably a touch on the cool side for SnPb. I spent years working with a Weller TCP series iron on my bench with a PTAA7 700° F bit in ( 370° C) and never had a problem with delicate boards unless some other bloody idiot had already cooked the pads before I got in there. A bit more temperature means less dwell time on the joint when soldering and less melting time when desoldering and there's a sweet spot where it actually reduces stress on the pads. However if you go too hot you will f--k up small pads on any board.If its a clone (or at least using clone bits), the bit to element fit is likely to be a lot poorer so it doesn't hold temperature as well, which means you will need to turn it up for heavier joints. I wouldn't advise going much higher than 750° F (400° C) or you will find yourself on the harsh end of pads dropping off and bits burning up. Don't forget to turn it back down afterwards. I used to keep a PTAA8 bit (800° F 425° C) handy for transformer tags, screening cans and other heavy joints, and while you *could* use it for ordinary work, if you were even slightly clumsy and dwelled too long, you'd risk board damage, and if you forgot it was on and took a coffee break, it would be badly oxidised ten minutes later when you wanted to get back to work.That sucker should work adequately with practice, though its nowhere near as good as a high quality all metal one. You *MUST* keep the chamber clean as solder dust buildup on the walls and plunger O rings seriously compromises the vacuum. If you put a wetted finger over the nozzle to seal it and trigger it, if its in acceptable condition, you can count to ten before the plunger reaches its full travel. Anything less, it needs TLC, and probably some light oil or PTFE drylube on the O rings. I find a few disks of waxed paper cut to be a close rubbing fit on the chamber walls put between the plunger and the spring go a long way to helping keep the O rings clean and the vacuum good. With the right soldering iron temperature it will mostly clear most pads with the sucker tilted 30° from vertical so you can get the iron's bit in there. Swapping between heating and sucking does NOT work well. If it doesn't suck easily in two or three attempts, for %DEITY%'s sake use desolder braid + fresh flux. If the joint is really crusty, start by adding fresh solder and flux *BEFORE* starting to desolder it - the pad will thank you!Some people like a silicone tipped sucker. Supposedly because its softer, it moulds to the board so you get better pick-up with less risk of board damage due to the tip hammering the pad when you trigger the plunger. If you want to try that, if you can get some clear silicone tube of about the same I/D as the O/D of the nozzle tip, simply push it over the existing nozzle and cut it a few mm longer with a really sharp blade. However before doing that, make sure its real silicone that stands up to touching the side of the soldering iron bit (not the tip as you don't want melted s--tty plastic on the tinned tip!). I'm used to PTFE tips and am unlikely to change.

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 11:42:20 am
content: Hi Ian,Yes, it's a genuine Hakko. I bought it from the Italian Hakko agents, 'Batterfly'. I know there are loads of copies out there, so I made sure I bought from a distributor listed on Hakko's website. Due to the Euro/GBP exchange rate, I got it for £30 GBP less than the UK distributor was asking, including delivery from Italy ! I was still suspicious but I'd be amazed if an official distributor would be stupid enough to sell fakes. It looks very well built and came in a nicely printed box etc. I haven't seen anything to suggest it's not the real deal.Having said that, I'd like to know if there is a surefire way to confirm !I've just ordered an Engineer SS-02 solder sucker as it gets good reviews and I prefer the look of it, so..... I'm getting my gear together slowly.So, you reckon hotter is better for desoldering ?

author: KJDS
author_rank: Super Contributor
timestamp: December 13, 2015, 11:59:40 am
content: The biggest bit you can comfortably get near the joint is better than more temperature. The bigger the bit the more heat in it, the quicker the solder will melt. Make sure the bit is properly whetted, usually with a little bit of fresh solder on it, and put some fresh liquid flux on the joint.

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 12:15:40 pm
content: Just be careful where you put your face or desk magnifier in relation to the plunger of that style of sucker when you activate it. It will give you a lovely shiner of a black eye!I prefer the style with a guard as I tend to hold it angled with three fingers close to the tip, with my little finger on the board for better control, fire it with the knuckle of my thumb, then flip it into a fist grip to re-cock it with my thumb one-handed. Without a guard, I'd have to be a lot more careful flipping it to avoid slinging it across the bench!Insufficient heat is bad - you cook the track and component for longer trying to melt the solder, then it doesn't stay liquid long enough for the sucker or braid to work. OTOH too much heat is also bad and can cause rapid track and component damage. I suggested temperatures above, and the easiest way of finding out if they suit your equipment + gaining skill is to take a scrap board and desolder anything interesting for parts. Try some fiddly little caps, resistors diodes etc, for practice, but don't bother keeping them once you've tested them to check you got them off without damage.Fine tip conical style bits are absolutely useless except for re-touching individual SMD pins of fine pitch ICs and 'flyspeck' passives, maybe to attach bodge wires because the heat transfer is so restricted by the tiny cross-section behind the tinned tip. For anything else, I prefer a chisel or spade style bit, and will use the very tip or corner for really small joints@KJDS: Nitpick: "whetted" means sharpened, usually on a whet-stone. I think both of us and the plating on the bit would prefer non-abrasive cleaning and "wetting" with fresh solder! I don't usually dish out spelling flames as my own is none too good, but when it disastrously changes the meaning . . . . .

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 12:49:23 pm
content: I had believed it was best to use a dry bit, so you're not adding more solder.. Or is that just for braid/wick ? .... And what does adding liquid flux accomplish? Is that for pumps, braid or both? How does that work?

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 01:16:31 pm
content: *NEVER* use a dry bit for desoldering. You need a little solder (not a great big blob) to get initial good thermal contact between the flat bit and the curved joint if using a sucker, and to get the braid (desolder wick) hot enough quickly enough if using braid. The only time a totally wiped clean (but freshly tinned) bit is appropriate is when removing a small amount of excess solder from SMD devices without using braid (e.g. cleaning up light bridgeing by dragging outwards).Flux helps the solder flow better, either into the sucker or along the braid. Without it, braid is fairly useless and sucking is more difficult and leaves a messier pad/hole.When dealing with s--ty old joints, the surface is often so badly oxidised that you have to add fresh solderto get the joint to melt cleanly so it can be sucked or wicked. If you've partially sucked a joint and cant get a plated through hole to come clean, its usually due to insufficient heat transfer to the lead and other side of the board. Remake the joint with fresh solder, add flux, if possible heat from the side with the most copper, and it will usually come clean on a second attempt. Faffing about trying to heat the lead and suck it again without adding fresh solder is invariably a disaster, often leading to pad loss. If you don't clear it on the second suck (with fresh solder), let it fully cool and grab the flux and the braid.

author: aneng
author_rank: Regular Contributor
timestamp: December 13, 2015, 01:25:48 pm
content: You need to use flux with braid?? More than it's already got in it? I had some cheap braid for years and it was rubbish, then I watched Dave's video on soldering tools and bought the Hakko and some Multicore (Loctite) braid and it seems to work well. I'm not using any additional flux though.... Should I be ? Any recommendations for brands etc?One thing I'm still getting used to is how to work out when and how far up to cut off the used end of the braid. I suspect I'm wasting a lot.Edit: I found my old Wavetek DM78A DMM from years ago...I stuck some fresh batteries in it and checked my 2 x 9V batteries in series on my op-amp circuit and it read 18.54V, and the CalTek CM5085 I've been using all along here read 18.42V. So, at that scale anyway, there's 0.7% difference between them.

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 02:17:40 pm
content: Add flux, Flux, ***FLUX*** when using braid on old joints, Its got enough flux in it for clean solder to flow up it, but not enough to deal with cruddy old oxidised solder on the board. On fresh joints, alittleextra flux does no harm and often helps. Some people like gel flux and some like liquid flux. Some people swear by flux pens, some swear at them.Then there's different grades of flux: R (Rosin), RMA (Rosin mildly activated), RA (Rosin activated) + various grades of synthetic flux(and the last resort of plumbers: 'killed spirits of salt' (Acid Zinc Chloride) which should *NEVER* be used for fine electrical work as it will eat through fine wires, tracks and component leads if any traces are left on the joint). Any flux more active than RMA that isn't explicitly specified as no-clean *MUST* be cleaned off properly to avoid corrosion. Most R and some RMA liquid fluxes are no-clean (i.e. will dry to a hard inert resin) but generally, gel and paste fluxes need to be cleaned off, and even R liquid fluxes are often better cleaned. Some fluxes clean up with Alcohol (IPA), some are water washable, but that needs HOT water, lots of it and the right detergent. See:https://www.eevblog.com/forum/chat/cleaning-pcbs-in-a-dishwasher/Personally I favour R type natural Rosin fluxes, don't like gel and prefer liquid in a felt tip flux pen for detail work or applied with a small brush for large areas, but many techs who've learned their skills with gel flux in a syringe would curse my choice and insist their $expensive$ favourite brand is essential. I detest any 'greasy' flux - I am *NOT* a plumber doing wiped joints in lead pipe using tallow on a moleskin! Ask 100 techs and you'll probably get >50 different recommendations at least half of which may be very good for the specific type of work they are doing.Beware of cheap flux from China - many gel fluxes have a similar colour and consistency to Vaseline (petroleum jelly), and one suspects that fake branded flux from China may well be no more than petroleum jelly, maybe with a touch of dye, and if you are lucky some Stearic acid. If you are unlucky, it will contain gutter oil and acid Zinc Chloride and will do more damage than it prevents.

author: KJDS
author_rank: Super Contributor
timestamp: December 13, 2015, 03:27:29 pm
content: On a minor point of grammar, whet doesn't just mean to sharpen, but also to excite or stimulate, as in whetting one's appetite. The amount of solder applied to an iron prior to using it for desoldering is more akin to whetting than to wetting.

