HP 1740A Channel B not working and flickering on A - Page 1
Forum section: electronics / repair

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 05, 2021, 05:06:46 am
content: I picked up an HP 1740A oscilloscope recently. I replaced the fuse and it now turns on just fine, and somewhat works. However, channel B doesn't work at all (only the flat line on the graticule shows, even when probing the 1V CAL signal). As for channel A, it works except for TIME/DIV larger than .2mSEC, it flickers rapidly even when not proving anything, and the RESET light flickers along with the displayed signal.I'm pretty new with all this. Any advice on what to check?Thanks!

author: lowimpedance
author_rank: Super Contributor
timestamp: May 05, 2021, 11:26:42 am
content: Firstly go over to the thread about restoration of a HP 1741 by forum member 'tekfan' and read that for any tips etc.Its in the test equipment section, and has been recently resuscitated !. Next download the manual from the link I posted in that thread. That site should have the 1740 but the 1741 is very close for the most part.The power supply needs verifying to be operational first. And do be careful working on the live innards as there is dangerous voltages present, know what you're doing.Actually the very first thing to do is check for loose or oxidized interconnections which could easily be the case with gear this old. Also dry joints are highly likely in the power supply PCB so check for those then measure the supply rails etc.Once the power supply has been cleared of any issues then move to the vertical input stage and check the resisters around the the hybrid for any that have drifted high, this Will cause bias shifts in the DC coupled stages and could easily cause the loss of the input signal getting to the CRT.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 05, 2021, 08:01:24 pm
content: Thanks for the reply! I had already checked the power supply voltages across the capacitors, and all six are very close to the reference values in the book (the scope actually came with the service manual). I just checked them again, and sure enough, they're all good.What do you mean by the hybrid? I've been checking resistors on the "vertical preamplifier assembly" right near the input, and those seem fine, but I'm not really doing so intelligently just randomly checking values and comparing to the book... Any help with what exactly I should be looking for?EDIT: I also just noticed that when switching to display B, the vertical position adjustment does not change the flat line position at all (it's above the graticule).

author: lowimpedance
author_rank: Super Contributor
timestamp: May 06, 2021, 12:05:05 am
content: The hybrid refers to the custom HP IC module which is A3A1 on the schematic page 8-19 (note ; unobtainium be careful measuring around it when powered, hopefully not already faulty!).Measure the volts on TP9 and TP10 are they the same with no input signal?. What voltage is on R36 and R61 wipers (vertical pos. pots). Does each side of those pots volts change when varying the pot position?.R64 and R66 100 ohm resistors, measure the volts on the end that connects to pins 36 and 34 (of A1) wrt 0V are they the same for each channel when selected ?.Measure the volts applied to pin 20 and then pin 1 of the hybrid amp when each channel is selected. These are the channel enable for A and/or B.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 06, 2021, 02:31:55 am
content: So I'm having some trouble figuring out where R36 and R61 are on the circuit board. I did however measure the rest.With no input, I get 0V on both TP9 and TP10. With the CAL 1V into channel A, I get 0.045V on TP9, and 0 on TP10; and with the CAL 1V into channel B, I get 0.045V on TP10 and 0 on TP9. Looking at the diagrams, this seems to make sense to me.Measuring the voltage from R64 and R66, on the ends that connect to pins 36 and 34, I get 11.67V on channel A, and identical on channel B. Presence or lack of input does not change anything. I'm not sure if that makes sense from the diagram, but it's at least reassuring that I'm getting the same on both channels.I then measured the voltage on pin 20 and pin 1, which appear to be TP5 and TP7 respectively. Presence or lack of input does not change anything. I get 4.863V on TP5 (pin 2) whether channel B or A is selected. However, on TP7, I get 2.801V when channel A is selected, and 4.867V when channel B is selected.Judging from the diagrams, those are the A and B on/off inputs into the hybrid. So it looks to me like the hybrid is never getting the signal to turn on channel B. Which is good I guess because it means the problem doesn't look like it's in the hybrid itself. Looks like the problem is somewhere in the circuit shown on 8-23. So thanks a lot for already allowing me to get this far!I'll try to do some more diagnosing on my own and will update if I manage to find anything. But I'll also keep checking the forum for further advice.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 06, 2021, 03:16:50 am
content: Quote from: PurpleAmaranth on May 06, 2021, 02:31:55 amR36 and 61 are the POT's mounted at the front and are the channel vertical position controls.Quote from: PurpleAmaranth on May 06, 2021, 02:31:55 amThose do make sense, if the voltage on each pin (36,34) was different that would indicate DC bias issues further up which would cause problems for the Hybrid, (the 11.67V is not coming from the hybrid), and most likely just flat line traces with any input signal. (specifically R1 to 5 going high value or Q1,3 faulty).TP9 and 10 seem consistent with the schematic.Quote from: PurpleAmaranth on May 06, 2021, 02:31:55 amThere are a number of logic IC's between the front panel push buttons and the various select lines so those would be a good place to start looking for correct logic function.Including shorted diodes around those IC's.As for the time base sweep issue thats quite possibly a mechanical issue with that switch assembly and the PCB's that form part of the switch. And that issue generally arisesfrom the original lubrication with age hardens and then interferes with mechanical and electrical operation. Solution is to completely strip/clean and lubricate then reassemble, making sure to complete exactalignment for each switch section/PCB. Not a fun job but when done you will likely never have to revisit again. Just make sure you take plenty of pictures of all parts concerned andmark the shafts/locking colars and switch sections positions before starting.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 06, 2021, 03:53:23 am
content: Alright, so I've started looking at the logic IC's, and checking the voltages on the NAND gate chip (U3), I get the following:Display A selected1) 4.951V2) 0.078V3) 4.864V (Output to B)8 ) 0.134V (Output to A)9) 4.375V10) 4.949VDisplay B selected1) 0.8862) 4.3753) 4.866V (Output to B)8 ) 4.868V (Output to A)9) 4.36710)0.422VSo to me, all the pins look right except pin 1, as I think it should have about 4.8V when display B is selected, and not 0.886. So I guess the next step is checking the components connected to pin 1. Maybe the diodes (CR18, CR19, CR20)? I'm not too sure where those are though.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 06, 2021, 04:53:06 am
content: OK make sure both CHOP and ALT are off (ie buttons are out) and then measure the logic levels on U3 pins 1, 2 and 3 again when selecting either Ch A or B.Pin 1 should stay at 0V , pin 2 is controlled by U2a and presumably should toggle with channel selection and thus pin 3 should follow.Channel B display button toggles pin 1 U2a CLR input with +5V or 0V, and thus Channel A toggles the PR input pin 4 the same way when selecting it.Use the component overlay grid reference to find component locations, see image adjacent to page 8-18.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 06, 2021, 05:25:46 am
content: I had made sure in the previous reply that ALT and CHOP were off, but I just checked the U3 voltages again just in case and I'm getting all the same voltage values. Pin 1 is never 0V, which to me makes sense, as both inputs need to be high for the NAND gate to output a high. Pin 2 however does toggle as you said.I just checked U2, and pin 1 is 0V when channel B is selected, and about 5V when channel A is selected (pin 5 has identical behaviour), and the opposite is true for pin 4, which is exactly as you said.I did make an interesting discovery though. I've attached a picture of some diodes. The second from the bottom is CR19. The right half of it has lost its orange colour. Is that possibly a sign of failure? It is the only diode that looks like that. If so, is there a way to check it, or do I just replace it? Resistance across it is 1.675 kOhms one way and 3.253kOhms the other way. If that's not relevant, what else should I check? The problem does look like it's between U2 and U3.I really wish I'd known the overlay existed before... it would've saved lots of time of checking each connection individually for continuity with the multimeter.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 06, 2021, 06:30:46 am
content: Quote from: PurpleAmaranth on May 06, 2021, 05:25:46 amUsing diagram 6 on page 8-18 pin 1 U3 has two connections, firstly via a 10k pull up to 5V and then out to the front panel push switches (ignoring CR19 just for the moment), so following the line that goes to the ALT switch which is shown in the OFF position thus the line continues up to the CHOP switch which on the diagram is shown as ON so imagine turning that switch OFF and then follow the connection, it will go to the momentary TRIG VIEW which shows the connection to be at ground (logic 0). Now with that condition CR19 cathode will be also pulled down and the anode should be at 0.6V. Which should then make U1a pin 1, U4a pin2 and U3c pin 10 also 0.6V (logic 0). Assuming they are not schottkeyYour front panel switches might need a clean !.As for testing the diodes, I assume you are using a DMM ? ! it should have a diode symbol on it indicating a diode test mode that can forward bias the diode junction giving a result showing the approx. forward voltage drop directly and is a much better way to test diodes and BJT's for quick go/no go.Quote from: PurpleAmaranth on May 06, 2021, 05:25:46 amAre you sure your not thinking of an AND function !!.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 06, 2021, 10:54:34 pm
content: Thanks for the reply and all the help so far! And sure enough, I confused NAND and AND gates. I measured everything you just talked about, but I'm not getting the same values. With Display B selected, I get 0.8V on the CR19 cathode (and 0.4V on the anode), 5V on U1 pin 1, 0.4V on U4 pin 2, and 5V on U3 pin 10. And with Display A selected, I get 5V on both terminals of CR19, 5V on U1 pin 1, 5V on U4 pin 2, and 5V on U3 pin 10.Pressing CHOP makes two lines show up on the screen, as does pressing ALT (also, only one of CHOP, display A, display B, and ALT can be selected at a time). This would seem to indicate to me that the switches work, but I could of course be wrong.With the DMM, I tested CR19, and I get 0.238V one way, and 0.566V the other way. Testing CR20, I get 0.570V one way, and 1.115V the other way.And just to compare, I also tested CR12 and CR17, and I get 0.250V one way, and 0.244V the other way. On CR25, 0.525V one way and 0.742V the other way. All these diodes are of the same model (1901-0040), so I was hoping that testing more than one would show what normal bias voltages were, but that doesn't appear to be the case. I'm guessing that must be normal, I don't remember what we learned about diodes in school very well. At least, it looks like none of them are shorted out are open-circuit, although I'm not sure how well they are working.Do you still think it's a switch issue or is there something else I should test?I know I'm repeating myself but thanks again so much for the help!

