Oscilloscope No Trace old Gould Advance OS3000 - Page 1
Forum section: electronics / testgear

author: TonyPh
author_rank: Regular Contributor
timestamp: March 17, 2014, 10:17:41 am
content: Hi.So I was fiddling around with a very basic 25kHz switching circuit yesterday and using my scope to see what was going on. All was working ok, more or less (more on that later).Now I can't remember exactly the sequence of events but perhaps after I turned the A timebase I lost the trace. I tried everything I know on the front panel with the cal signal and my own signals to get the trace back but I can't see to get it. The scope is triggering as I have a one shot lever that illuminates a neon and it goes off when I feed a signal in, or twiddle the trigger knob.When I switch the power off I get no green flash on the CRT that I used to get.In a darkened room I can see the soft glow from the very back of the tube. No amount of twiddling seems to get any kind of illumination out of the screen and I am just getting what I assume to be the phospor glow from ambient light. This glow is only visible in total darkness.I've had the scope around a year and have had no major issues, although it did take me a while to get a trace the first time because I didn't know how to do it.I am a basic hobbyist who perhaps shouldn't have bought an old scope but I couldn't resist the lower price.I've had a quick probe around inside and to the marked voltages, -15, +15 and +150v and they seem pretty good on the board that sits next to the CRT. Large resistors are getting. The board under a high voltage acrylic sheet also seem to be ok.The screen was very bright and I normally use it quite low down on the intensity scale.So is there any advice that I should be following? Where should I start my investigation? What sort of things should I be looking for?The other slight issue that I've noticed lately is sometimes the trigger seems uneven. I often use a 25khz PWM signal for some of the things I do and the waveform is not even.It looks a bit like this. The signal I'm feeding in is a repeating signal and is normally stable. This seems to happen on one or two time settings and not when I switch into x10 mode. Slower timebases work fine, faster ones seem to be ok. It's just the one or two I use most.Would there perhaps be a link between the two faults.Many thanks.

author: tautech
author_rank: Super Contributor
timestamp: March 17, 2014, 11:28:30 am
content: Have you changed triggering from Auto to Normal, in which case you will not see a waveform unless Trig level is adjusted to trigger.If scope is faulty and you have a manual, normally it describes how to check DC test point values for fault finding with a multi-meter.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 17, 2014, 11:36:04 am
content: Thanks for the reply.Yes I've tried Auto, Normal and Free Run on the triggering side of things. The scope is triggering as far as I can tell. It just doesn't display anything.I do have the manual for it but it really isn't helping that much, or maybe I don't understand it.

author: kc9qvl
author_rank: Regular Contributor
timestamp: March 17, 2014, 11:41:59 am
content: Does turning up the Intensity and pressing the Beam Finder do anything.?

author: TonyPh
author_rank: Regular Contributor
timestamp: March 17, 2014, 11:52:07 am
content: Even in a totally dark room I see no change with a full turn of the intensity from zero to max.I don't think there is a beam finder button/dial on my scope.

author: iDevice
author_rank: Regular Contributor
timestamp: March 17, 2014, 07:53:55 pm
content: Seems to me that you've lost one of the CRT voltages or the post acceleration.The facts that you don't see the glow when switching off tends to supports this assumption, otherwise I would advice to search around the Z modulation circuit (flyback blanking).You should measure the cathode voltage, wehnelt, anode and so on.But the problem is that you need a HV probe as the wehnelt for instance is generally around -1kv and post acceleration at several kilovolts.A standard multimeter is usually specified at 1000v DC, beyond that the protection kicks-in and you will short-cut the point you measure.If it happens when measuring the wehnelt voltage you may destroy the CRT because the wehnelt will become an anode and will melt itself and the cathode if the cathode current is not sufficiently limited, so be cautious.There are ways to check these voltages supposed presence by measuring other LV points of reference, but it asks knowledge of the device and the schematics.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 18, 2014, 10:16:07 am
content: Thanks iDevice I think you might be right as I tried injecting a negative voltage to the Zmod port on the back and got absolutely no change.I'm going to do a little teardown and see if I can't spot anything obvious. I'm a little bit daunted by this but it should be interesting at least. I do like the smell and warmth of the old electronics and the CRT is just lovely to look at so I'd really like to get it working but don't have access to any high voltage test stuff, and cost of HV probes would be more than the scope is worth.Maybe I should start looking at getting a £200 DSO scope.