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 04:12:25 pm
content: @KJDS: Whetting as in sharpening your appetite? Humour aside - I thought it as well to make it clear NOT to abrade the bit, (unless of course its Saturday evening, the bit plating has already failed, and you just need to get through to Monday morning and the courier delivery of some new bits).@Aneng: I forgot to mention trimming dead solder-wick for best results and economy. Fold it over, the stiff bit is saturated with solder and mostly should be cut off. Keep a couple of mm of it because the trace of solder stops the braid end fraying and improves heat transfer to the joint. If its still soft but full of burnt flux, you've either been using it wrong or have had to totally clean a significant number of SMD pads. That's also dead and should be cut back to clean braid.With enough good flux, you can even get good results with fine copper screening braid off scrap coax, pulled tight and flattened, as long as its still bright copper or bright tinned, not oxidised or corroded, and as long as it isn't too heavy for the joint. That can be a significant saving if you have to desolder a heavily soldered long joint on a screening can, and is certainly worth trying if you've run out of solder-wick over the weekend.

author: Brumby
author_rank: Unknown
timestamp: December 14, 2015, 12:03:40 am
content: PRACTICE.That's the way you will get the 'feel' of soldering - and the more you do, the more the advice (like that above) will make sense.Grab some scrap boards and maybe some ancient components. See how much easier clean metal is to work with. Try a couple of different types of flux to see which works for you. See how different tip shapes work and don't work in various situations. Experiment with temperature settings. Cook components, wreck traces and lift pads - doeverythingwrong - where it doesn't matter and you will develop a sense of when to back off before destruction.Along the way, your soldering skills will improve.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 14, 2015, 04:00:10 am
content: Quote from: Brumby on December 14, 2015, 12:03:40 amI would add---------CLEAN,CLEAN,CLEAN!Flux is great,but if your tracks, components or SOLDER have grot on them,you will be battling to get good solder joints.As an experiment,get a piece of paper towel,wet it with solvent,& rub it along your solder.You will find a build up of black "Gunk",if your solder is old.(New solder will leave a mark too,but only a slight one.)When you are using it,clean a few inches of the solder before each lot of joints.Get an eraser,& cut into it a short way.Use the cut to clean "through hole component" leads---you will again see "Gunk".Use the other end of the eraser to clean tracks---clean afterwards with solvent.When you use flux cored solder,every time you complete a joint,the end of the solder becomes solid,so the flux is not available for your next joint.For this reason.High Reliability Hand Soldering courses suggest you snip off the end of the solder after each joint to make fresh flux available.If you use separate flux,you can get away without this step.De-solder braid:Cut the solder clogged end off leaving a tiny bit of solder,or just cut it right off & apply some solder to start the capillary action,If the braid doesn't seem to be working,it may be old,or just poor quality.With the latter,apply some liquid flux to it-----it quite often does the job.Years ago when I did the Courses,they suggested reducing the iron temp for desoldering.At work,I had a WTCP,so couldn't do this.I found out "quick in & out" does the trick with a fixed temperature iron.All that said,Brumby'sadvice is pure gold.Practice soldering & desoldering on scrap boards,new boards,new components,old components.paying due attention to all the tips folks have given.You will be surprised at the quality you can achieve.

author: aneng
author_rank: Regular Contributor
timestamp: December 16, 2015, 12:32:55 pm
content: Sorry for the delay in responding... Work has been a bit hectic ! Until I can get back to the scope later this week, I'm wondering whether now might be a good time to have a quick look at the signal generator I blew up - as I can dial in specific amplitudes and offsets etc, and this would definitely come in handy in checking the calibration of the scope.The schematic is here;http://www.jyetech.com/Products/085/Schematic_085F.pdfand it was C4 that exploded. I've subsequently replaced C4 and U2 (a 5V regulator - I didn't get the exact replacement but a little 100ma one which allegedly matches the spec, a TS78L05CT).It works, (i.e. it powers up and the menus work), but the waveforms are badly distorted. I was just wondering if any other components could have been fried by my connecting the power up with reversed polarity ? It was 'only' £25 so it's no huge loss if it's unrepairable.I just thought that if it was possibly an easy fix, it would come in handy for finishing the scope repair.

author: Ian.M
author_rank: Super Contributor
timestamp: December 16, 2015, 12:51:18 pm
content: You could have blown the negative supply generator (U1 MC34063) or just about any or the analog chips that use the V+ and V- rails. If U1 is good, check if the output of the R-2R ladder DAC makes sense (it will need low pass filtering) and work through it from there. If its bad, you may need to change one or both of the ATmega MCUs (as its possible that incorrect voltages on the analog section could have damaged an I/O pin) which means you'll need the firmware and a programmer.

author: aneng
author_rank: Regular Contributor
timestamp: December 16, 2015, 02:26:14 pm
content: In that case, it's going in the scrap and spares bin! I'll write the £25 off as a stupidity fine!

author: aneng
author_rank: Regular Contributor
timestamp: December 16, 2015, 02:36:47 pm
content: So then... What's my next step with the scope? Do I need to be hunting down this possible intermittent on Ch1 (I've got some freezer spray now - not that it looks like a thermal issue.) before we can move on to calibration ? Can I sort out the focus yet?My new ss20 desolder pump has arrived too! ...but the cheap headband magnifier is going back as it's junk.

author: Ian.M
author_rank: Super Contributor
timestamp: December 16, 2015, 03:27:07 pm
content: I'd do some checking around the negative supply generator (U1) before stripping the DDS signal generator for parts. It is totally fixable as all the firmware is available, but even if you decide you don't fancy that much SMD rework and scrap it, the board has a CP2101 Single-Chip USB to UART Bridge on it that could easily be hacked for logic level serial to anything that needs it for a quick test.(I'd want to use a module that didn't remind me of failure for stuff that was going to see frequent use, rather than a hacked disaster)On the scope side of things - yes you absolutely *MUST* hunt for the intermittent and fix any other known issues that are too severe to ignore, *BEFORE* calibration, as if the calibration error is related to the intermittent, you wont get a stable calibration to 'take'. Any general preset twiddling or unsuccessful attempt at calibration before locating the fault will make it harder to find. Unless you are sure that some other bloody twiddler has been in there before you, any preset that needs significant adjustment to bring its controlled parameter into calibration should raise suspicion, and the cause for it be located and if possible, fixed, before moving onwards.

author: MarkL
author_rank: Unknown
timestamp: December 16, 2015, 06:10:55 pm
content: Is Ch1 currently working?When it is/was working, is it also approx 16% low?Since you have it handy, I would try the freezer spray anyway on R31 and IC12. You can also try freezing the board on roughly the +10V path from L1 up to R31.You successfully measured +9.76V on C31, but that is a different path from L1 which starts on the bottom. How does the top pad on L1 look?I wonder if R31 is being intermittent? Did you ever catch a reading on the other end of R31 (the +10Va end) during a failure?

author: aneng
author_rank: Regular Contributor
timestamp: December 16, 2015, 10:58:15 pm
content: Quote from: MarkL on December 16, 2015, 06:10:55 pmWell... it wasn't when I turned it on ten minutes ago, so I started prodding with a wooden stick around C12 and -whether the prodding was the cause or not- Ch1 suddenly started working again. I tried prodding the board in the other direction (from behind) to see if I could get it to fail again, and it wouldn't. I froze the areas you suggested - no change, still working ! GRRRR !!! However.... at least I know there's a good chance it will go faulty again.QuoteYes - it is perfectly matched to Ch2.QuoteFrom the vantage point of the installed board, it's not easy to see, but it doesn't look like L1 has a trace that connects on the top - I could be wrong though. Another late night in work, so I'm in no state to dismantle the boards tonightI can't wait for this week to end; once Friday is over, I'm off until the 30th Dec !! I'll get the boards off and check (hopefully, work/tiredness permitting) tomorrow.QuoteI don't think so, but now I know it's important, I'll measure it when (if!) it fails again.Which brings me to my next question; given that the failure can be a bit short-lived, what should I be measuring when it does fail ? Maybe I could rig up my extension cable/test point to various points in readiness ? I'm using an 8-core cable, so I can easily get it ready to measure 8 different points.

author: MarkL
author_rank: Unknown
timestamp: December 16, 2015, 11:42:08 pm
content: Quote from: aneng on December 16, 2015, 10:58:15 pmI've attached the L1 area from one of your previous photos. L1 is right next to P3. It's hard to see the condition of the soldering on the pad on top (the same trace that ended in the nasty pad under C12 that you fixed).QuoteI would measure the two ends of R31 as the highest priority, in-circuit while the failure is happening. That would at least tell which side the problem is on. Assuming we're on the right track with the bias voltage missing.Does the problem usually clear after being on for a few minutes?The other thing I would suspect is that IC12 wasn't getting its +10V on pin 14, but you measured that long ago while it was consistently failing.If I get more choices, I'd also probe IC12 pin 6 and 11, and IC13 same pins so we can compare. This is a total long shot because I don't know what those pins do. I don't know if it will provide any useful clues. The only reason I'm looking at them is that they are attached to +10Va and they might be providing some kind of internal positive bias to IC12 and we're not seeing any positive voltages during the failure.I hate intermittents.

author: aneng
author_rank: Regular Contributor
timestamp: December 16, 2015, 11:48:07 pm
content: @Ian.M : That link for the Signal Generator schematic was for an older revision. I found the correct one for mine here:-http://www.jyetech.com/Products/085/Schematic_085G.pdfAs for checking U1 - I don't know how toMy knowledge at this point is such that I can follow a schematic and measure voltages if they are listed ! Without a nice list of TP voltages and/or waveforms, I have no idea what to look for. I have emailed the manufacturer to that effect, but I doubt that will yield anything.I've poked around with the scope (my first ever use of a scope to try to find a fault !!!) and followed the output backwards on the schematic, and have found the point where it starts going wonky ! The right-hand side of R9 (bottom right of the schematic, under all those resistors), the waveforms are fine... no visible distortion. The other side of R9, the bottoms of the waveforms start getting distorted, cut short if you will. The waveforms I'm getting at this point though are nowhere near as wacky as the ones on the output itself, so I think maybe there is something wrong with the bits to the left of R9.