author: lowimpedance
author_rank: Super Contributor
timestamp: May 07, 2021, 12:45:14 am
content: Quote from: PurpleAmaranth on May 06, 2021, 10:54:34 pmOops I meant U1 pin 2.Looks like the CHOP and ALT functions are been correctly enabled , except the B channel vertical !.With the power off using your DMM low ohms range can you measure the resistance from U3 pin 1 to 0V and confirm its connected.Measuring the diodes in cct (no power !!) using the DMM diode function may give odd readings depending on the other component connections but usually you would get a reading when forward biasing and open (no reading) the reverse. Seems to be not the case here though.Ok I opened up my 1740 so hopefully we can compare actual hardware to be sure. Now I have discovered that my initial assumption of the TRIG view was incorrect so ignore that part including having CR19 pulled low as a consequence sorry about the red herring.Starting again!....Now I have measured U3a pin1 to be at 5V for all selections of display A or B and either CHOP or ALT (excepting the possibility of pulse wave forms that I cannot see on my DMM !, most likely in CHOP etc).And U3 pin 2 toggles low when selecting Ch A and high when selecting Ch B, and of course pin 3 follows pin 2. Thus selecting B channel in the hybrid amp.U1a pin 1 stays at 5V for either Ch A or B but U1 pin 2 toggles low for Ch B and high for A.CR20 has 5V either side for either Ch A or B. CR19 anode is 0.3V for Ch B selected and 5V for A.What happens again on your scope ?Is A versus B OFF in the Horizontal button area ?.is R108 and R97 10K ?.So why does pin 1 go low on your scope for Ch B... !!....hmm

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 07, 2021, 01:45:35 am
content: Alright, so I made a bunch of measurements!Resistance from U3 pin 1 to 0V: 6.50kΩU3 pin 1: 5V with Display A selected, 0.9V with Display B selected, 2.9V with ALT, 1.1V with CHOPU3 pin 2: 0.1V with Display A selected, 4.4V with Display B selected, 2.2V with ALT, 2V with CHOPU3 pin 3: 4.9V with Display A selected, 4.9V with Display B selected, 4.9V with ALT, 4.8V with CHOPU1 pin 1: 5V with Display A selected, 5V with Display B selected, 5V with ALT, 0V with CHOPU1 pin 2: 5V with Display A selected, 0.4V with Display B selected, 2.7V with ALT, 0.9V with CHOPCR20 anode: 5V with Display A selected, 0.9V with Display B selected, 2.9V with ALT, 1.1V with CHOPCR20 cathode: 4.9V with Display A selected, 4.9V with Display B selected, 4.9V with ALT, 4.9V with CHOPCR19 anode: 5V with Display A selected, 0.4V with Display B selected, 2.7V with ALT, 0.9V with CHOPCR19 cathode: 5V with Display A selected, 0.9V with Display B selected, 2.9V with ALT, 1.1V with CHOPA vs B is OFF. Only MAIN is ON.R108: 6.23kΩ one way, 6.40kΩ the other wayR97: 6.33 kΩ, 5.73kΩ the other wayCould these wrong resistor values be causing the problems? Also, out of curiosity, are those fluctuations in resistance values reading one way vs the other way normal? I know that theoretically they should be the exact same, but of course, that's only theory... And about the diodes, which show a reading in both directions, does that mean they're Zener diodes? We never actually touched a circuit board in school, just books! So it's pretty cool seeing these principles in action.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 07, 2021, 03:56:30 am
content: The diodes are not zeners, would use a different schematic symbol. I'm sure you could find a nice symbol chart somewhere on line.The wrong resistor values do hint at something is not right in this area and my best guess at this point would be that CR19 has gone 'leaky',replace with a 1n4148 or 1n914 etc generic type small signal diode will do. Do measurements again after replacement.Also get yourself a copy of the HP part number cross reference PDF, most handy finding actual part numbers for the HP coded parts!.search up....,Transistor - Diode Cross Reference - H.P. Part Numbers to ...