author: iDevice
author_rank: Regular Contributor
timestamp: March 18, 2014, 10:00:15 pm
content: Don't forget that it could be as obvious as the HV oscillator circuit nor working anymore.The HV transformer usually generates all the voltages needed for the CRT, so if it stops at the primary which works at reasonable voltages you can check with you dmm.Obviously, many times, you need another scope to fix the first one ;-)In this case checking the oscillation would be the first step.Now, many scopes are feeding the CRT heater from a specific winding of the HV transformer, so a good way to check if this a possible cause is to check if the heater is glowing, but unfortunately some other scopes are feeding from the power supply directly, so you need to check the schematics to now in which case you are to determine if the heater check is an indication of HV circuit failure.There are other simple tricks like using a few turns of wire close to the transformer, connected to a ac voltmeter or to a DC but with rectifier, and so on.But really, the schematics should be your first quest as it will show you easily where to measure and more important where it is safe to do without HV probe.Even without any clue, one of the things you can do is check all caps around the CRT circuits, these components are the first things to go bad and particularly tantalum that tend to die by shorting.I've restored a few vintage scopes and I always been very fun projects, so have faith ;-)

author: TonyPh
author_rank: Regular Contributor
timestamp: March 18, 2014, 10:43:40 pm
content: Thanks again for the advice.I've read and understood the power supply and EHT section of the manual a bit more and see that a the unsmoothed +15v tap drives the EHT oscillator while a separate +6.3v tap actually powers the heater. The EHT part is protected by a fuse so maybe it's as simple as that. Will whip the base cover of tomorrow evening and take a look.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 19, 2014, 07:36:19 pm
content: Here are some photos of what I'm looking at.The fuse FS832 was blown as expected. I replaced with another 500mA which also blew quickly. I wonder if this is blowing because of a fault in the EHT or downstream in the PSU. The 20v rail feeding the fuse seems stable enough, altough I've only measured it with a DMM. All other fuses are intact.The PSU section looks fine to me. There is a transistor at a bit of an odd angle but it seems to be clean and making a contact. Not a great deal of electrolytics in there and the ones that are there seem to be ok visually.The bright up boardThe CRT connector, at least I assume it is. I've pushed and twisted but it's not going anywhere.The other side of the caseAnd finally part of the EHT.The 'snot' seems to be all over the voltage multiplier section (at least that's what I think it is after reading the manual) and I'm wondering if it is allowing shorting or something. The snot is fairly firm but can be easily scraped off.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 19, 2014, 08:44:46 pm
content: Here's the rest of the EHT section.The MJE520 B-C-E junctions are 1 diode drop but I am getting 1.1v drop and 2.2kOhm across C-E, but maybe that's because the transistor is still in circuit. I've not really done in circuit testing before and don't know the tricks.

author: iDevice
author_rank: Regular Contributor
timestamp: March 19, 2014, 09:25:10 pm
content: Ok, we are progressing...A blown fuse is good, at least it shows in which direction to search.Now, by looking at your pictures, the animal is not from yesterday, it seems to be from an era before the tantalum nightmare. So it may be something else than a cap.The snot you see on the multiplier circuit is probably candle wax or silicon. Back in the days, candle wax was an often used trick to stop corona on HV circuits. So I guess a previous owner has done that.Anyway, the next step now is to try to find what makes the fuse blow, so the first thing to check is if it is due to a short or simply higher current drain than normal.The best case is obviously the short somewhere. You have to check the resistance between the output side of the fuse and ground. the fuse is .5A and supply 15V, so that means that measured resistance must be higher than 30 Ohms.This is a ballpark figure as your Ohmmeter wont put 15V on the line so you may measure something higher with a low voltage if the some circuits are not polarized.But what's important is that you get a figure like a few Ohms or less then it's good, well it's not but I mean, it indicates a short, so it is easier to find.You then have to trace back to the lowest resistance location by following the tracks.A milliOhmmeter is of course of a great help here. A good dmm usualy has a resolution around 10 mOhms which is enough, a .1 Ohms resolution is more challenging when you approach the target.If it's not as obvious as a few tens of an Ohm short then it's more difficult to track as you have to analyze the schematics and measure many points to see if what you measure makes sense against what you would expect with the circuit values.That said, there are obvious usual suspects in such circuits, like the power transistors or flyback diodes driving the primary taps of the transformer. I would check there first as well as any other transistors. But a short somewhere on a secondary circuit could induce a high current from the oscillator primary circuit as well.Unfortunately, troubleshooting a switching power supply is one of the most difficult task as many things depends on each other. And these HV CRT circuits are actually very similar in that respect.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 19, 2014, 10:24:50 pm
content: Thanks again iDevice.I measure 1.1 Ohm on one DMM and 0.7 Ohm on another. So that's pretty close to a dead short. That's on the EHT side of the fuse as it were. On the PSU side it's around 5k to ground.As an aside I checked one of the main oscillator transistors in the EHT and it was fine, diode drops as expected and no continuity where there should be none.I also repaired a 1N4148 (D833) near the EHT fuse as it looked like it had a blob of solder on it. I now think that someone snipped it at some point to isolate it and then just resoldered the leads together again.Do you think it's worth replacing all the electro caps, including the 4000uF 40v reservoir caps? They don't look bad at all. I've seen worse caps on 2 year old devices. I guess this scope never saw much activity.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 19, 2014, 11:37:48 pm
content: Hmm, no. I've just opened the EHT again and the +20v in there is 2.2kOhm from ground. Went back to the Power supply and its showing the same there. I must have been shorting the pcb to the case. The mounting hole and ground is very close to the fuse holder.Oh well so much for a dead short.Later today, once I get some more 500mA fuses, I'm going to disconnect collector or emitter from TR813 and see if the fuse still blows.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 20, 2014, 05:54:39 pm
content: Ok. Disconnecting TR813 collector stops the fuse from blowing.