author: MarkL
author_rank: Unknown
timestamp: December 16, 2015, 11:55:16 pm
content: Quote from: aneng on December 12, 2015, 11:12:45 pmHang on a moment. I think you already took a reading on R31 during a failure above.This says R31 is fine since the +10Va is not getting to it.Take a look at the L1 soldering. There just isn't that much between the two!Sometimes an intermittent will show up as a few ohms on your DMM because there's a sliver of a connection. With the power off, you can try taking a resistance reading between the +10Va side of R31 and the top pad of L1 and then the bottom pad of L1. You should subtract the resistance of your meter leads, which you can measure by shorting the leads together.I'm focusing on that side of L1 because that's where the +10Va splits off into two paths. One goes to R31 and the other to C13. And you said C13 has +9.76V.

author: aneng
author_rank: Regular Contributor
timestamp: December 16, 2015, 11:59:48 pm
content: Quote from: MarkL on December 16, 2015, 11:42:08 pmHey - you're right ! There is a pad on the top ! I'll look at that tomorrow.QuoteI haven't managed to pin down any pattern to it yet - it's only happened twice so far. Both times that it cleared, I was poking the board around C12 - but that proves little as it could still be thermal. My logic says if it's a bad joint, then surely I should be able to 'unfix' it by prodding the board in the other direction, but nothing I do makes it faulty again.QuoteYou can have as many choices as you wish !! I will wire up cables to as many points as you feel require testing Sir...unfortunately not tonight though... I have a two hour drive in the morning to get to work by 09:30 and it's midnight here now.Thanks ever so much for all your help - and the others who have contributed that I haven't acknowledged yet (soldering tips etc). It really is greatly appreciated.

author: MarkL
author_rank: Unknown
timestamp: December 17, 2015, 12:03:24 am
content: Quote from: aneng on December 16, 2015, 11:48:07 pmI agree with your analysis.Check that U3 has the right power on pins 4 and 8. If it does, it shouldn't be messing up the signal from R9. It's dead.

author: Ian.M
author_rank: Super Contributor
timestamp: December 17, 2015, 01:19:59 am
content: I've done a bit of digging round their site and it seems that TP13 should be about -13V and TP14, +14V. Your results may differ but if TP13 has at least -12V, U1 is almost certainly good. It feeds power to U3 and U13. U3 is a bit special:http://www.ti.com/lit/ds/symlink/lm6172.pdfas its high speed high slew rate with 50mA output drive capability, which is used to let the FG085 drive low impedance loads, but U13 is a bog-standard TL084 which only seems to be used for feedback to the DDS synth.

author: aneng
author_rank: Regular Contributor
timestamp: December 17, 2015, 10:13:57 pm
content: Quote from: Ian.M on December 17, 2015, 01:19:59 amOh.My results certainly do differ ! TP14 is +14.10V, but TP13 is +0.6V. That's definitely different from -13V. !I've just ordered a replacement U1 !

author: aneng
author_rank: Regular Contributor
timestamp: December 19, 2015, 05:40:57 pm
content: Hmmm. I ordered one off eBay and then went and bought one from RS Components as I couldn't wait all weekend ! I just fitted the replacement U1 and things are not good. The display backlight pulses on and off at about 3Hz - so I'm assuming that's the power supply protection circuit kicking in and breaking off the juice as it's all drawing too much current ?I removed U2 to cut off that side of the circuit, removed and tested both C10 and C3 (both fine). I removed D2 and tested that, but I'm not sure how to interpret the results of my tester !This is what it says for D2 (an IN5819 - a "Schottky Barrier Rectifier");Is this knackered or good ?I also tested the resistance across the main power input terminals - 2.2 Ohms. 15v @ 2.2 Ohms = 6.81 Amps !!I have to admit, I am enjoying fixing these things, even if - ironically - they are tools that are supposed to help me fix other things ! However, I'm very aware that all of this has been dragging on for a while now and I don't want to take advantage of folks' generosity and knowledge by looking for help with this Signal Generator as well as the Scope. I'm quite prepared to bin the Signal Generator and focus on the Scope only if people are getting bored of helping !

author: Ian.M
author_rank: Super Contributor
timestamp: December 19, 2015, 06:06:35 pm
content: The Schottky looks to be good. Its likely that the OPAMPs U3 and/or U13 may have been damaged. U1 produces the -13V supply VRAW- which feeds TP13 and the OPAMPs via R18. The only other thing it goes to is the output clamping diode DN1. If you remove R18 and DN1 the -13V supply VRAW- should come up, and if it doesn't you know the problem is round U1.Once you get the -13V supply up, check and refit DN1, and assuming there was no fault found so far, you can refit R18 and try and get V- (TP13) up. The only things it feeds are its decoupling cap C10 and the two OPAMPS. As U13 is the cheaper one, I'd remove that one first.

author: aneng
author_rank: Regular Contributor
timestamp: December 19, 2015, 07:53:28 pm
content: Bloody hell.... this SMD stuff is a pain isn't it !! I have removed DN1 and R18 (in addition to the previously removed U2 and D2). DN1 tests ok. However, applying power now resulted in the same pulsing of the PSU voltage, shortly followed by a smell.... the smell of my newly installed U1 toasting nicely. Whether it's survived the ordeal I can't say, but maybe that one off eBay may be needed after all !I'm getting closer to the point of putting this down to experience and just spending another £25 on a replacement as the board is beginning to look a bit battered now, what with my clumsy SMD skills, and all the electrolytics on the wrong side of the board, not to mention my DIY heatsink arrangement for the replacement for the SMD U2 which is now in a TO220 case !One last shot I think and then it's destined for the parts bin !What could be causing U1 to get so hot ?

author: MarkL
author_rank: Unknown
timestamp: December 19, 2015, 08:13:59 pm
content: Quote from: aneng on December 19, 2015, 05:40:57 pmNever bored! But I am wondering if there was anything interesting with that top pad under L1?

author: aneng
author_rank: Regular Contributor
timestamp: December 19, 2015, 10:11:55 pm
content: Quote from: MarkL on December 19, 2015, 08:13:59 pmThis is the best shot I could get....Doesn't look too great does it ?Now.... the question is - do I wire up the extension leads to check for dodgy voltages on the points you requested earlier - now - or do I touch-up the pad on L1 and hope the intermittent doesn't come back ? My gut feeling is to leave L1's pad alone at this time. What do you think ?

author: aneng
author_rank: Regular Contributor
timestamp: December 20, 2015, 01:18:55 am
content: I have some results... but don't get excited.I have a full set of voltages from the scope in failure mode and the same points in operational mode - and they're almost identical.The only changes are a couple of hundredths of a volt difference that applies across all measurements and in the same downward direction, so I'm presuming this is just the thing warming up a bit and nothing to do with the fault.R31 (IC12 Pin14 side) = +9.67VR31 (C9 side) = +6.94VIC12 Pin 6 = +2.435VIC12 Pin 11 = +2.320VIC12 Pin 14 (for the hell of it as I had spare wires) = +9.69VIC12 Pin 3 (same reason) = -9.88VIC13 Pin 6 = +2.424VIC13 Pin 11 = +2.425Those are voltages for it in the faulty mode. The voltages for working mode are the same, bar, as I said, a couple of hundredths down.It seems to be possibly heat (or time) related - but despite freezing and heating everything and anything, I can't seem to isolate it. At least it's no longer intermittent - it's consistently recreateable. If you watch the video below, you can see what it does from startup on CH1.Basically, the vertical position knob does very little to the trace for the first four minutes, then it slowly starts to have more effect. The trace starts to jump a little on its own until at 4:22 it suddenly jumps to the position set on the V-Pos knob. However - notice that the trace never goes off the top of the screen when the V-Pos knob is at its extreme (unlike Ch2 which goes off the screen at both limits of the knob).(I've had to leave the video uploading - it's huge for some reason... it'll take a couple of hours I think !)https://youtu.be/4u4j_4QsyLI

author: Brumby
author_rank: Unknown
timestamp: December 20, 2015, 09:19:26 am
content: Tried to view the video - but I got the message: "This video is private".You'll need to change that.

author: aneng
author_rank: Regular Contributor
timestamp: December 20, 2015, 09:39:27 am
content: Oops ! Sorry about that. Changed to 'unlisted' instead - it should work now.Edit: Update; I've just turned the scope on to confirm that the behaviour is predictable, and - guess what - it's not.It's just started up in 'working' mode (albeit with the Y-Pos dial on CH1 unable to get the trace completely off the top of the display). However - for the first time, I thought I'd investigate what CH3 & 4 do - as it's not obvious how to even adjust anything for them. There are two knobs on the side of the case towards the rear, labelled CH3 Position and CH4 Position. I can't find any other controls for these channels, so I'll have to have a flick through the User Manual to see how you are supposed to use these.Interestingly - when I put the scope in Quad mode, I get four lines - but neither CH3 or CH4's Position knobs have much effect on their traces. They just sit there. I can't even get them to display much of a signal, but as I have no idea what I'm doing with these channels yet, that's probably down to me, but.... I would have thought the position controls should function.Given the identical voltages in fail/working mode and the behaviour of channels 3 & 4, I'm wondering if we're not looking in the wrong area ?Are there any areas that Ch1, 3 & 4 have in common - but not Ch2 ??!!