author: lowimpedance
author_rank: Super Contributor
timestamp: May 07, 2021, 04:04:37 am
content: Just a thought, try lifting the anode leg of CR19 first and see if you can enable Ch B (don't bother with CHOP or ALT).I assume you have an iron!. What brand of DMM are you using ?. Basic LAB tools etcAlso as a side note when I powered up my old 1740 I noticed the buttons and particularly the timebase switches needed quite a bit of 'exercising' to work of some contact oxidation after sitting for many years collecting dust.So yes all your switches will need some cleaning /lube if you intend to use this CRO regularly.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 07, 2021, 04:17:28 am
content: Ah true, I'd forgotten about the special Zener symbol. I'll try lifting the anode leg of CR19 and possibly replacing it tomorrow. I do have an iron. DMM brand is MAXIMUM, it's Canadian, probably doesn't exist elsewhere. I found the cross reference PDF yesterday but I'll make sure to save it. I am wondering though, how would CR19 being leaky lead to wrong resistor values?

author: lowimpedance
author_rank: Super Contributor
timestamp: May 07, 2021, 04:38:05 am
content: R97 and 108 would have a leakage path to each other on that connection side and the other terminals are common to the 5V rail etc. effectively creating a sort of parallel connection but a bit more complex that a straight short.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 09, 2021, 05:52:11 pm
content: Finally got around to replacing CR19, and it works! Both channels now work, which is great! Thanks for all the help with that. Now the other issue was the flickering when I set Time/Div to anything greater than .2 ms, every lower Time/Div setting works. Do you have any idea what that could be?And a probably dumb question: the scope came with two probes, but only one of them came with the "grabber" on it, the other one just has the pointy end. Is it possible to buyouts the grabber, are they a standard size?

author: lowimpedance
author_rank: Super Contributor
timestamp: May 09, 2021, 11:44:15 pm
content: Quote from: PurpleAmaranth on May 09, 2021, 05:52:11 pmAs I noted earlier the problem is quite likely to be mechanical as the whole time base switch arrangement in these series scopes are not the best design mechanically speaking. The grease/lube ages rather poorly (Don't we all !) and basically gums up and prevents good contact operation. You will be well served to strip down the rotary switches/PCB's and clean and re lubricate with suitable contact lube, (many choices here so search the forum for opinions aplenty). Just remember to document everything before starting to make sure it all goes back with correct alignment/axial position etc.Next would be use contact cleaner on the push button switches and exercise them at the same time, (Only apply enough spray into the switch to do the job).Quote from: PurpleAmaranth on May 09, 2021, 05:52:11 pmAssuming they are the original HP probes your best bet would be second/third hand on ebay etc, other wise buy a new 100Mhz probe with similar capacitance so it will properly compensate on the 1740.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 11, 2021, 02:26:30 am
content: Alright, so I took the whole Time/Div switch assembly apart today, which as you said, was not a particularly fun job. I sprayed the contacts down a couple times with Deoxit, made sure it all looked good, put it back together, and... it still flickers when Time/Div is greater than .2 ms. Also, when I turn it even more, it looks like the trace is "sweeping" on the screen from left to right. This happens on both channels. Any other ideas as to what to try next?

author: lowimpedance
author_rank: Super Contributor
timestamp: May 11, 2021, 05:14:43 am
content: When you say "flickers" , do you mean intensity over a scan (trace movement from L to R) ?. Would be easier to give suggestions if you were able to post a quick video even a few pictures.Does carefully flexing the PCB's associated with the time base make any difference to the trace as well as switching the time base selection push buttons ?.Also the alignment of the time base switch sections is critical to correct operation, just gentle side ways movement without actually changing time base speed on my 1740 is enough to upset the sweep particularly on the slower settings.The delayed time base is not selected etc I assume , so just the main only.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 11, 2021, 05:46:40 am
content: Here's a video of the issue:I tried rotating the knob very slowly from one setting to the next, but that doesn't change the flickering. Pressing on any button doesn't seem to change anything either. However, pushing/flexing the top of the rear timebase PCB (Assembly A8) towards the rear of the scope does stop the flickering up to 2mSEC. When I increase it past 2mSEC, it still flickers, but the more I flex the PCB, the better it gets, although I'm afraid of flexing too much and breaking it...Does that mean the the contacts aren't 'making contact' properly? If so, is that even fixable?Thanks!

author: lowimpedance
author_rank: Super Contributor
timestamp: May 12, 2021, 01:44:18 am
content: What actual setup of the timebase switches and buttons are being used (pictures!, could be handy). The brightness of the screen seems rather excessive, is that just the effect of the videoing ? or is the intensity far to high !?. It very much reminds me of the storage version of the 1740, the HP 1741 !. We are taking about a 1740 ?.So PCB assembly A8 seems to have an effect. Maybe the collar needs moving up a little to give a bit more compression of the switch contacts. Also check the connector XA7 for dry joints or poor contacts, this also means checking the main PCB all the timebase PCB's plug into.I still suspect a mechanical issue rather than electronic failure given the effect of A8 which is not very well fixed in place and just kinda hangs there.These types of issues can be hard to diagnose without the hardware in front of you though, so my best guess is concentrate around the A8 PCB and interface connector.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 12, 2021, 04:00:00 am
content: I've attached a picture of the front, hopefully that helps. It's a 1740 for sure, and brightness is much less than it looks like in the video!I tried moving the collars really tightly to the switch plates, but that didn't change anything. I also tried pressing the two plastic switch plates together as hard as I could with my hands, trying to apply uniform pressure all around, but that didn't change the flickering either. I therefore assume it's likely not the switch contacts? Might of course be wrong though.I checked for continuity from each the 10 connector solder points on A7 to each of those on A8, and I get continuity for each of them. I looked at the solder joints I could see and they look fine to me, but I can't see the bottom 3 on A7 as they're too low. To look at them, I'd have to take out A7, which looks tedious to do... It sounds odd that it'd be the connector because it works on Time/Div lower than .5 mSec/Div. But at the same time, since flexing the PCB improves it, it has to be the connector or the switch contacts right?One other thing, which I don't particularly care about but I figure I should mention just in case it's related: Occasionally, and seemingly randomly the green background will "turn off" and only the trace stays on. Sometimes it comes back on its own, sometimes I have to turn it off/shake it around a bit, and sometimes I just have to let it rest. Not a big deal since the trace is still there though.Do you think I should take out A7 to inspect the solder joints? Is there any other way to check them or anything else to check first?Thanks!EDIT: I noticed that the red RESET light also flashes when the screen flashes, if that helps