author: iDevice
author_rank: Regular Contributor
timestamp: March 20, 2014, 08:29:38 pm
content: Quote from: TonyPh on March 19, 2014, 10:24:50 pmNot necessarily, what happens with age is that they dry out and so loose capacity and increase ESR, so you may see hum in power supply rails or lack of decoupling but they rarely die by shorting or leaking.So it is not of primary concern for fixing the problem. A systematic recap may be considered later.But that doesn't mean they don't have to be checked at least for shorts.If you want to evaluate them without desoldering anything, a garden variety ESR/RLC meter could be handy.

author: iDevice
author_rank: Regular Contributor
timestamp: March 20, 2014, 08:38:03 pm
content: Quote from: TonyPh on March 19, 2014, 11:37:48 pmWell I guess it was too good to be true.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 20, 2014, 08:57:09 pm
content: I've checked C816, C812, C813 and R818 and they are measuring close to nominal with my DMM in capacitance mode. I've got no shorts across the caps. I'm getting megohms basically.I will look into a ESR/RLC meter. I was thinking of building one at some point. Its a pain getting these caps off the board.

author: iDevice
author_rank: Regular Contributor
timestamp: March 20, 2014, 09:09:27 pm
content: Quote from: TonyPh on March 20, 2014, 05:54:39 pmRight, as far as I can decipher the schematic (picture is a bit short on resolution), this transistor seems to be the one regulating the power delivered to the oscillator.It is driven by the regulation feedback loop from R802 and by the EHV setup pot.The actual oscillator is constituted by TR814 and TR815 and the primaries of the transformer.Did you check these two transistors ?Your first step could be to open a side of L801.At this stage you would be able to power up with a good fuse and check the working of the whole regulation circuit.And while still disconnected, but then with power off, check resistance between point 13 and ground, this should be open because neither TR814 and 815 should be polarized.Or you could actually do that first, because I suspect the problem to be there, unless there is a short somewhere after emitter of TR813.Then I would also check all the diodes in the secondary circuits as well as these caps there C804, C807, ...Also the voltage multiplier if you can.The fact is, like I said earlier, any short on the secondary can lead to oscillator stopping or overloaded.If the oscillator stops, then current drain is very high because the primary windings of the transformer are like a short under DC current.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 20, 2014, 09:29:19 pm
content: Strangely there are two 150uH in series on my board rather than just the one. I've disconnected the lead on the transformer side as its easier.So point 13 to ground shows several hundred kOhm.

author: iDevice
author_rank: Regular Contributor
timestamp: March 20, 2014, 09:35:03 pm
content: Quote from: TonyPh on March 20, 2014, 09:29:19 pmSure, wherever the easiest, it doesn't matter, the idea is to separate the regulated power feed circuit from the oscillator circuit.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 20, 2014, 09:39:10 pm
content: Ok. With inductors disconnected I get several hundred kOhm to ground from point 13.The scope didn't blow the fuse when the inductor was disconnected, obviously I reconnected the collector I dosconnected earlier TR183.Would the oscillator work without the load though?