author: MarkL
author_rank: Unknown
timestamp: December 20, 2015, 08:05:12 pm
content: Thanks for the video.To start, the voltage readings on R31 were very interesting. They are not the same as before when we suspected a bias issue. They are now within the expected operating range, and match your previous readings on Ch2 back in post #158.Because results have changed, I would back up a little. I would check to see if the two inputs on IC12, pins 2 and 15, are around +0.42V (which is what you found on Ch2), and that IC12 outputs, pins 8 and 9, are around -1.3V each (again what you found on Ch2, post #122). It would also be worth watching these voltages through the progression of the two failure modes you demonstrated on the video. (I'll call them failure mode A and failure mode B.)The top pad on L1 *looks* ok, but I would probably reflow it with some fresh solder and some extra flux if you have it. Just to be rid of it as a possibility.I think we're facing multiple problems in Ch1. I'd like to see if we can get stable readings on the Ch1 OUT+ and OUT- that also match those on Ch2. If we can do that, we should look further into the vertical pre-amp section for more issues.Is Ch1 able to display a signal of the correct magnitude all the time through both failure modes A and B?The thing that's perplexing is we have one completely working channel, Ch2, which exonerates a large amount of common circuitry. Never say never, but I don't think there's a common cause that's creating the same problem in Ch3 and Ch4.And keeping in mind, when both Ch1 and Ch2 were working, they were both 18% or so low. That could be a side effect of something else that's gone wrong that *is* common to all.Ch3 and Ch4 should work as regular input channels, except they only have two attenuator settings: 0.1V/div and 1V/div. If I recall, the 1V/div setting also makes Ch3 or Ch4 the input trigger for the A sweep or B sweep, respectively. When used with a 10x probe, it's 1V/div and 10V/div. When I had this scope, I built a x5 external attenuator to give me 5V/div with the probe without having to select Ch3 or Ch4 as a trigger input. We can get into that later if you're interested, assuming we get it working.So, when you use quad mode, you should be seeing Ch3 and Ch4 with 0.1V/div, and the position adjustments should allow the full range. In my scope, several times I forgot to take the knobs off the Ch3 and Ch4 positions controls when taking the cover off, which then pulled the metal top off the base of the pots (bent the tabs). You might want to check for physical damage.I liked this scope a lot when I had it, but having the position control for two of the channels on the side near the very back of it was a fail.Since we have a working Ch2, we may want to use it to do some probing on Ch1 to see if we can get some of the service waveforms. This may sound odd to troubleshoot a scope with itself, but it's entirely possible as long as you look at signals that are unrelated to the internal operation of the sweep. (That's in general. You can also look at any signal that is synchronized to the sweep, as long as you understand when your probing channel is being switched on and off by the sweep and what affect that has on what you see.)EDIT: Fix ambiguity.

author: aneng
author_rank: Regular Contributor
timestamp: December 21, 2015, 11:24:08 pm
content: OK, I have more readings ! It seems that having the ethernet cable connected to the various pins on IC12 & 13 screws with the trace. It must be acting as some kind of antenna, picking up RF perhaps ? As you can see from the (much lower resolution this time !) video below... This incidentally is in "working" mode.Right... onto the voltages;Seeing as I had spare wires, I took readings from IC13 as well for comparison.Bear in mind that in order to confirm the switch had taken place from "Failure" to "Working" mode, I had to wiggle the V-Pos knob, so there may be slight variations between the readings in each mode due to me not getting the V-Pos knob back in exactly the same position - and indeed slight variations between the relative positions of the V-Pos knobs on Ch's 1 & 2.IC12PIN FAIL MODE WORKING MODE==============================2 +0.687 +0.6748 -1.429 -1.4959 -1.366 -1.39815 +0.692 +0.677IC13PIN FAIL MODE WORKING MODE==============================2 +0.438 +0.4348 -1.296 -1.3739 -1.325 -1.33715 +0.448 +0.445I have cleaned up and resoldered L1 and L2 (it was there, next to it so why not !)Lastly, I have to confess another error on my part... Due to the PCB trace layout around ICs 12 & 13, I (stupidly now that I think of it !) made the assumption that pin 1 was on the bottom left of the board for IC12 and bottomrightfor IC13...i.e. a mirror image ! You can see my error from the pic;This means that any measurements I've previously given for IC13 are not from the pins stated. The previous measurements I took were luckily from opposite pins and the voltages almost identical, so that won't have had any effect on things. The readings in this post are, correct !One last thing... in order to encourage it to switch from failure to working, I prodded the Vertical Preamp board in the middle and it suddenly switched over - so maybe it's just a shitty solder joint somewhere ?Update: I switched the scope off, dismantled the boards (yet again ! - I can do this blindfold now !) and removed the ethernet extender cable. Reassembled and switched on - it went straight into fail mode. I very gently prodded the Vertical Preamp board (this is not the one we've been working on), and it switched immediately to 'working' mode. I'm definitely suspecting this board now.In other news; I went back in the thread and followed your advice about shorting out TP2 to stop the CAL oscillator. The DC signal on the CAL terminal reads 316.7mV on one meter and 317.6 on the other. With TP2 unshorted and measuring the P-P value of the CAL signal, I get 250mV on both channels (though until Ch1 kicks back into fail mode, I can't get you a reading for that just yet). I tried it with the probes and with a directly-wired BNC connector and the results were the same.Further Update: I've been looking closer at CH3 & 4 in the past half hour, and this is what I've found; CH3 doesn't seem to work at all - it's a flat line, unaffected by the CAL signal on its input. CH4 does display the CAL signal (but it barely registers half a division P-P). Furthermore, the V-Pos knob for CH4 behaves very much like CH1's did right at the very start of all of this - i.e. it moves the top half of the waveform only, and then only a fraction of a division. CH3's V-Pos knob is batshit-crazy - it seems to have three positions (as I discovered using the beam-finder button); off the top of the screen, bang-smack on top of CH4's trace, or off the bottom of the screen. There is no gradual transition; it's like a three-position switch. Even though the pot itself turns smoothly, the trace jumps to each of the three positions in turn.I can't help thinking that whatever is causing this is also something to do with CH1's problems. It seems too much of a coincidence to have V-Pos issues on multiple channels (well.... except Ch2 !).

author: MarkL
author_rank: Unknown
timestamp: December 22, 2015, 01:59:13 am
content: Ok, these readings from IC12 and IC13 would imply to me that both channels are probably working.Pins 2 and 15 are probably picking up noise. They are fairly low impedance, but the signal at those pins gets amplified quite a bit so that a 23mV pk-pk signal there results in a 5 division deflection. (According to the test traces.)As far as the readings error, it didn't make a great deal of difference since the chip is symmetric. 2 and 15 are the differential input pairs, and 8 and 9 are the differential output pairs.If you've found that prodding the vertical amp board makes it work, start hunting!!I think it's interesting there's a voltage difference at the outputs between working and non-working. As we talked about before, with differential pairs what matters is the difference between the two. So, to embellish your readings a little:IC12PIN FAIL MODE WORKING MODE==============================2 +0.687 +0.674 \15 +0.692 +0.677 | inputdelta -0.005 -0.003 /8 -1.429 -1.495 \9 -1.366 -1.398 | outputdelta -0.063 -0.097 /IC13PIN FAIL MODE WORKING MODE==============================2 +0.438 +0.434 \15 +0.448 +0.445 | inputdelta -0.010 -0.011 /8 -1.296 -1.373 \9 -1.325 -1.337 | outputdelta 0.029 -0.036 /The inputs are very close, as they should be with no signal coming in.The absolute outputs show a more negative voltage when working. Perhaps there's a problem with one of the -10V rails. Take a look at the emitter resistor on Q1 on the vert pre-amp board, it looks like "-10Vc"? This schematic is blurry.I would expect the difference on the outputs to be smaller, but without knowing the offset specs for the ATM-1040, maybe they're ok. The delta in the outputs also show a variation, which may be another clue.It appears in the video that Ch2 also suffers from insufficient range. Is that correct? Does its range change at all in the extreme positions working to non-working? With the delta in the output above, I would expect it would.Great checking the cal generator. At least we now know the amplitude of the cal signal to a decent level of confidence.On the Ch3 jumping, that sounds like a mechanical issue, but I'll take any input as a clue. Check to see if you have -10V on the center wiper of the Ch3 pot. The schematic hints that Ch1 and Ch3 share some power rails, as does Ch2 and Ch4.Will continue to look at all this new data.EDIT: Typo.EDIT#2: To clarify, if you were wondering, I'm interested in these changes in voltages on the output of IC12 and IC13 because I suspect they are being influenced by current being sunk by the transistors in IC1 and IC2 on the pre-amp board. There's a correlation in that voltage from working to non-working, and by tracking that to the reason we might be able to find the problem. That's one way. Poking at the board with a stick can be equally as effective.

author: aneng
author_rank: Regular Contributor
timestamp: December 22, 2015, 07:22:01 pm
content: Quote from: MarkL on December 22, 2015, 01:59:13 amIt is. I've checked the schematic for the CS-2070 and the circuit there is identical.QuoteIn the video, yes, but that was down to having the ethernet cable attached (as was the fuzzy trace). Once removed, CH2's V-Pos range returned to normal. Ch2 has never suffered from insufficient range. My definition of 'non working' mode is when CH1's vertical position knob refuses to have any effect. I don't even look at Ch2 as being faulty, other than the 16% low reading, which I'm presuming is something elsewhere.I'll have a look at the places you suggested and also check some voltages on the Vert Attenuator board in failure mode (if I can).

author: aneng
author_rank: Regular Contributor
timestamp: December 24, 2015, 09:54:09 pm
content: Well, I decided to probe a bit further around the V-Pos controls for Ch3 and Ch4, as on closer examination they were in a right state.Here's a pic of Ch4's control before I removed it (Ch3's was actually in a worse state !)...Ch3's had -10V on the wiper, but Ch4's had -2V (could have been +2 - can't remember now !) - either way, it was way off. It also had the same voltage on all three pins AND had no connection to the wiper either !Once I got both pots off - they are completely kaput - both of them. I didn't break them removing them, they were like that on the board - readings all over the place, mostly OC with the occasional blip to near-shorts. The way they had been butchered was a sight to see.. I only lost two pads taking them off, so all the other missing pads in the following pics were there before.This is the underside of Ch4's pot before I removed it...This is what's left after I've cleaned it all up...I can't help feeling that this couldn't have been helping with the issues on Ch1.. Only time (repeated power-cycling with enough cool-down time between) will tell whether these were the cause of the problems with Ch1. I've powered it up twice so far and not yet seen the fault, but it will need a lot more testing than that before I can come to the conclusion that it's fixed it. Either way, we're still 16% down on the readings.I'm not likely to get time to do any further testing for a couple of days, not least because we're eating Christmas lunch off the table I've currently commandeered for oscilloscope repair !So in the meantime, a big thankyou once again, and a Merry Christmas to all

author: Ian.M
author_rank: Super Contributor
timestamp: December 24, 2015, 11:11:27 pm
content: Now *THAT* is going to be fun to fix . . . . NOT. I'd bodge in some fixed resistors to set the CH3,4 trace positions midrange and get the voltages right for now.Doing it right is going to involve obtaining mechanically compatible new pots and a lot of copper foil and epoxy work, + probably eyelets for the signal pins so you may want to put that off till you've debugged the rest of the board.