author: lowimpedance
author_rank: Super Contributor
timestamp: May 12, 2021, 04:32:55 am
content: Make sure the delayed timebase switch is in the off position, outer dark brown ring, ie full clockwise so there's no bright up trace on the main sweep.I noticed that the blue screen filter is missing which can also explain why the video looks too bright.As annoying as it will be A7 might have to be removed just to be sure. However a good cleaning of all the interface connectors with deoxit would be the first step.On my 1740 I have just found the connector on A11, (just behind A8), to be dodgy as a slight lateral movement will cause the sweep to collapse to the right hand side last 1/4 of the screen!.So check all assemblies connections before moving onto the PCB's, scanning all over for any suspicious joints or corroded component legs etc.Be mindful when working on the scope powered up for any shock hazards.As for the bright green screen ,that is related to the scale illumination which is probably a noisy pot or issue in that cct. (your picture shows the control set approx. half way).to get rid off that for the moment turn the scale illumination fully CCW.Add some photos of the time base PCB assemblies installed in the scope and of the switches etc. Might be able to spot something else.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 13, 2021, 04:09:19 am
content: I didn't know about the blue screen filter. I found some pretty cheap ones online, so if this flickering ends up getting fixed, I'll order one.Turning the scale illumination fully CCW makes the background "grey" exactly like what I was describing happened sometimes for no apparent reason. Interestingly, turning it fully CW makes the pot rotate as well, and the terminal attached to the yellow wire touches the CRT housing and makes it really bright.Anyway... I removed A8 and inspected it all and it looks all fine to me, although I've attached a whole lot of pictures of it both removed and back in, as well as some pictures of the back of A7, and close-ups of the interface connector pin solder joints. I sprayed Deoxit in the interface connector twice before reconnecting and putting it all back together, but that hasn't changed anything unfortunately. I am wondering though how it's possible for the connector to be the issue, given there is no flickering for timebase selections smaller than .5 mSec/Div. Would that not instead indicate a problem with the switch? Although as I said, pressing on the switch plates changes nothing, only flexing the PCB eliminates the flickering.Do you see anything suspicious in the pictures? Do you think I should remove A7 to inspect those solder joints (and post some pictures)?EDIT: Here's a link to the pictures:https://ibb.co/album/c6PnLh

author: lowimpedance
author_rank: Super Contributor
timestamp: May 13, 2021, 10:20:03 am
content: Those PCB connectors were never a good choice , not even gold plated, so I always make 100 % sure they are clean.As my 1740 showed with A8 causing trace collapse at switch on until the unit warmed up and the initial indication was the time base switch with with operation seemed to fix it!.Any way your thoughts are heading in the right direction I think. Looking at the switch contacts on the photos where you can see some of the PCB gold plated contact it does look rather dirty , not sure if that's an artifact of the photo taking or its real!. I would take the switch housing of the PCB for another photo of the gold switch traces before giving it a good clean with a cotton tip .There is one other possible failure that I have encountered and that is component legs can impart some stress into the component giving internal intermittent connection which you overcome with the flexing. Although I would start at the switch first. Given you can stop the flickering with mechanical movement there is a bad connection somewhere.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 13, 2021, 10:29:52 am
content: One last thought , don't discount a dry joint particularly around components that get warm to hot as that thermal cycling over the age of this unit could easily introduce those and the joints still might look good but are not. Real close inspection with magnification needed. Unfortunately this is going take some effort top in down the culprit.It will be worth it though as these are nice vintage CRO's with a crisp bright display.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 14, 2021, 02:54:12 am
content: So I took apart the switch to look at the gold plating (which I had already sprayed with Deoxit before). Here are some pictures:https://ibb.co/album/5MXsG9. The gold is very hard to take a picture of, but it all looks really clean. The one thing is that there are two tones of gold, one a shade darker than the other, and this darker shade is not totally uniform, and seems to fade a bit in some areas in the counterclockwise direction, but the lighter shade which is present still looks very good.I looked at each of the components' solder joints with a magnifying glass, and they all look perfect as far as I can tell. I did attach a picture to this post, and as you will see, part of one of the light green PCB lines (I don't know what they're called) appears to have what looks like solder or something of that colour on it, is that normal?I haven't put it back together yet in case there's anything else to take a look at in particular. Is there any other way of testing apart from visual inspection, I don't seem to see anything visually wrong, although I'm hoping the pictures can tell you something!Thanks!

author: lowimpedance
author_rank: Super Contributor
timestamp: May 14, 2021, 06:34:52 am
content: Quote from: PurpleAmaranth on May 14, 2021, 02:54:12 amFrom the photo's at least the gold contacts appear dirty or worn looking at the darker 'tone' which is where the wiper contact slides. In the end its too hard to tell actual condition from the pic's. However assuming the photo's are representative of condition they certainly need a good cleaning. I have used a soft white pencil rubber to gently remove crud from gold contact surfaces on PCB's with no issues, make sure its not one of those combo PEN/PENCIL types but a PENCIL only. After that clean with IPA and re-lube the contact surfaces again. Don't forget the contact springs on the black rotors (be careful not to flick one out of its position, as it will likely disappear on the floor forever.Quote from: PurpleAmaranth on May 14, 2021, 02:54:12 amThat trace on the left half way looks odd , check for continuity, also give it a swipe with the iron and a small amount of fresh solder to be sure proper bridging of any corrosion induced breaks.Also be mindful that although the joints look okay that there still could be a component leg that is 'loose' in the middle of an otherwise acceptable solder fillet. I had one such type of joint in an old fluke voltmeter that had an intermittent fault caused by such a bad joint !. Could only see it by visual inspection and moving the component up and down through the solder joint and observing the lead moving !.I will say though that this type of failure in not common, usually a dry joint is fairly obvious.Quote from: PurpleAmaranth on May 14, 2021, 02:54:12 amWell after cleaning the switch contacts and cleaning up that suspect trace as well as being happy that all components on that PCB are properly soldered its re assemble time then back to trying to track down the poor/intermittent connection, (if still present!), by carefully prodding and flexing sections of the PCB/components and surrounds with a plastic 'stick' etc. trying to isolate the issue area .

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 14, 2021, 06:35:56 pm
content: Hello, I'm just lurking on this topic because I am working on a HP 1727A that has a couple of problems, you might have seen the topic for it on this forum.Although the 1740 is a bit different than mine I am still interested in how it turns out!

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 15, 2021, 04:13:59 am
content: So I took the switch plates off again, and rubbed the gold plating and the little metal springs on the black rotors with IPA until they looked perfectly clean, and then a few more times for good measure. That does appear to have gotten rid of the vast majority of the "two-toneness" and made it a much more uniform gold colour. (It does looks like there were some very light scratches now that it's clean, but nothing major). The q-tips did get black, so I guess it needed it! I lubed it back up, put it back together, tried it again... and it's still flickering. So I assume it's not the switch that's the problem?I checked the suspicious-looking trace, and there is continuity. Do you still think I should add some solder to it anyway?I looked carefully at all the components again, and C11 (on board A8) looks a bit odd. I've attached a picture of one of its legs, it looks like a "drip" of metal on it. And although this isn't really visible in the picture, the leg has turned a hue of purple very near the connection to the green side of the capacitor (same side as the drip). And a last thing is that it wiggles when I try to roll it move it back and forth, significantly more than C18 which looks very similar. Does any of this sound suspicious? Should I take A8 back out and test C11's capacitance using the DMM? Will that even work if it's connected to the rest of the circuit?Otherwise, what else is left to do? Prodding with a plastic rod? Taking out A7 to inspect the solder joints?And Alex, I've actually been lurking on your thread as well! I hope you manage to fix the problems you have, you sound like you know lots more than I do! The 1727A looks very similar to mine, and I believe it's just the storage version. Even the inside looks very familiar.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 15, 2021, 06:18:31 pm
content: I realized I can access C11 without taking out A8, so I tested the capacitance and I get 102μF, so looks like it's not C11. I guess the next step is carefully prodding with a plastic rod to try to isolate the issue