author: TonyPh
author_rank: Regular Contributor
timestamp: March 20, 2014, 10:04:49 pm
content: Here's something.Even though point 13 was not short to ground, the wire I took off wire 13 is, and to the order of 2 or 3 ohms.I can't see that it would go to ground unless the C-E junctions on either TR814 or TR815 have a short.Am I right in thinking this?

author: iDevice
author_rank: Regular Contributor
timestamp: March 20, 2014, 10:45:58 pm
content: Quote from: TonyPh on March 20, 2014, 10:04:49 pmOk, but I'm not sure I understand.The wire you took off that seems to be shorted to ground, where does it go to, the transformer taps or to L801 ?QuoteExactly, that's why I asked to check that, point 13 can only refer to ground through the two transistors, so if you have a short there or something approaching, then bingo !That said, when nothing is connected to point 13, meaning L801 disconnected of course.Now if it's the right side of L801, or a wire going from point 13 to this L801 that is shorted, then you have to look there.You know, sometimes, it can be as simple as a wire touching ground, or a small piece of debris making a short.I had one scope power rail failing once because one connection of a front face potentiometer was touching chassis. That was on the other side of the scope from PSU area. Took me an hour tracing the short.The root cause was due to rotation of this pot.Ok it's time to go to bed on my side of the world

author: TonyPh
author_rank: Regular Contributor
timestamp: March 20, 2014, 10:56:17 pm
content: Thanks iDevice. It's time for my bed too. UK here so only an hour behind you.The wire I took off is at position 13, so on the transformer side of the inductor. Reading from the PCB back towards the TR813 it was not a short.Measuring from 13 towards the high voltage I found the short.TR814 is short across C-E junction.D813, D814, TR815 are all looking fine, as are R819 and R820.So that's the fault I suppose, unless something upstream caused it to blow. Will see if I can obtain a replacement part.BFY50 seems eerily familiar. I'm sure I bought a couple of these, or maybe the 51, 52 versions a while back for a project I never started.Edit: It's a pair of BFY51. Might be good enough to try with them. Will check the data sheet.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 21, 2014, 06:37:00 am
content: Hmm BFY50 - 80v.....BFY51 - 60v with a slightly different gain. I think I will order some BFY50 and wait for them to arrive before fixing.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 21, 2014, 04:52:30 pm
content: Found this rattling about inside the scope as I was tidying up today. It's only a small potentiometer adjuster, or rather half of one that's had it's handle broken off.I wonder if this is the culprit.I thought I had lost it when we moved house 6 months ago.

author: iDevice
author_rank: Regular Contributor
timestamp: March 21, 2014, 05:18:47 pm
content: Quote from: TonyPh on March 20, 2014, 10:56:17 pmAh ok, nice, same side of the pond but still some water between usQuoteOk, brilliant, I was expecting something like that, that's why I directed you in this area.QuoteWell at least we are moving forward.But we may not be out of trouble though. Transistors don't usually die of old age, so something happened.Like I said, it could come from a debris or something wrong on the secondary side that loaded too much or for too long. So it would be wise to still check things on that side, otherwise the replacement transistor may be short lived.

author: iDevice
author_rank: Regular Contributor
timestamp: March 21, 2014, 05:24:52 pm
content: Quote from: TonyPh on March 21, 2014, 04:52:30 pmHmmm, could be, but the chance that this tiny blade could do some short seems slim, but probably one possibility to consider.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 25, 2014, 07:07:32 pm
content: Finally got some BFY50 today so as I had just a few minutes this evening so I soldered two of them in place. The little collar around the one that had blown was actually a little bit melted from the heat.Turned on. Blown fuse. Hmm. I was hoping it would just be that but a part of me knew it wouldn't be.Replaced with a 800mA fuse (original was 500mA) and tried again.Fuse is not blowing so clearly something is still amiss but it's not producing a dead short this time.Still a dead screen. No change to apparent brightness in dark room on switch on or switch off.What I did notice on one power on was the soft light from the back of the CRT which normally glows quite dim started off bright and went dim after a few seconds. This only happened on one power on. That's probably unrelated as I think that is the heater and it comes from a separate secondary on the main transformer.Will pull the oscillator board out again tomorrow evening and just double check those BFY50 and their connections as the board didn't respond well to being worked upon so maybe I pulled a trace and didn't make the connection good after or something. Was in a bit of a rush.