author: aneng
author_rank: Regular Contributor
timestamp: December 25, 2015, 01:45:00 pm
content: Well, I know removing those two pots has made no difference - the scope just started up in fail mode again. However... I have definitely established that a gentle touch with a wooden skewer in the centre of the Vertical Preamp board and it immediately switches to 'working' mode. It was a very gentle prod, the board deflected in no more than 0.5mm. Vertical Preamp board it is then !...Update; Now this really is weird.... I've just tried it again after it being off for several hours and it started up in fail mode, but no amount of prodding of the V.Preamp board could get it to switch to working mode -untilit had been on for a few minutes, then a minute touch on the V.Preamp board and it sprang into life. How the hell can you have an intermittent joint that only becomes fixable by prodding once it's warmed up ?!!One other thing I've noticed - whether or not this is related, I don't know - but the CH2 INV button sometimes will not light up at all - no matter what you do. Other times, it works perfectly and consistently (lighting and extinguishing as required) - but - CH2's display doesn't invert. It twitches a bit but stays put. I was expecting it to invert the waveform - i.e. with the CAL square wave, the displayed phase would appear to shift 180 degrees.

author: aneng
author_rank: Regular Contributor
timestamp: December 27, 2015, 02:20:22 am
content: OK - I think I'm coming to the end of my patience with this scope. In frustration at the intermittent connection somewhere on the V.Preamp board I decided that given my current level of knowledge and test gear, it would take me far longer to try to pinpoint which joint was dodgy than it would to just shotgun the whole board. So, that's what I've done over the past two evenings. I've been very careful, cleaned up every joint with a fiberglass pencil first, then applied fresh flux, then touched up the joint with the minimum of (or no) extra solder. Cleaned all the flux off the board and it looks like this now....After reinstalling it and switching on, several things have changed. First, the good news;The intermittent appears to have goneCh1 now displays the correct p-p voltage for the CAL signal - i.e. it's no longer reading 16% low.Now the bad news;Ch2 no longer functions at all - flat trace that does not move with the V-Pos control, nor displays a waveform.Ch1's V-Pos control takes the trace from waaaay above the top of the screen at one end to about a third of the way from the bottom of the screen at the other end.It's no longer possible to get Trigger Lock on the CAL signal unless the H.Sync selector is in 'V Mode'.Selecting Ch1 as the Trigger source results in a waveform with no gaps (i.e. it's just two horizontal lines with little notches where the square wave corners should be) - Selecting V Mode instead displays a proper square wave.I've also just noticed that Ch1's trace occasionally twitches vertically as if there's still an intermittent connection.Unless all of this is somehow significant in a good way (yeah right !), I think that further attempts to repair this thing are possibly an exercise in futility. It cost me £20. I think maybe now is the time to perhaps put it on the shelf, buy another scope and maybe return to it another day when I have more knowledge and don't have to keep bothering the kind folks here with noob questions.

author: Brumby
author_rank: Unknown
timestamp: December 27, 2015, 03:20:52 am
content: aneng- I (and I'm sure many others, judging by the view count) have been watching your adventure with interest and are keen to see you victorious. Your frustration at this point is understood and nobody would blame you for wanting to walk away from it for a bit.Your 'good news' is good indeed. To see the end of an intermittent is absolutely brilliant and to see the calibration signal come up to par is also rather comforting.Your 'bad news' isn't all bad. What you now have are some (hopefully) stable faults that are 'black and white' - instead of varying shades of grey - which could make further fault finding a lot easier.I will offer you my encouragement and retire to the sidelines once again.

author: aneng
author_rank: Regular Contributor
timestamp: December 28, 2015, 11:00:19 pm
content: Thanks Brumby ! I'm not going to give up on the scope actually, but.... I think I need to pull in some reinforcements - in the form of another scope. After all the time I've spent on this, I'm bloody determined to find the fault and fix it - whatever it takes !!! However, as I also wanted a working scope (and this was meant to be it), I've decided to buy one off a friend for £50 delivered. It's a Metrix 803B (I think it's a French make)..Hopefully I can use it to tackle the Trio with a better chance of pinning down the fault.Meantime, until I get my hands on this, is there any point in continuing with the FG085 Function Generator ? The new U1 got extremely hot last time I tried it, but I do have another one spare now. Is there anything I can test further with this or should I be taking a large hammer to it in frustration at this point ?

author: Ian.M
author_rank: Super Contributor
timestamp: December 29, 2015, 12:26:05 am
content: Put the function generator aside till the new scope comes - it will be much easier to debug with a working scope.

author: aneng
author_rank: Regular Contributor
timestamp: December 29, 2015, 09:28:25 am
content: Will do.Is this how it starts, gear acquisition ?I bought the scope to fix the amp, I bought the function generator to fix the scope, I bought another scope to fix the scope... :-)Is my spare room going to look like Dave's lab by the spring ?

author: Ian.M
author_rank: Super Contributor
timestamp: December 29, 2015, 10:41:50 am
content: Yep^2.(apart from those idiots who buy the gear without learning why they need it just because they wanna be cool)

author: Deathwish
author_rank: Unknown
timestamp: December 29, 2015, 02:46:04 pm
content: Quote from: aneng on December 29, 2015, 09:28:25 amI prefer the term Acquisition agent, sounds much posher

author: aneng
author_rank: Regular Contributor
timestamp: January 08, 2016, 03:43:34 pm
content: Right then !! I hope everyone had a Merry Christmas and a Happy New Year !!I'm now the proud owner of one of these...I've given it a quick test and it seems fine as far as I can tell. It's got calibration stickers on it from Dec 2010 and the insides are far more modern than the Trio. This chap has an album here of the insides:-https://plus.google.com/u/0/photos/+AndrewHull-penguin-droppings/albums/6094881872199591361SO ! - Back to it !!I think I'd like to get the signal generator working if possible, so at least I'll have a) a known input for the Trio and b) another working scope to read the outputs from the various boards. That seems the logical order to me, though I'm happy to defer to the more experienced eye.

author: MarkL
author_rank: Unknown
timestamp: January 08, 2016, 06:32:45 pm
content: Congrats on the new scope!I agree that you should tackle the function generator first. The Trio isn't going anywhere, and the function generator can only help with the debugging.

author: aneng
author_rank: Regular Contributor
timestamp: January 08, 2016, 08:24:26 pm
content: Could someone wind me up and plonk me down on a good part of the schematic to start at please ? I'm still in the spoon-feeding stage!The last time, I got as far as replacing U1 but the replacement gets insanely hot (I have another spare).http://www.jyetech.com/Products/085/Schematic_085G.pdfFrom my previous post on this repair (!): "I have removed DN1 and R18 (in addition to the previously removed U2 and D2). DN1 tests ok. However, applying power now resulted in the same pulsing of the PSU voltage, shortly followed by a smell.... the smell of my newly installed U1 toasting nicely."

author: aneng
author_rank: Regular Contributor
timestamp: January 10, 2016, 12:18:41 am
content: I got all my gear out on the kitchen table again (!) and started working on the Function Generator. I looked up the datasheet for U1 (MC34063) and to my surprise, the junction temperature is 140 degrees Celcius ! That's crazy hot ! So, my concern about it overheating may be a red herring. Either way, I plugged it in and U1 got very hot as before, but this time I ignored it and poked about a bit with the new scope. I didn't see anything noteworthy (nor would I when I remembered I'd removed D2 and R18 - effectively isolating everything downstream of U1 !). However, ten mins later I touched U1 again and it was room temp - and I also noticed what appears to be a ring of solidified something around the centre of L1's windings (excuse the pic, it's the best I could get)..I have a funny feeling that stuff is the result of some serious overheating of L1 - it's a bubbly crispy resinous deposit that I can scrape off.The fact that U1 is now cold and L1 looks suspect makes me think that U1 has burnt out and maybe L1 too...I do have another U1 spare, but... I think I have to face it that the Function Generator is just too much hassle to fix, and that I'm going to have to buy another one. I could do without spending any money right now, what with the new scope etc, but I don't think we're going to fix this thing are we ?

author: aneng
author_rank: Regular Contributor
timestamp: January 10, 2016, 02:53:50 pm
content: Update: Hell no ! I'm not going to be defeated by a simple function generator !! I'll order replacements for both Op-Amps, get a new SMD 5V regulator and a spare inductor just in case. I'll also cut off the 20 pin header so I can desolder the display and fit the electrolytics on the correct side of the board (the display gets soldered on over the caps, so you can't do anything with the display in place and I've already discovered how impossible it is to desolder a 20 pin header that's soldered flush to both the board and the display !If I give up over a little circuit like this, there's no point in me pressing ahead with my new hobby. I WILL fix you Function Generator.... !!

author: rch
author_rank: Regular Contributor
timestamp: January 10, 2016, 08:23:31 pm
content: I am no expert, but I have built things that didn't work! There are more fault possibilities with something newly built than with a fault in something previously working. I suggest you check the resistance of the various circuit meshes without some of the active components if you are rebuilding parts of it. Make sure the resistance, either with a low voltage multimeter, or with a higher voltage one which is likely to turn on diodes, makes sense given the circuit diagram. Once you have put something in the wrong place, you will find you will never see it however hard you look! Then if you can find no error start it with a low voltage bench supply (voltage and current limit gradually increased to reasonable levels from well below normal working voltage) - use it instead of the regulator and see if appropriate voltages appear and watch for anomalies as you raise it. That way you may find what is wrong before anything burns.

author: aneng
author_rank: Regular Contributor
timestamp: January 10, 2016, 11:27:26 pm
content: Does it matter what type of inductor I replace L1 with (as long as it's 330uH of course) ? It's just that I can't seem to get my hands on a little SMD one the same physical size and package; This is what's on the board (package type);I can get radial leaded ones that look like a resistor - will one of those do ?