author: lowimpedance
author_rank: Super Contributor
timestamp: May 16, 2021, 02:16:43 am
content: While that may seem tedious, its one of the tried and tested methods for isolating mechanicaly introduced failures. And if temperature related then add heating and freezing to the mix.As it is affected by flexing the PCB assembly that's currently the best avenue of isolating the area.Besides a plastic rod such as the outside barrel of an old ball point pen etc is better than poking fingers around the live parts.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 16, 2021, 07:29:18 am
content: Well... I spent some more time on it and although I've narrowed the problem down, I'm now very confused. I set the Time/Div to 2mSEC, which is where the flickering starts to happen. By flexing the PCB in many different directions, I figured out that the problem came from the lower right quarter of the PCB (A8) when looking from the front of the scope.I also found that the flickering stopped when that area was pulled towards to the front, not pushed towards the back. So I took an old pen and a thicker wooden rod and first tried to press on each component in that lower right area, but that never stopped the flickering. I then tried pushing each component side to side, but that also did nothing. So I turned my attention to the switch, even though I was fairly confident it wasn't the switch given the time spent on it already.I tried pressing the front plate of the switch in the lower right quarter area into the board, but that changed nothing. So then, although not expecting anything interesting to happen, I tried lifting the lower right quarter of the rear switch plate up a bit (between the two pin-like metal structures that act as stoppers) with a fingernail, and somehow that eliminated the flickering.I tried it again, many times and every time, the trace stabilized.I made sure the PCB wasn't bending at all, and then tried to figure out how this was possible. My first thought was that maybe lifting the bottom right of the rear plate caused the top left of the front plate to press down harder, but pressing on that had no effect. So I took off the rear plate and checked that all the springs were making contact, and they all seem to be nice and springy so I don't think that's a problem. I did notice however that the area I'd been lifting was the area with the springs. I put it back together, but left the C-clip off, as well as the round stoppers that go on the switch axle.From there, I tried varying the pressure, and I found that nothing I do on the front plate has any effect. But the rear plate can have its area opposite the contacts completely off the board, and its area with contacts just slightly on, and it works, but if I press down a bit more, it stops working. I found that there's a very specific amount of pressure that works optimally. Not enough and I get a dot on the screen, too much and I get the flickering.I'll spend some more time on it tomorrow in the hopes of finding something notable, but do you have any idea how it's possible that lessening the pressure on the contacts would eliminate the flickering? That sounds very counterintuitive to me. Anything you'd recommend trying in particular to further isolate the problem? It definitely appears to be in the switch.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 16, 2021, 09:53:04 am
content: Random thought, what about a trace fracture inside the switch area or at one of the via's. As to the contact pressure.Did you get a sense that the PCB gold contacts were heavily worn, can't really tell from the picture.

author: Brakeboard
author_rank: Newbie
timestamp: May 17, 2021, 07:57:35 am
content: Hi guys, Been loitering on this thread. I have a 1741A, the same as the 1740A but with storage.I have no Vertical or position function on my scope. It's so clean inside it seems worth salvaging. Thought I will be lucky and find a simple power supply issue. just like David's repair vlog: EEVblog #803But alas I've narrowed it down to the custom HP A3A1 amplifier IC. An unobtainium part it seems. Is there any way to fix this? Or do I need a parts machine to resolve the issue?Any thought would be appreciated.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 17, 2021, 09:47:52 am
content: Are the DC voltages at its outputs balanced?. Note that these voltages are set further up the chain by resistors, (don't have the manual in front of me currently, so can't give part no.s ATM.).I think I mentioned about this in the other thread noted in my second post here.Upshot is check the resistors for drift ,usually high and can easily upset DC bias conditions.This could make the hybrid A3A1 seem dead is no signal displayed with an input applied.I had a 1741 with this exact fault, fixed the bias and bingo both channel's working again.Of course if the hybrid amp is toast then a parts unit is the only solution. Hopefully its not toast !.EDIT : check R1-R5 on the vertical O/P board A5 for incorrect values to start.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 19, 2021, 12:43:10 am
content: Not sure if this is relevant or not to the flickering problem, but I found a solution to an intermittent issue on my scope by cleaning the gold contacts on the PCB rotary switches. They were covered in a layer of grease that over time hardened into a nasty tacky paste. I am not sure if this is true of your scope but it might be worth a look, a light rub with acetone and a cloth cleaned mine right up.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 21, 2021, 12:32:31 am
content: It's been a while since my last reply... things got busy! I havent come up with much more since then... If I remove the C-clip that hold the two switch plates together, and then move the rear switch plate totally outwards such that no contact is made, there is very rapid flickering that is at the limit of what my eyes can perceive, so looks more like rapid dimming and brightening. As I move the rear plate inwards, I get a slightly thinner line, and a stable display, and then if I press it up against the board (where it should be), the usual flicker is back.So I move the rear plate slightly outwards again, and by varying the pressure in different areas (in particular pulling the area opposite the contacts outwards), I get a stable display. If I then hold the switch in this position relative to the board and move the A8 PCB and the switch assembly jointly in one direction or another, the display remains stable. It therefore seems that the problem is not with a PCB component or with a trace fracture within the PCB, but rather with the switch itself.Given that nudging the front plate in any way has no effect on the flickering, I suppose the issue is with the rear plate or the rear gold contacts. So I tried bending the little springs outwards just in case they weren't making good enough contact from having flattened over time. But that changed nothing. The assembly looks perfectly clean. But I am in the process of scrubbing it all with IPA and then I will lube it up and reinstall it. However, given I've already done this, I don't expect much.It's hard for me to tell the condition of the gold contacts. They look nice and shiny and golden, but when holding the PCB at a certain angle to the light, I do get reflections akin to a grey/black 'mosaic'. And it does look like there are some circular scratches. Having not seen good condition contacts, I really do not know. But they're definitely not in particularly bad condition. I'll try reassembling after rubbing with some more IPA. But if it still doesn't solve the issue, I really don't know what to try next. Deoxit makes a product called Deoxit Gold which they claim chemically removes any oxidation on gold, so I may try getting some of that as a last resort.Any other ideas?And Alex, I'm a little wary of using acetone on a PCB, are you sure it's alright to use?

author: lowimpedance
author_rank: Super Contributor
timestamp: May 21, 2021, 12:47:58 am
content: Unfortunately its getting to the point where you will need access to another oscilloscope to monitor the waveforms as shown in the manual etc to see what happens to them when the issue occurs.Its good to be wary of acetone, If you really had to then small amount on a cotton tip and apply carefully at the problem spot only. I have never needed to use it on any electronic (ie PCB ) assemblies myself, of course metal mechanical stuff OK as required.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 21, 2021, 12:58:53 am
content: I used acetone because it's what I had, rubbing or isopropyl alcohol would probably be better, but for the 5-10 seconds that any acetone would be in contact with the PCB I doubt it would really be much of a problem. There is no silkscreen you need to worry about wiping off either (on the 1727 at least).

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 22, 2021, 01:22:13 am
content: Put it all back together, and it's still flickering. I'm out of ideas. I don't know anyone with an oscilloscope that I could borrow. If I manage to get one somehow, I'll post again. Thanks for the help so far!