author: iDevice
author_rank: Regular Contributor
timestamp: March 25, 2014, 08:30:18 pm
content: Well, like I said, transistors don't usually die of old age, so I feared that it wouldn't be so easy.Now, you need to find a way to see if the oscillator is actually...oscillating.This is where a second scope becomes handy of course.But you really should verify everything on the secondaries side.It could be a HV diode short.Be aware that checking HV diode can't be done with a standard multimeter because HV diodes are made of several junctions in series, so you need an test circuit with a higher voltage than a normal multimeter can output.But even if it's easy to do, what you are looking for is a short so at the moment it's not an issue.Anyway, like you said, check the board again, in case you just nicked a trace.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 26, 2014, 08:34:08 am
content: Thanks iDevice.The BY187 used in the voltage tripler and the grid and cathode supply are 11,500v 2.5ma with <26V forward voltage so if I do need to check their junctions then I will need to find a high enough voltage source.I have a few centre tapped 18v transformers so will probably use one of those if I need to test them properly, although it may actually be easier to just use a couple of 24v switchmode supplies in series as long as I keep the current low.I have no access to another oscilloscope so I'm out of luck with that, unless I can get my laptop soundcard scope working. I never had much luck with it when I tried before but I didn't have scope probes at the time. I'm thinking of getting a low end DSO by Siglent, Rigol etc. in a couple of months but would still like the old scope working.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 26, 2014, 07:23:59 pm
content: I cleared the majority of the wax from the multiplier board.It was actually fairly simple. Mostly it flaked off with a little gentle prising.Early in circuit indications are that none of the diodes anything near a short. I've tested with two meters and they appear as open circuit in both directions. The caps all seem to measure 0.1nF, again in circuit. Using my DMMs I can detect no short across any of the multiplier caps. The resistor, tested in circuit, measures a shade over 1MOhm as it is supposed to.The BFY50 I put in yesterday are still intact, there is no short across C-E. Tested in circuit. I was a bit concerned about my soldering of the new BFY50s but they look absolutely fine and even if the track is lifted slightly it is making connections to where it needs to.Now on to the Grid and Cathode supply before I attempt any out of circuit stuff.

author: tautech
author_rank: Super Contributor
timestamp: March 26, 2014, 07:35:55 pm
content: Be aware the multiplier board provides around 10 KV for PDA. The wax is to prevent tracking and leakage.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 26, 2014, 08:35:39 pm
content: Grid and Cathode all looks fine, in circuit tested.The diodes show open circuit when measured with my meter. The caps show open circuit, or very high resistance and the measure of capacitance is close to nominal. The resistor shows a resistance as per schematic.I checked the wiring back to the bright up board from the Grid and Cathode outputs and they make a connection. Not sure what happens to the voltage once it gets there but at least it's got a nice low resistance path to wherever it is meant to go.I suppose next I have to get some forward voltage into those BY187 diodes. I have a centre tapped 15v/15v regulated power supply I made for an audio circuit which I think I will utilise, rather than make something up specially.So my question is this...Is it safe/ok to leave the diodes in circuit to perform the diode test? I will add the 30v to the anode and a 10k resistor (I'm thinking 3mA should be fine to allow through the diode) to the cathode and link it to the power supply and measure the voltage drop across the diode.Or should I take the diodes out? It doesn't look easy to do this. If required which end is best to go with?If no fault is here is this pointing towards the oscillator or the CRT being dead?The bleeding resistors, 2 x 1M in series are intact.@Tautech.Yes I know. I intend to borrow one of the wifes candles and replace once I've finished in the area. It was so easy to remove I'm put off using anything more sticky and nasty, like silicone.