author: rch
author_rank: Regular Contributor
timestamp: January 11, 2016, 12:22:32 am
content: I'd be tempted to get the right size magnet wire and rewind it. You could check it for shorted turns first, if you know what it's equivalent resistance should be and you've got a suitable inductance meter. The problem with a new one is to know what DC current rating, power rating and magnetic properties it has. They are interrelated and depend on the frequency it is used at and, of course, the core material. I'm assuming this is a power inductor, it's a lot simpler for RF inductors!

author: MarkL
author_rank: Unknown
timestamp: January 11, 2016, 01:23:07 am
content: Having the exact replacement inductor is likely not critical. You could replace it with a larger form factor power inductor as long as it has the same or larger current rating.However, that inductor looks ok to me. It takes a lot of continuous DC current to cook an inductor. The bubbly stuff looks like glue to hold the outer layer of windings in place, perhaps needed during reflow.The edges also look like they're chipped, but that shouldn't be a big problem.If in doubt, you could pop it off the board and test it with your multi-tester. I think it can measure inductance (if I'm looking at the right one). Or if not check the DC resistance with your meter.The MC34063 internal junction temperature of 140C does not mean the chip normally runs at that temperature. In most instances if you can't keep your finger on it, it's running too hot and there's something wrong.I would start with the basics and start verifying voltage levels on the + and - rails, and now with your new scope you can also verify the ripple voltage on them as well. You can do this by setting the scope to AC coupling and looking at the rails with a higher sensitivity.

author: MarkL
author_rank: Unknown
timestamp: January 11, 2016, 07:08:19 pm
content: Quote from: aneng on January 08, 2016, 08:24:26 pmLooking at the schematic...Without D2 in there, the MC34063 can't work correctly. D2 is needed to switch L1's energy into C3. With no voltage being created across C3, the feedback loop through R1 is also not working.You can leave out R18 since that isolates VRAW- from the rest of the circuit. There is some filtering on the other side of R18, but it shouldn't be required for the operation of the MC34063.DN1 is just acting as a voltage clamp. You can leave that out for now too.The MC34063 will try to create +1.25V between pin 4 (chip GND, not board GND) and 5 (COMP). Following the voltage divider consisting of R1 and R2 should result in a VRAW- of about -11.5V.Your first task should be make sure you can get the right VRAW-.

author: aneng
author_rank: Regular Contributor
timestamp: January 14, 2016, 10:06:40 pm
content: Hmmmm.... Well, I sorted the board out, soldered the caps on the correct side of the board (i.e. under the display) -triplechecked the polarity of all the caps. Fitted a new U1, Refitted D2, again triple-checking the polarity. Examined L1 (which by now had lost one of its leads in the winding) - so I gently unwound L1, cleaned up the wire with flux remover, rewound it by hand, measured it with my component tester which saw it as a resistor of just under 6 Ohms (but not an inductor ?), and refitted it to the board. The only components missing from the board were U2 (the 5V reg), R18 and DN1.I plugged in the supply, got the meter on -VRAW to measure it, hit the power button and watched as my new U1 burst into flames.It appears to have ignited just by pin 1.I am not a happy bunny this evening !Edit: I've just removed the charred remains of U1 and I'm checking the resistance of each of U1's pads on the board to ground, as there is presumably a short somewhere that's allowing lots of current to set fire to U1.... This is what I've got;Pins 1,6,7 & 8: 9.4 MOhmsPin 2: 1.2 OhmsPin 3: NCPin 4: 1.24 KOhmsPin 5: 2.13 KOhmsI don't know if this helps point the finger at anything ?

author: rch
author_rank: Regular Contributor
timestamp: January 15, 2016, 12:00:51 am
content: Quote from: aneng on January 14, 2016, 10:06:40 pmWhat voltage does your measuring device produce on the low ohm ranges? If it is much more than 250mV then diodes may be affecting some of the readings. Is the resistance reading from pin 2 the same whichever way round you put the leads? Excluding a current path through D2. 1.2 ohms is suspiciously low, but disconnect the earth end of L1 and repeat the measurement. With the positive lead on pin 2 the resistance should now be very high, and then you can check the resistance of L1 (with one end isolated) on this meter. It may be lower than the component tester reads. But if it still reads 6ohms you may have a short nearby pin 2. If at the same time you are measuring this with one end of L1 lifted the resistance is *high* at pin 2 then the short is probably in L1. If pin2 to earth still measures 1.2ohms with the earth end of L1 lifted then the short is elsewhere (or L1 screen if it has a third terminal).As to the lack of inductance on the component tester, it may not read below a few mH, but if it is supposed to show 300uH then there must be suspicion of shorted turns or a short to earth (if it has a screen pin) in L1.

author: MarkL
author_rank: Unknown
timestamp: January 15, 2016, 01:35:28 am
content: Oh, no! So sorry the smoke got out!I hadn't noticed before that the designers disabled the over-current sense input (Isc) on pin 7 on the MC34063. The usage of this chip should include a current sensing resistor. It's in every circuit example in the data sheet and app note.Assuming you're up for ordering another (or multiple) MC34063, you could improve the design by including the current sense resistor. Something on the order of 0.6ohm will limit the current to around 500mA.But if there is a dead short at pin 2, as rch suspects, it might have died anyway.After the next MC34063 victim is installed, you might want to power it up carefully next time through a 10 or 20ohm 5 watt resistor until you're sure nothing is shorted. Or you could even try a 9V battery for a short time, since they are very limited in current.But if you were looking for reasons to expand your equipment collection, a variable DC power supply with settable current limit would be the right ticket for this work.

author: aneng
author_rank: Regular Contributor
timestamp: January 15, 2016, 02:15:10 pm
content: Oooh ! Another bit of kit to get !I'm definitely up for building my own... I've done a bit of googling and found this design -http://www.tuxgraphics.org/electronics/201005/bench-power-supply-v3.shtmlHow useful is it to have a bench supply that can do negative voltages too ? ( -ve, 0, +ve ) ? What sort of voltages & current should I be aiming for, for general repair work ? I've got a bag of 15V Toshiba laptop bricks I could use..... Would it be possible to use four, so I could get -30V to +30V ? (Can you do that ? - connect the outputs of psu's in series ?).If the -ve voltage is a bit pointless, then I could just do a 0 - +15V (or 30V if I can series them).I like that design because it shows the realtime current consumption as well as the limit. It would be nicer with a rotary dial though....Are there any more popular designs out there that everyone's using ?

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2016, 04:09:55 pm
content: If you had a +/- 12V bench supply you could simply hack the function generator to remove the negative rail generator and feed it from the bench supply.If the laptop PSUs have three pin mains connectors, you cant stack them as the output 0V will be connected to mains earth. If they have two pin mains connectors, you can, but any connected in series *MUST* be identical and you need a beefy diode across the output of each so that if one goes into current limit, the others cant reverse the polarity of its output circuit while trying to push current through it to the load. You may be better off using EBAY adjustable DC-DC converter modules off a single beefy 19V laptop supply, followed by linear active ripple cancellation.Bench supply design and construction is a *BIG* complex subject once you want something better than a basic LM317/LM337 linear supply and is fairly popular here - start a new topic or join an existing one.

author: aneng
author_rank: Regular Contributor
timestamp: January 15, 2016, 05:51:23 pm
content: Cheers Ian - will do. One last thing on the subject of PSUs - I've had a quick look on eBay and there are loads that mention, for example, "0-30V" yet the photo of the unit shows three output terminals; -ve, GND and +ve. Does this mean that the GND is actually 50% of the +ve terminal ? - i.e. if it's set to 30V, what you actually get is "-ve" (0V), "GND" (+15V) and "+ve" (+30V) and you are supposed to connect the Gnd of the equipment being worked on to the GND (+15V) of the PSU, thus giving you -ve and +ve relative voltages on the other two terminals ?If so, and you had something connected up like this, it would have to be isolated from any 'real' earth whilst powered up wouldn't it ?What about measuring with a scope under those circumstances ? You would just connect the probe's Gnd clip to the board's Gnd and everything works off the same principle ? Is that how it works or have I invented some insanity here ?

author: MarkL
author_rank: Unknown
timestamp: January 15, 2016, 06:40:54 pm
content: I would have to see the specific unit, but most power supplies that have three terminals labeled with +, -, and GND would mean that the + and - are floating and you can optionally ground either one of them to create a positive or negative earth-referenced supply.Like this one, for example, has three separate, floating sections:http://www.ebay.com/itm/321969294816Most of the time you wouldn't need to worry about grounding one of the terminals. If the circuit is floating because the power supply is floating, you define ground in the circuit when you connect your scope probe ground to it.

author: aneng
author_rank: Regular Contributor
timestamp: January 15, 2016, 11:29:13 pm
content: So if I'm going to need +ve and -ve voltages, I'm going to have to get some fancy dual-rail supply then ? Hmmm....How do people normally deal with gear that needs +ve & -ve (wrt ground) supplies ? 99% of the bench supplies I'm seeing on ebay are single polarity only.

author: MarkL
author_rank: Unknown
timestamp: January 16, 2016, 01:05:37 am
content: A large majority would just go for a triple supply. Triple supplies often have a separate, isolated lower voltage output for driving the logic portion of your circuit, and then a higher voltage section with a +, -, and common that can be programmed to track eachother. Here's a good example:http://www.ebay.com/itm/272095170994And here's one with less current output:http://www.ebay.com/itm/281899660880I haven't used a Leader power supply, but these are the most ok looking ones I came across first. Maybe someone else can attest to Leader performance.But in general I wouldn't try to go too cheap on a lab power supply. A quality one will serve you for many years.

author: aneng
author_rank: Regular Contributor
timestamp: January 16, 2016, 12:09:12 pm
content: Ok, I'm going to need a bit more spoon-feeding on this I think ! When you say 'triple supply', is that a psu that outputs -ve, 0v, +ve (i.e. the 'triple') ?I'm not sure what "programmed to track each other" means either.How can I tell which ones on eBay are triple supplies ?Thanks in advance !