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 23, 2021, 04:08:20 am
content: Well, looks like I'll need more help anyway. I was playing around some more with the switch assembly on A8, gently prying with a screwdriver, and with the 1V cal connected to channel B, in the hopes of finding, well, literally anything. That probably wasn't smart given a screwdriver is metal. But it was going fine, and then suddenly the trace disappeared. And now no matter what I do, I can't get a trace on the screen. There's nothing that shows except the green background. I don't know if it was my faut or just a badly-timed coincidence...What should I try first?Edit: I see no visible damage, and there were no sparks when it happened, so I'm leaning the coincidence route, but I don't actually know of courseEdit #2: I should've mentioned. I get no trace on either channel.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 23, 2021, 07:53:46 pm
content: check the +120V power rail, if the rail is above or below a certain threshold the HV is disabled, causing no image. If +120 is ok, then I don't know, just an easy thing to check.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 23, 2021, 11:29:58 pm
content: The 120V is not on A8 PCB, but easy to test anyway and while checking voltage rails also measure the +/-15V on A8 (C19 and 21 pos and neg respectively wrt TP3).If all present then its quite possible one of the transistors has popped, use your DMM diode mode to check for junction shorts etc.Symptom sounds like the main sweep trigger is not happening.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 24, 2021, 01:20:43 am
content: I tested the voltage rails. All are fine. I just checked the +/- 15V on A8 and those are fine too. So I started checking the transistors by following the instructions here:https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07.On Q12, I get: BE 0.75V, BC 0.75V, EB 2.657V, CB OL, CE OLQ11: BE 0.755V, BC 0.753V, EB 2.776V, CB 1.258V, CE 1.04VQ10: BE 0.701V, BC 0.678V, EB OL, CB 1.377V, CE 0.805VQ9: BE 0.752V, BC 0.750V, EB 2.353V, CB 1.917V, CE 0.707VQ3: BE 0.332V, BC 2.407V, EB 0.333V, CB 0.719V, CE 0.809VI didn't test the others because they aren't labelled EBC, is the order of leads still EBC for all of them? Do any of those values look suspicious to you? They don't seem to correspond to my link but I suspect that's because they are still connected on the PCB.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 24, 2021, 04:51:12 am
content: The values seem to be reasonable given there may be interconnections and other components causing odd readings, like other semiconductor junctions , low value resistors and largish capacitors which charge up when your probes are connected (usually noted as slowly drifting value change on DMM).The other transistors may follow but Q7 does not !. Note the small 'dot' in the component side copper next to each transistor, that is the Emitter. Use that as a guide to work out what the others are by cross referencing to the schematic.Next steps would be to measure the voltages on the transistor leads and compare with the values noted on the schematic, investigate any major differences.Double check the front panel button selections for auto trigger / main sweep etc. Does beam find button result in any bright dot/trace on screen ?, Is the RESET lamp on or off?.In NORM mode the RESET lamp is on when waiting for a trigger event and during the trace sweep (when triggered) and will go off during the retrace and re arming of the trigger cct. Randomly rotating the TRIGGER level pot below the reset lamp will cause a trigger to happen and a sweep (RESET lamp should blink).If you select NORM mode sweep and EXT. trigger in then hold a piece of bare wire to the EXT trigger input BNC centre pin you should be able to trigger a sweep.BTW as you can still get the scale illumination (green background) the HV is OK. as this CRO uses a floodgun in the CRT for the even back ground glow as opposed to using a row of lamps above and/or below the front of the tube. Although in most normal situations that is generally turned off, so no green background.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 24, 2021, 08:20:07 pm
content: Interesting about the Emitter corresponding to that dot, I didn't know that.So I measured the rest, and here's what I'm getting:Q8: BE 2.186V, BC 1.447V, EB 0.717V, CB 0.707V, CE 2.075VQ7: BE 0.567V, BC 1.079V, EB 0.567V, CB 0.722V, CE 0.818VQ6: BE 0.601V, BC 0.625V, EB 0.997V, CB 0.961V, CE 1.396VQ5:Q4: BE 2.031V, BC 1.446V, EB 0.718V, CB 0.710V, CE 1.899VQ2: BE 1.922V, BC 0.534V, EB 0.674V, CB 0.532V, CE 1.977VQ1: BE OL, BC OL, EB 0.724V, CB 0.717V, CE 1.901VQ5 is apparently a JFET, which I’ve never heard of before, but looks like it’s similar to a MOSFET, I’ll have to read more into it. I tested that using the resistance setting, although I’m not sure it tells us much… But at least nothing seems shorted out.Q5: GD & GS 6.86MΩ, DG & DS rapidly increasing in the MΩ range, DS & SD about 190ΩDo any of the above values look odd?Pressing BEAM FIND results in a bright dot on the screen.RESET lamp is off.Selecting NORM mode, the RESET lamp is still off, and rotating the TRIGGER level below it does not change anything, the RESET lamp stays off.Selecting NORM and EXT, and putting a piece of bare wire into the EXT trigger input BNC centre pin changes nothing, RESET lamp stays off, and screen stays blank (only green background).Good to know for the HV!I’ll measure the transistor voltages and post again once I’ve got them.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 24, 2021, 09:41:53 pm
content: Alright, so this took a little while, but here are the transistor voltages, with the voltages that they're supposed to be according to the schematics in brackets, I put ... when no voltage was indicated.Q1: E 1.699V (1.7V), B 1.32V (1.4V), C 0V (0V)Q2: E 1.699V (1.7V), B 0.98V (1V), C 0.89V (0.7V)Q3: E 0.89V (0.7V), B 0V (0V), C 0.862V (1V)Q4: E 7.58V (…), B 14.15V (1V), C 0V (0V)Q5: G 0.863V (1V), S 14.66V (…), D 2.256V (0.8V)Q6: E 0V (0V), B 2.256V (0.8V), C 14.14V (1V)Q7: E 14.17V (13.3V), B 13.35V (12.7V), C 14.14V (1V)Q8: E 13.64V (1.5V), B 14.15V (1V), C 0V (0V)Q9: E 12.92V (1V), B 13.63V (1.5V), C 12.90V (14.6V)Q10: E 5.193V (3.5V), B 5.88V (0.5V), C 12.93V (14.6V)Q11: E 11.64V (0.2V), B 12.31V (0V), C 14.82V (15V)Q12: E 1.072V (0.8V), B1.477V (1.5V), C 5.193V (3.5V)Q13: E -9.66V (-9.7V), B -9.13V (-9.2V), C 0.86V (1V)As you can see, many of the transistors have very wrong values. Any ideas what could be wrong?

author: lowimpedance
author_rank: Super Contributor
timestamp: May 25, 2021, 01:02:19 am
content: Certainly looks like something has gone 'pop' !.Did you set up the controls as per step 1 and 2 on page 8-26 of the manual before measuring ? , repeat if not.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 25, 2021, 04:07:46 am
content: Yup I set them up exactly as indicated.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 25, 2021, 04:34:30 am
content: Looking at the voltages I have a suspicion that Q6 is toast and I cant find a cross reference to a recognizable part numberMaybe email Sphere electronics and ask if they have one using the HP part number 1854-0019, otherwise you may have to make an educated guess at a replacement, but which specs were important.carefully remove Q6 and bodge in a generic NPN, 2n2222 etc , watching out for correct orientation and see if any difference in voltages at least.Whats your level of experience with electronics, new/hobby etc looking to set up a small collection of gear ?. Maybe look at a digital oscilloscope. Plenty of discussion on the forum for the hobby end, RIGOL / SIGLENT etcThese old boat anchors can take some effort to get running and maintain which of course is a great way to learn but a balance needs to be found between learningand getting on with other projects.Not to say this unit is a goner yet !. Perhaps another parts unit from this series might turn up and using bits from each resurrect a good one. I have resorted to this method myself