author: tautech
author_rank: Super Contributor
timestamp: March 26, 2014, 10:19:30 pm
content: As for the HV diodes, you just need to check they conduct forward and block reverse. Even a DC voltage forward will do measured with a meter. The result will be supplied voltage less Vf of the diode. Then check they block in reverse with a meter.The HV oscillators are often self starting and age drift of components can prevent them starting. For the few dollars of components don't be scared to replace anything suspect. The catch will be that when finished you will have to adjust the HV output and will need a meter capable of up to 3 KV

author: TonyPh
author_rank: Regular Contributor
timestamp: March 26, 2014, 10:45:47 pm
content: The first two EHT diodes drop 15.88v and 16.43v respectively when forward biased. They both have no reverse current that I can measure. Measuring with 30v input to the cathode with a resistor, 0v to anode. When measuring voltage dropped across the resistor I get next to no reading. I think that method makes sense as if there were current flow there'd be a voltage drop across the 10k.Managed to get to the third HV diode by sticking my tongue out at the right angle. It had a 16.20v forward drop, no reverse current measureable. It's looking fine too.

author: tautech
author_rank: Super Contributor
timestamp: March 26, 2014, 11:13:36 pm
content: I would look hard at the oscillator circuit, in particular the RC's that kick the oscillator off, then its running is over to the feedback circuit that you have repaired. Pull, measure and substitute components, take nothing for granted. Even check caps with your ohm meter and compare against a known good one. Without the right test gear be inventive.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 26, 2014, 11:27:06 pm
content: Thanks Tautech, you've been very helpful. I really do appreciate your input.I've checked a few caps for shorts on the oscillator section and already checked TR813 which seems to be the main current transistor as it's box mounted right before a two series inductor. I didn't replace anything. I didn't check some of the smaller transistors, namely TR811, TR812 and TR816 but will certainly do so tomorrow. I'm not sure what substitutes I have for the active components that would be suitable but I will see what I have. Fortunately the passives should be easily replaceable and susbstituted.The actives.TR811 BC107TR812 BCY70TR813 MJE520TR816 2N3905 - Well I'm thinking 2N3906 here as I have plenty of these. Higher gain though.I could always buy replacements, if they are still available. Should only cost a few quid.

author: iDevice
author_rank: Regular Contributor
timestamp: March 27, 2014, 08:26:32 pm
content: I don't know the ac bandwidth of your dmm, but it costs nothing to try to check if you read something on the bases of the two oscillator transistors, even their collectors should be safe.The only thing we could gain from this is to know if the oscillator runs, but if you can't read anything then it is either not running or the frequency is too high for your dmm.Now, it can also be done with a cap connected to the collector side of the winding then to a diode anode then to ground. Then a dmm in dc range between ground and the anode of this diode to see if you get a negative voltage, which would only be possible if it oscillates.A bit crude but it should work and without scope you have to use your bag of tricks.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 27, 2014, 09:51:14 pm
content: I have a pretty poor DMM, just Ohms and volts. The one I had capacitance on is broken at the moment and another I was using I had only borrowed from work. I've ordered another DMM which has a Hz function as well as duty cycle. It's probably not a wonderful DMM but it's about all I can really justify the spending.I tried measuring AC on the the ctransistor bolted to the case, the TR813. I got a sub 1v reading. Measuring Dc varied but was around the 12v range with DC. According to the manual it is meant to be 15v, but that could be peak-peak.I've also ordered a PCBScope, a Picaxe thing which may help in lieu of a proper scope.Here's some closer images of the oscillator board.Of note for me are the two black plastic transistors. These are not the ones featured on the schematic. Schematic calls for BC107 and 2N3905. The board hols BC147 and a BC22 something.I've replaced the 33uF and a 10uF electrolytic caps but this large capacitor has me puzzled. It is much bigger than the other 10uF and is written twice. It is referred to as a Mullard 12uF 121-16129 so it's strange how 10uF is written upon it.Also of note is the brown marking on the inductors. In the image above there is clearly some heating, when compared with this image I took a few days ago.I also guess this is indicative of a straight current flowing through the inductor rather than an oscillating one.I also noted that while powered up and with case mounted TR813 collector connected, the TR815 I replaced (I may have thought this was TR814 but I mixed them up) got very hot. The little plastic cradle it sits upon started to melt and give off a little smoke. The transistor is still intact but there is clearly too much current going through it, or it's not being shared between the two transistors.QuoteSo something like this? Or on the other side? Ceramic type 100nF ok?