author: Brumby
author_rank: Unknown
timestamp: January 16, 2016, 12:32:15 pm
content: A triple supply is one that has 3 separate outputs. A common arrangement is to have two variable voltage outputs (often 0 to 30V at up to 3A) and one fixed voltage output - maybe 5V at 5A or something like that.Each output should be isolated from each other, so that you can connect them however you want.For a split power supply, you connect the positive output of one supply to the negative output of the other - and this becomes your 0V rail The free negative becomes the -V rail and the free positive becomes the +V rail. You would normally adjust the two voltages to be the same, so that your 0V rail is right in the middle.In adualtrackingpower supply, this adjustment toBOTHvoltages is done by ONE control,automatically keeping them the same- which just makes life so much easier.

author: Ian.M
author_rank: Super Contributor
timestamp: January 16, 2016, 12:40:51 pm
content: A tracking supply has two sections with a common voltage control (and often a common current control) so if configured in series, taking 0V from the link you get symmetrical +ve and -ve rails, great for analog stuff, especially OPAMPs. Better quality ones have cross-coupled current limiting so that an overload on either rail reduces the other rails voltage symmetrically.

author: aneng
author_rank: Regular Contributor
timestamp: January 16, 2016, 12:53:22 pm
content: I can see I'm going to need to learn more about negative voltages and grounds etc because I'm hitting a 'layman's logic block' here ! e.g. If I connect two psu's in series, say, 15V each, then I'm going to get 0V at the -ve terminal of the first, 15V at the +ve terminal, and, as it's connected here, at the -ve terminal of the second, then 30V at the +ve terminal of the second. Therefore, my '0V' is really +15V isn't it ?If I were to connect this to a circuit that also had an earth connection, that would cause current to flow from the circuit '0V' line to earth (the 'real' 0V).I know there's a hole in my knowledge here! Where can I go to grasp this concept properly ? Is it in the Forest Mims book ? (I'll look now...).

author: Ian.M
author_rank: Super Contributor
timestamp: January 16, 2016, 01:37:14 pm
content: There is no such thing as an absolute 0V. *ALL* voltage measurement is differential. Picking a circuit node, calling it 0V and measuring all voltages with respect to that point is merely a convenience. Sometimes the node is picked for you, e.g. Earth ground is commonly taken as the 0V reference point, and oscilloscopes usually have the BNC shell grounded so forcing you to ground the 0V node of your circuit. Thefore tpyr two series connected floating 15V PSUs cold be considered as a 30V PSU (+ve or -ve, tapped at 15V) or as a +/-15V supply, and if your circuit includes level-shifting between sections, what you consider the PSU to be can vary between section. e.g. it is sometimes advantageous to make the 5V logic supply the same as the 12V power rail and provide a logic Vee/Vss rail at 7V that acts as 0V for the logic section - because it makes it easy to control P channel MOSFETs as high side switches. If one of the supply rails is connected to the case, chassis or earth, it is generally foolish NOT to use that as the 0V reference point.

author: Brumby
author_rank: Unknown
timestamp: January 16, 2016, 03:08:59 pm
content: I've used a couple of 9v batteries for this illustration, but you can look at each of them being a floating power supply...Question: What's the difference between these 3 circuits?Answer: Where you put the 'common' (black lead) of your multimeter. The circuits themselves are identical and any other circuitry off to the right will not care where you put your multimeter leads.

author: Brumby
author_rank: Unknown
timestamp: January 16, 2016, 03:40:36 pm
content: While there are situations where a few more details might be relevant, the following has been kept really simple - to demonstrate the basics....These batteries are just like 'floating' power supplies (usually a pair of red and black terminals) - they aren't connected to anything.The (usually green) earth terminal IS connected - but only to the earthing pin that goes back through your power point. It is not connected to any of the red or black terminals ...unless you make a connection.Now here comes the head spin....With the circuit above applied to two floating supplies, you can connect the earth to any one of the three measuring points without affecting anything. FURTHER, if you keep the common lead of your multimeter connected to whichever of the three measuring points you used before, the readings will not change.There are two simple rules here:1. That all voltages are measured from aspecific reference point.and2. A voltage measurement is only valid if the reference point and the measurement point are electrically connected through a circuit of some kind. (Unconnected windings on a transformer don't count as they aremagneticallycoupled.)

author: aneng
author_rank: Regular Contributor
timestamp: January 16, 2016, 05:47:01 pm
content: Thanks for that guys... it's a lot clearer now. SO, taking my function generator as an example, if I were to connect two 15V supplies as per the diagrams above to give me VRAW (+15V), 0V, -VRAW (-15V), what would the scope do if I measured the output of the unit (say a sine wave) with the ground clip of the probe connected to this arbitrary '0V' point - given that the shield of the BNC connector is likely connnected to mains earth ?(actual circuit notwithstanding - I'm assuming the output would centre around the 0V line for the sake of this argument).Would it show as a +15V offset or not ? ...and, presuming the power supplies were isolated from any earth, would any current flow if I connected the +ve terminal to mains earth ? If I've understood your posts correctly, I'm assuming no current would flow. ?

author: MarkL
author_rank: Unknown
timestamp: January 16, 2016, 06:30:08 pm
content: Quote from: aneng on January 16, 2016, 05:47:01 pmYour assumption is correct. It would not have an offset, assuming you asked the function generator to output a waveform with 0V offset.If you connected +ve to mains earth no current would flow, BUT as soon you connected your probe ground clip to the 0V point on the circuit, you have now created a short between +ve and 0V since they are both at earth potential. The shorting path is through the earth ground in the scope and power supply line cords.

author: aneng
author_rank: Regular Contributor
timestamp: January 16, 2016, 10:12:23 pm
content: Ooh... now.... err.....Right, so it wouldn't have an offset because, although the waveform would be centred around +15V relative to the -ve lead of the first power supply (i.e. the middle point in your middle example pic), because there is no circuit between the function generator and mains earth, there is no potential difference between circuit 0v (i.e. the centre of the waveform) and the mains-earth-connected ground clip of the scope ?My brain is aching at this point.."you have now created a short between +ve and 0V since they are both at earth potential."The... what... but....... eh ?Oh... there it goes... it's imploded.

author: Brumby
author_rank: Unknown
timestamp: January 17, 2016, 02:42:18 am
content: Breathe .... relax .... You are not far from understanding!I refer you to this part of my previous diagram - as this is the more typical way of expressing a split power supply...Quote from: MarkL on January 16, 2016, 06:30:08 pmRestated: If you connected the +ve terminal (the one at the top) to the earth terminal on the power supply, then nothing nasty happens. The +ve terminal is now directly connected to mains earth, through the power supply cord going back to the power point. No current would flow.You then bring out your oscilloscope, plug it into another power point, whereitis earthed as well. For several reasons, oscilloscopes have this earth firmly connected throughout, including to the 'earth' connection on your probe.QuoteConnecting the probe's ground clip to the 0V point (the one in the middle)...Do you see the problem...?QuoteThis is something about which you should be very careful - as you can start blowing up things! So much so, that Dave has done a video on just this issue:

author: KJDS
author_rank: Super Contributor
timestamp: January 17, 2016, 08:09:23 pm
content: Have a look through the PSUs I have listed on ebay. PM me if any take your fancy and I'll do a good deal on it.

author: aneng
author_rank: Regular Contributor
timestamp: January 18, 2016, 03:09:36 pm
content: Right... I get it now - thanks Brumby (and others !). !@KJDS Thanks for the offer - I'll have a look.As for getting a bench PSU, I think I'm going to need one regardless. I would prefer to buy a commercially-made one I think as they look SO much better than the DIY ones! Not to mention it would take me weeks to build it.So, in Dave's video about setting up a lab, he suggests building a dual-tracking psu and says there is loads of info on the forum. However, I've searched and can't find any threads that have designs for them. Am I not looking in the right places or what ?I don't really want to spend more money than I can afford, but.... if I'm not going to save a great deal by building one (i.e. at least 50%), then I may as well just go straight for a manufactured one. I've seen this one on eBay, but it is a bit more than I wanted to spend at 108 GBP, however... it looks like it would last me a while...

author: aneng
author_rank: Regular Contributor
timestamp: January 18, 2016, 04:09:15 pm
content: Naah... changed my mind. On closer examination, you need to move a slider on the display to switch between volts and amps. That's a bit 'meh'.

author: nfmax
author_rank: Super Contributor
timestamp: January 18, 2016, 04:20:09 pm
content: This one, maybe?http://www.ebay.co.uk/itm/Farnell-L30BT-/151951331949?hash=item236101526d:g:hqAAAOSwa-dWmM7BWith luck, it may even need repair!

author: aneng
author_rank: Regular Contributor
timestamp: January 18, 2016, 07:10:07 pm
content: Thanks nfmax, that would have been a bargain if it had been a bit nearer to me. It's collection only, and unfortunately, a 270 mile round trip.Tell me guys, I can see that having a dual tracking PSU is the most desirable type of bench supply to have. Why is it then that 90% of the PSUs on eBay are single output and I can't find a design on the forum to build my ownDo most jobs require only a single output ?

author: MarkL
author_rank: Unknown
timestamp: January 18, 2016, 07:37:30 pm
content: Quote from: aneng on January 18, 2016, 04:09:15 pmHere's one that displays volts and current for both channels at the same time. And there's a third output with 5V @ 3A. The third output has no readout and is not adjustable, but that's fairly common and could be used for powering digital circuits.http://www.ebay.co.uk/itm/261646567155The specs are ok if you can trust them, and 5A is more than you'll ever need to power small projects. I've never heard of MLINK, but the design looks a lot like other makes. No doubt a Chinese OEM.Quote from: aneng on January 18, 2016, 07:10:07 pmDepends what you're doing. If working on digital circuitry, then usually one output is fine. If you do a lot of work with analog and op-amps a dual supply can be a big help instead of having to derive your own 0V reference. Hybrid analog/digital often will need a dual or triple supply, and sometimes more.You'll never regret buying more outputs than you think you need.