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 25, 2021, 05:54:43 am
content: I don't have any transistors at all, so I ordered some 2N2222 which I'll go pick up tomorrow, I'll let you know the voltage measurements with that replaced. I'll then email Sphere asking if they have a replacement for 1854-0019.Quote from: lowimpedance on May 25, 2021, 04:34:30 amI don't have much experience at all, really new to all this. But I'm studying electrical engineering, so I figure I really need to get acquainted with the practical side of things. I've read lots of good stuff about some RIGOL models, and I'm sure the same is true for SIGLENT. But although they're 'only' about $400 or so, I really can't afford that at this stage.I'm really enjoying working on this a lot so far, although you've (thankfully!) done most of the actual analysis. But I've learned lots already, from replacing that diode to reading circuit diagrams.I really hope we manage to fix it.I have been looking for parts units but haven't found much yet that's not more than triple what I paid for this one.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 29, 2021, 06:25:17 pm
content: Alright, so things got busy and I didn't get to replacing Q6 with a 2N222 until today. But now it's in, with a pretty questionable soldering job... but at least it's in.Measuring the voltages yields the following:Q1: E 1.652V (1.7V), B 1.316V (1.4V), C 0V (0V)Q2: E 1.652V (1.7V), B 0.97V (1V), C 0.643V (0.7V)Q3: E 0.643V (0.7V), B 0V (0V), C -0.738V (1V)Q4: E 1.601V (…), B 0.89V (1V), C 0V (0V)Q5: G -0.739V (1V), S 14.67V (…), D 0.684V (0.8V)Q6: E 0V (0V), B 0.685V (0.8V), C 0.887V (1V)Q7: E 13.42V (13.3V), B 12.69V (12.7V), C 0.889V (1V)Q8: E 1.586V (1.5V), B 0.889V (1V), C 0V (0V)Q9: E 0.95V (1V), B 1.584V (1.5V), C 14.61V (14.6V)Q10: E 3.994 (3.5V), B 0.434V (0.5V), C 14.61V (14.6V)Q11: E 0.164V (0.2V), B 0.382V (0V), C 14.84V (15V)Q12: E 0.802V (0.8V), B 1.473V (1.5V), C 3.993V (3.5V)Q13: E -9.68V (-9.7V), B -9.13V (-9.2V), C -0.74V (1V)Thee almost all look much better to meTesting the old transistor out of the circuit with the DMM yields the following:EB OL, CB OL, BE OL, BC 0.63V, CE OL, EC OLLooks like the Emitter-Base junction is 'dead', right?There's still no trace on the screen, but the RESET light is now ON when the settings are selected as per the A8 schematic section of the manual.I'll email Sphere to see if they have a proper replacement for Q6. Anything else to check or do? Or just try to find a suitable replacement for Q6 for now, and then see where that takes us?

author: lowimpedance
author_rank: Super Contributor
timestamp: May 31, 2021, 01:36:44 am
content: Q6 is toast, Having a further look I find some mention that its specs are similar to the 2n2369 which is also obsolete but that gives a starting point to choose an alternative.See what Sphere recommend but remember they may be rather pricey !. Otherwise look up the 2n2369 and get a similar spec. type. ie, Vceo > 15V (I would aim for 30V and up), approx. 350-500mW PD, an FT~500Mhz and if in a TO-18 package great but I guess a TO-92 will also do.As for the voltage readings things look better, but noted the -ve collector voltage on Q3 is that correct?.Does that voltage change when you are sure you have supplied a trigger input?.Note damage to Q5 may have also occurred which is not as easy to replace (2n5245), may be Sphere can help here as well.

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 31, 2021, 02:40:09 am
content: Alright, I'll take a look at the 2N2369, thanks for that.What do you mean by "when you are sure you have supplied a trigger input"?And yup, -ve collector voltage on Q3 is correct, I measured several times and it's definitely -ve.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 31, 2021, 03:04:47 am
content: An input signal applied and the trigger source selected for that channel and the trigger level such that the trigger generator will trip thus causing a sweep.Although in Auto a free run a sweep should be visible. Analysis would be much easier with another oscilloscope to see what the trigger signal is doing P/OXA7 etc

author: PurpleAmaranth
author_rank: Contributor
timestamp: May 31, 2021, 03:19:27 am
content: Found someone not too far from me selling 2N2369A left over from a project. I'll see if I can get some there.Could Q5 be the cause of the -ve voltage?I see what you mean about the trigger, I'll go check it. I really wish I had another scope to check all this, I'll see if I can borrow one but I seriously doubt it.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 31, 2021, 03:51:20 am
content: I'm not sure the 2369 is the best choice as its Vceo is only 15V which also happens to be the VCC rail !. At this stage I would probably leave the 2222 in for further fault finding.Its possible that Q5 is busted which would prevent the sweep ramp from happening when a trigger signal arrives at Q1 base. Which is why we need to see it with an oscilloscope.Being able to eliminate any failures on PCB A7 would be useful/desirable.I suggest reading the circuit operation details in the manual for further info. Section 4.

author: PurpleAmaranth
author_rank: Contributor
timestamp: June 01, 2021, 09:56:31 pm
content: Not much luck getting anyone to lend me a scope. However, Sphere replied and they have some 1854-0019 that they can send me to replace Q6 with. I haven't looked at section 4 yet though, I'll do it this evening. Is there anything else we can do without a second scope? Should I ask Sphere for a replacement to Q5 too? Should I try desoldering Q5 and checking it with the DMM out of the circuit?

author: lowimpedance
author_rank: Super Contributor
timestamp: June 04, 2021, 12:20:27 am
content: Quote from: PurpleAmaranth on June 01, 2021, 09:56:31 pmOkay I took the cover off my 1740 and measured the gate of Q5 and mine was also negative !?! so don't worry at this stage with Q5 we'll assume its still ok.Next thing to try is select single sweep at say 1/2 second TB speed and press reset (LED ON). Connect your DMM to R1 on A8 (which is the Trigger in from A7) and you should read approx. 1.3V while measuring trigger a sweep with the trigger level pot (turning back and forth a good amount) to see if the trigger line goes low to around 0V.

author: PurpleAmaranth
author_rank: Contributor
timestamp: June 05, 2021, 03:57:01 am
content: Quote from: lowimpedance on June 04, 2021, 12:20:27 amThat's interesting! I wonder if there's an error in the manual...Anyway, I selected single sweep at .5sec, pressed RESET, and LED turned on. Then on both sides of R1 I get about 1.32V. Triggering a sweep by rotating the trigger level pot, I see the dot on the screen go from left to right, and both sides of R1 go down to about 0.03V. And once the sweep is done, we're back at 1.32V, and RESET light is OFF. So looks like Trigger in from A7 is fine. Does that mean the problem is necessarily on A8?