author: oldway
author_rank: Super Contributor
timestamp: March 27, 2014, 10:25:29 pm
content: For me, it looks like a bad HV transformer.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 28, 2014, 10:36:01 am
content: Quote from: oldway on March 27, 2014, 10:25:29 pmI'm hoping it isn't that.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 28, 2014, 04:24:54 pm
content: Right. The PCBScope I ordered yesterday arrived lunch time today.It's a bit rubbish but here's a trace off a 5v 20kHz square wave. I used my scope lead and a few wires so it wouldn't ever be that clean.Then the Gould's internal cal 1v/1kHz square wave (and the software measured exactly 1kHz)I'm not quite sure why the horizontal time base shows the wave as nearly identical. Changing the s/div seems to have no effect on the wave but I know it's around the 20kHz as it was measured and calculated on the Gould when it worked. I haven't bothered to learn the software at the moment but it is very buggy and keeps crashing on both PCs I've tried to use it on. Edit: It does eventually show a more accurate trace once you go into the uS region.Here's the output on the emitter of the TR183. I had to use the x10 on the probe to attenuate the signal for the PCBScope. There is some movement but it can't be more than a few volts of oscillation.The 'osicllation' was similar when I had the TR183 totally disconnected.For the moment I have only replaced the electrolytic caps on the oscillator board.Now that I've got it in a quiet room I've noticed that when I switch on the power something in the EHT box seems to be making a noise similar to crackling for a few moments, which then gradually dies away. I'm wary of shoving my ear too close.

author: tautech
author_rank: Super Contributor
timestamp: March 28, 2014, 06:15:06 pm
content: You can disconnect the low side of the HV multiplier for testing. Often HV supplies will make some noise.

author: iDevice
author_rank: Regular Contributor
timestamp: March 28, 2014, 11:03:21 pm
content: Quote from: TonyPh on March 27, 2014, 09:51:14 pmTR813 is only the supply regulator, it handles DC current only and any AC component coming from the oscillator is choked by the inductor that follows.So this not the place to check the oscillator.You could check after the inductor though, there should be plenty of AC component there.QuoteWell it shows excess current for sure. Now it can come from the fact that the oscillator is not running, in this case, indeed, the DC current would be quite high, but even when oscillating, a bad HV transformer (shorts between windings) could overload.QuoteIt's certainly not good. Maybe the HV transformer is bad after all.But did you check the base voltage of TR814 and 15 ?QuoteYes exactly, but now that you have a PC scope, there is no need anymore.Just wander with the probe around the oscillator circuit after the supply inductor.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 29, 2014, 06:18:16 pm
content: Still trying to get this PCBScope working properly.I'm picking up this from the collector of TR815.It's clearly not 20v/div. I'm not sure what the swing is and how accurate that 'waveform' is. I may have neglected to switch into x10 on the probe on that one.This one is more to scale.I think this PCBScope is defective as it captures only when it feels like, has frequent USB disconnections and software hangs. A DSO Nano would have been better.So there is, I think, some ocillation at least. I would expect it to be cleaner though. The PCBscope did pick up my 20kHz signal ok so when it works for the briefest moments maybe it isn't that far off.-----------------I haven't been home much today so haven't check the base of the TR814 and 5 yet. Will do so shortly.I've run my meter across the wires leading to and coming from the HV transformer and I see no shorts, although I am getting some lower rend resistance across windings. I will do a list of the values and which wires they come from and see if that makes sense to anyone.

author: tautech
author_rank: Super Contributor
timestamp: March 29, 2014, 08:27:30 pm
content: Be conservative with your settings for your USB scope. Turn on 10x, AC coupling and Averaging.Expect to see HV oscillating in the 20 - 50 KHz region. With these and DSO's pay attention to your trigger settings to get a representative waveform.The HV transformer primaries and feedback will be low resistance, that's why the oscillator section is normally fused.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 29, 2014, 09:18:24 pm
content: It doesn't seem to have AC couplingWill have to check the manual to see if I can enable it.Unless I'm missing something obvious. I'm used to a lever that gives a satsifying thunk when I switch it.Anyway armed with your advice I managed to get this TR815 collectopr signal by using a wire rather than my probe. I guess the software isn't compensated for the probe. The waveform started fairly spikey but resolved to the waveform shown.There's a lot of DC content in the signal and not a lot of AC. AC coupling would be really handy as it is tricky to find the AC content with the trigger marker with the huge DC.I'd prefer to use a proper scope but this is all I have.Edit:Edit:Same signal, just more captures.At 5uS and a lot of averaging.At 10uS and not so much averagingMaybe the waveform is such a mess that averaging just tidies it up.It looks like a 15kHz wave with 500kHz content modulated on it.