author: nfmax
author_rank: Super Contributor
timestamp: January 18, 2016, 10:43:06 pm
content: Dual tracking supplies can be useful, but they almost invariably have a common 0V output, that is, one output gives you plus some adjustable voltage, while the other gives you minus that voltage (or some adjustable fraction of it). You can't connect the two in parallel if you want more current, they are always in series. On the other hand, at least in good quality supplies, they will turn on and off together - i.e. you never get, even for a moment, one supply much higher voltage than the other. This can matter with sensitive circuits.Good power supplies also never overshoot when turned on, the voltage rises steadily from zero to the set value, never going above it. Good supplies also have low levels of noise on their output, they have accurate meters, and either multi turn controls or separate coarse and fine controls so the output voltage is easy to set. They are also reliable, and naturally cost more. The advantage is they go on for donkey's years so a 40 year old supply like that Farnell is quie a feasible proposition.For someone like yourself, just starting out, I would recommend a dual independent (isolated) supply, covering up to about 15 or 20 volts at about 1 amp, with either digital or good quality analogue meters, and adjustable current limiting. Like that Farnell, in fact. You don't really need simultaneous measurement of voltage and current for both supplies (though it is nice) because you generally use the voltage meter only to set the supply output, then measure current while your test circuit is operating. On the other hand, an adjustable current limit is very useful. This allows you to set a maximum current that your circuit can draw from the supply: if it tries to take more, by some mischance, the supply will automatically reduce its output voltage to keep the current at the limit value. This can help prevent tears!Designing a good quality (bench) power supply is harder than it looks. eBay is full of the offerings of people who have yet to learn this lesson. You will probably do better with a second hand unit from one of the leading manufacturers - HP (Agilent), Thurlby-Thandar, Tektronix, Keithly, Farnell, or Hameg are likely candidates, but there are many others.

author: aneng
author_rank: Regular Contributor
timestamp: January 18, 2016, 11:27:42 pm
content: Thanks for all that info both. Is the 'tracking' bit just a convenient way to match the two outputs then ? i.e. if I got a two (independent) output supply that didn't have tracking, I presume I could just manually set the voltage and current limit for each output separately, achieving the same result (with the exception you mentioned that one rail could be different or on/off compared to the other) ?That one you linked to looks alright Mark. Shipping isn't too bad considering it's in Barcelona ! If what I've surmised above is correct, then I may go for an old-school unit for a budget price - unless of course I see something that looks too sexy to resist ! I'm a sucker for good looking equipment !

author: Brumby
author_rank: Unknown
timestamp: January 19, 2016, 01:06:26 am
content: Quote from: aneng on January 18, 2016, 11:27:42 pm10/10

author: MarkL
author_rank: Unknown
timestamp: January 19, 2016, 03:47:11 pm
content: Quote from: aneng on January 18, 2016, 11:27:42 pmThat one looked liked a reasonable new unit, but old school would be great and, in my opinion, actually preferable if you can find one in your price range and from a known reputable manufacturer. HP/Agilent, Lambda, Sorensen, TTI, Power Designs are the ones I would seek. Leader, Tektronix, Xantrex, Kikusui are also decent. Leader makes the LPS-151 and LPS-152 which are priced well and I think would make a nice "old school" first power supply, although it doesn't have all-way metering.I would also try to find a linear supply for your first supply. They have less ripple and noise than switching supplies. Many listings don't say, but you can usually tell because they are heavy and have cooling fins sticking out the back. Most old supplies will be linear.

author: nfmax
author_rank: Super Contributor
timestamp: January 19, 2016, 05:01:54 pm
content: Note:TTI = Thurlby-Thandar InstrumentsLambda, Sorenson & Power Designs are more common in the USA than in the UKI forgot to mention the linear/switching difference & I agree with Mark you want a linear supply. You should also not bother with a (computer) programmable/remote controlled supply at this stage.

author: aneng
author_rank: Regular Contributor
timestamp: January 20, 2016, 05:08:36 pm
content: I've found an old school one that appears to do 50V and 0.5A that I might get for a bargain. I know that Dave suggests 1A in his "Setting up a lab" video, but I'm hoping 500mA will suffice for now?

author: MarkL
author_rank: Unknown
timestamp: January 20, 2016, 06:17:39 pm
content: 500mA is a little lean, but it would certainly get you through the current project of repairing the function generator. I agree with Dave and would opt for something with 1A or more given the choice. I guess it depends how much of a bargain it is.But no matter, if you stick with doing electronics projects I can guarantee it won't be your last power supply. There are some people on this forum who can never have enough of them. (You know who you are...)

author: aneng
author_rank: Regular Contributor
timestamp: January 20, 2016, 11:17:48 pm
content: I'll know how much of a bargain it is come Sunday... All will be revealed then (hopefully).

author: Brumby
author_rank: Unknown
timestamp: January 21, 2016, 12:10:50 am
content: Quote from: MarkL on January 20, 2016, 06:17:39 pmDoes the old laptop power unit I keep on hand for 12v up to 4A count?

author: aneng
author_rank: Regular Contributor
timestamp: January 24, 2016, 03:45:26 pm
content: Well, the 500mA old-school one I was watching went for stupid money for such an old device, so I let that one go. There's a few more coming up this week so everything is on hold really until I get a dual-output PSU. I must say, I'm a sucker for the LED displays, especially if they are different colours like this one...

author: Brumby
author_rank: Unknown
timestamp: January 25, 2016, 02:48:07 am
content: That's a pretty typical power supply. It has some useful features built in, such as output selection: Independent, Series (for dual rail - and it seems it turns into a tracking supply) plus Parallel for those higher current requirements.Not sure about the quality side of things (I have no experience with Tenma), but a supply like that will be useful to have on any workbench.

author: aneng
author_rank: Regular Contributor
timestamp: February 09, 2016, 01:12:17 pm
content: Well... Things are still on hold until I get a power supply! I held out for several on eBay but was outbid, and since then they have become rarer than rocking horse poop. Typical!After a couple of weeks of waiting, an old Farnell unit has turned up.. Faulty, of course! I do have the schematic for it though, and it looks relatively simple compared to the Trio scope. The question is... Do I bid on it, and end up with another box to fix? :-)He had to fix the supply to fix the function generator to fix the scope to fix the... Hang on - I can't even remember what the original thing was I got the scope for!I must say, I am tempted - if it goes for a cheap price. Apparently one channel is pegged at 30v and the other doesn't work at all.The problem is - you all know where I'm going to come for help don't you? :-)

author: Brumby
author_rank: Unknown
timestamp: February 09, 2016, 01:41:18 pm
content: Whirlpool?There's also a song on my mind - but making the lyrics fit isn't coming easily.............

author: Brumby
author_rank: Unknown
timestamp: February 09, 2016, 01:45:45 pm
content: Quote from: aneng on February 09, 2016, 01:12:17 pmPlease ... if you remember, let us know. I can add it to the song lyric....

author: Performa01
author_rank: Super Contributor
timestamp: February 09, 2016, 01:55:30 pm
content: I think it's hard enough to fix some broken gear even in a reasonably equipped lab, but buying something broken when all you already have is a bunch of broken gear ... I don't know ...What was your budget again?You might want to have a look at this eBay link:http://www.ebay.at/itm/ELECTRONIQUE-DOLOISE-D-LAB-3-1-ALIMENTATION-POWER-SUPPLY-2x-20V-1-5A-1x-7V-D495-/291666530447?hash=item43e8ae108f:g:HAAAAOSwFqJWogFWThis shop has tons of bench power supplies, so make sure you check the other items. But the one in the link seems already pretty decent.Disclaimer: I am not related with the seller in any way nor have I ever bought something from there, but I certainly would give it a try!

author: Ian.M
author_rank: Super Contributor
timestamp: February 09, 2016, 01:59:01 pm
content: OTOH a broken linear power supply that doesn't have any digital controls and doesn't use a phase-angle controlled TRIAC preregulator, can be fixed with no more test equipment than a selection of high power load resistors and a couple of good multimeters.

author: Brumby
author_rank: Unknown
timestamp: February 09, 2016, 02:17:07 pm
content: Quote from: Ian.M on February 09, 2016, 01:59:01 pm+1

author: aneng
author_rank: Regular Contributor
timestamp: February 09, 2016, 02:48:48 pm
content: Quote from: Performa01 on February 09, 2016, 01:55:30 pmInteresting ! I've had a good look around the eBay shop and there are plenty of old-school units there that would definitely fit the bill but.... they're all in France and I'm in the UK and the postage pretty much equals or exceeds the cost of the item on most of them !My budget ? Probably no more than 80GBP. That seems to be the figure most of the digital supplies I was looking at on eBay went for. Less would be better (i.e. if I can get this faulty Farnell until for under 25/30 GBP then it might be worth a shot), but if a digital type unit appears and it looks like I can get it for 80GBP or less then I'll go for that. It's just a question of finding the buggers now !As for trying to fix broken kit with more broken kit - I'm not completely surrounded by piles of smoking PCBs ;-) I do have a fully working oscilloscope, two DMMs, a good soldering iron, a little component tester gadget and the most important tool in my toolbox............. you guys !! :-)Oh - and @Brumby, I've remembered ! - It's my mum's HiFi Amp :-)

author: Performa01
author_rank: Super Contributor
timestamp: February 09, 2016, 04:12:16 pm
content: @anengYes, I've noticed that shipping costs are high in that shop. It's a bummer, as I thought they really have some interesting vintage stuff at mostly reasonable prices.As for the broken - broken thing, yes, I knew you got a 2nd scope - which is even working, woo-hoo!Thankfully your soldering iron isn't broken too - I figure it would be quite difficult to fix a broken soldering iron by means of a ... flat iron maybe?As you sure have already guessed, I was a bit exaggerating, and having said that, I agree with the others that it shouldn't be too difficult to fix an old school linear power supply.Anyway, good luck in finding something useful and be sure I will be following your efforts!