author: lowimpedance
author_rank: Super Contributor
timestamp: June 06, 2021, 11:25:57 pm
content: That looks consistent with mine, next measurement would be to look at the collector of Q6,(TP1), to see if there is a positive going ramp up (DMM should show something, better with slow timebase speed.).I suspect there is as you state you now see a trace scan, a 'dot' is to be expected (there may be a slight tail depending on the brightness setting). What do you see?....Rememver to press reset between each trigger attempt.Quote from: PurpleAmaranth on June 05, 2021, 03:57:01 amThe problem could still be to do with the switch at higher timebase speeds, so if TP1 is good and you see a sweep then the next thing to do is work your way through each timebase setting one at a time from slow upwards till you no longer get a trace (make sure your brightness is set appropriately, ie more as the sweep speed is increased.).

author: PurpleAmaranth
author_rank: Contributor
timestamp: June 07, 2021, 05:27:40 am
content: Alright, so I measured all the DC TP1 voltages, and checked for a sweep. Below are the results, but to summarize quickly, RESET lamp turns ON every time, then OFF once the sweep is triggered, and a trace scan appears on screen for all timebase settings.2 SEC: DMM starts at 0.885V, goes steadily up to between 11 and 12V, then suddenly switches back to 0.885V. Dot was slowly making its way across the screen from left to right while the DMM reading was increasing.1 SEC: Same as for 2 SEC, started/ended at 0.890V.5 SEC: Same, start/end at 0.898V.2 SEC: Same, start/end at 0.918V.1 SEC: Start/end at 0.947V, getting too fast to tell how high DMM reading goes during the sweep, but definitely increasing50 mSEC: Start/end at 0.997V, too fast to even tell if DMM reading is increasing or decreasing, but still changes20 mSEC: Start/end at 0.882V10 mSEC: Start/end at 0.889V5 mSEC: 0.898V2 mSEC: 0.919V1 mSEC: 0.948V.5 mSEC: 1.001V, 'dot' moves so quickly it looks like a line.2 mSEC: 0.881V.1 mSEC: 0.888V, had to turn up the brightness setting50 µSEC: 0.896V20 µSEC: 0.916V10 µSEC: 0.944V, turned brightness up all the way5 µSEC: 0.994V2 µSEC: 0.880V1 µSEC: 0.888V.5 µSEC: 0.897V.2 µSEC: 0.917V.1 µSEC: 0.945V, had to lower background brightness to still see the scan.05 µSEC: 0.997VLooks like the sweep is working, right?

author: lowimpedance
author_rank: Super Contributor
timestamp: June 07, 2021, 06:38:55 am
content: So at faster timebases you see a normal trace as expected ?. There will be a point reached rather quickly where the DMM will not see anything happening at TP1 and then a oscilloscope is needed.It therefore looks to be working, can it display the cal signal ?. What is the current fault ?.

author: PurpleAmaranth
author_rank: Contributor
timestamp: June 08, 2021, 02:29:45 am
content: No I only see the moving 'dot', as if it's scanning, which looks like a line for fast timebases. But as soon as it's done scanning, the screen goes blank (just the green background), whereas before there used to be just a constant horizontal line when no input was applied. Using the CAL as an input does not change anything, nothing is displayed.Also, I should be able to get access to a modern and functional oscilloscope for a couple hours next week; if we still haven't sorted this out with just the DMM.

author: lowimpedance
author_rank: Super Contributor
timestamp: June 08, 2021, 06:17:49 am
content: Is single sweep still set ? put the scope into auto/continuous and apply the cal signal to channel A and trigger source from A, what happens ?.BTW the green background is not needed, scale illumination to minimum, unless that is not possible and also a fault.Its getting rather hard to diagnose remotely. You might need to post another video showing the controls and screen.

author: PurpleAmaranth
author_rank: Contributor
timestamp: June 08, 2021, 07:46:52 pm
content: Well, I feel pretty dumb... It turns out I forgot to put it back in AUTO mode, so it actually works just fine. Looks like it was just Q6 that had to be replaced, sorry for wasting your time with the rest. I'm in the process of ordering an original Q6 replacement from Sphere, but the 2n2222 seems to be working pretty well, the trace looks good! I've attached a picture.So the one thing we have left is the flickering for timebases larger than 1mSEC. You'd said that there wasn't much more we could do with the DMM about that. What should I try looking at with the second scope which I'll be able to use for a couple hours later this week (hopefully) or next week?

author: lowimpedance
author_rank: Super Contributor
timestamp: June 08, 2021, 11:34:54 pm
content: Ahh looks much better, I would certainly use the second oscilloscope to check all the time base circuit test points comparing to the manual. Do this at time base settings that produce a good scan and those that cause flickering. While monitoring those test points use aninsulatedtool to prodd/flex the suspect PCB area's and observe what happens on the 1740 screen and on the monitor CRO. Take pictures with your phone for records etc. Trigger the 1740 on its cal signal BTW. Also use 10:1 probes on the second oscilloscope which have been properly compensated to its input with its own cal signal !.This will be a good learning exercise on using a CRO as well, so look at the 1740 manual circuits for the timebase/trigger area etc and read the relevant manual circuit operating principles before hand and make a note of the test point waveforms you want to measure etc so your ready to go when the other oscilloscope becomes available. Have fun !Once the 1740 is working properly a good clean and it will be as good as new.

author: PurpleAmaranth
author_rank: Contributor
timestamp: June 14, 2021, 04:42:53 pm
content: So apparently what I've been calling flickering is actually perfectly normal and to be expected on any analog scope.I went to the lab to try probing with a second scope, and I was told there was no point at all, and that nothing is wrong with it. So I guess that's good, although I wasted a lot of time on it in that case. But at least I learned a lot. Thanks for all the help! Everything else seems tow work on the scope as well.

author: lowimpedance
author_rank: Super Contributor
timestamp: June 14, 2021, 11:25:54 pm
content: Excellent, I was having some difficulty with the fault description and was thinking along those lines too. Trying to diagnose more esoteric issues remotely can be a challenge!. Nothing beats being able to have hands on.While it seems time might have been wasted, it actually was valuable learning of many aspects of the fault finding/repair process which will useful for all future work, whatever that may be.As for the HP, I would give the Low Voltage Power Supply a one over for dry joints etc particularly around the transformer connections and the bridge rectifiers, generally where you get high thermal cycling and/ or mechanical stress. Also if you ordered a replacement for Q6 put that in too.And lastly a nice clean and a blue screen filter. ...........Then put it to good use !.

author: PurpleAmaranth
author_rank: Contributor
timestamp: January 04, 2022, 06:16:31 am
content: I somehow missed this reply until now, months later. I'll get the blue screen filter, and check for dry joints. I didn't end up buying the replacement for Q6, but it's been working great since June with the 2n2222, so I'll leave it at that for now. Thanks again for all the great help!

author: scopeman
author_rank: Frequent Contributor
timestamp: February 07, 2022, 09:15:40 pm
content: Just a short comment. Enjoy that scope. I have a mint one that was willed to me by a good friend. I was with him when he placedthe order for it with Jack Molcan the local HP Salesman with HP when HP was a real company. I has also bought my first HP calculator an HP 45 (I still have it) from Jack.My friend paid over 2K for the 1740A and Jack delivered it to his house when it came in.I pull it out and use it once in a while. It is still in cal and works like new. Real HP quality. I still have the original shipping box.It's a great scope and a great teaching tool as well.All the best!SamW3OHM