author: tautech
author_rank: Super Contributor
timestamp: March 29, 2014, 09:54:55 pm
content: Has the manual any waveforms for HV PCB test points? Probably 19V @ 30 KHz sinewave at the emitter of TR813 or similar.EDITPin 13 on transformerIf so, set scope for those and don't go looking for what MIGHT be there. There should be around 20 V across TR815.Study the schematic and write down/record resistance for each of the primary and feedback windings. Total of 2 (hopefully each pair leg is equal) pairs.Have you an transistor tester in your multimeter? That can confirm the gain of each transistor is within datasheet specs.Check TR811 to 816 for peace of mind.Replace R818 and C813, D813,814 and R819,820 for peace of mind.The RC of R818 and C813 is crucial to correct operation.If the Regulator part of the PCB is not right(TR811,816,812,813) it will not go.Also the 15V from PSU PCB(plus or minus?)A heavier than normal load on the secondary might screw things, so check any component that could pull it down. C815 for instance. Maybe caps in the HV secondaryPersonally I would replace all electrolytics on the HV board and check all resistances and caps.L801 will discolor with normal operating heat.The Good thing is you can muck around with these until you get them going and you have not affected critical scope settings, only the EHT voltage, which can be re-adjusted to spec.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 29, 2014, 11:10:11 pm
content: There's no waveform for anything in the EHT section. The closest is in the bright up board. Typical really as that would have been handy.I will check through the board and makes replacements. I doubt I have substitutes for much of it so will likely have to be either creative or buy new. I know for certain I have no large wire wound resistors and this (R818) is one of the hotter components as the board works currently.Maybe I will get a chance to make some measurements tomorrow.I suppose I've been procrastinating on taking all the components off the oscillator board but it is inevitable that it needs to be done.Only my broken DMM has a transistor gain checker. It was only a broken switch. My other two DMM don't have the feature, although I am waiting on delivery of a new DMM which has the function.Edit: Actually one of the DMM I have here has the transistor checker, I've just never noticed or needed itit also has frequency

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2014, 12:34:06 am
content: Just check the WW resistors only, likely to be ok.Sorry the schematic you posted has no component values so I got the RC wrong for R818, C813

author: TonyPh
author_rank: Regular Contributor
timestamp: March 30, 2014, 09:00:06 am
content: 390 ohm and 0.01uF for R818 and C183.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 30, 2014, 04:46:42 pm
content: Primary resistances.Pos 13-16 - 1 OhmPos 13-17 - 1 OhmPos 16-17 - 2 OhmPos 14-12 - 1 OhmPos 12-15 - 1 OhmPos 14-15 - 2 OhmTest leads together 0.5 OhmConfirmed on both meters.Swapped all diodes for new.Swapped R819, R820 for new.Swapped TR812 for new.Replaced C813, C814, C816.Tried a D882 for TR813, it melted.Checked all resistors for values.Swapped TR814 and TR815 for new again.Swapped TR816 - schematic calls for 2N3905, was BC212 fitted, for a 2N3906Swapped TR811 for a 2N3904, schematic calls for BC107

author: oldway
author_rank: Super Contributor
timestamp: March 30, 2014, 05:26:16 pm
content: You are loosing your time: bad HV transformer.I have an HP1740A with the same problem and this happen often with Philips scopes too.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 30, 2014, 06:44:03 pm
content: If the HV transformer is bad is there any way to recover the scope?

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2014, 07:09:18 pm
content: You could check secondarys for open circuit and all windings for isolation from each other and the core. Results will not be conclusive with a multimeter.Also check secondary caps and pots for shorts.I used to have some oscillator transformer specs (R&L) in a scope manual, but unfortunately now sold with that scope. From the best of my memory the inductance of primaries was in the uH range which suggests resistance would be low too.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 30, 2014, 07:43:31 pm
content: Right.Here's my findings.Maybe not conclusive but there's no short there and the 100 Ohm and 4k suggest a lot of windings.Maybe things would be different at high voltage though.

author: oldway
author_rank: Super Contributor
timestamp: March 30, 2014, 09:02:48 pm
content: Generally, the problem is an internal short circuit in a few turns of the HV coil.Only solution is to replace this transformer.It's not possible to measure if the transformer is bad or not with a multimeter.

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2014, 09:21:46 pm
content: OK no OC. I have fixed one or two that had OC in a secondary that was in the first few turns, that I just removed and re-connected. Worked fine.Sorry oldway, but I would not give up quite yet.Not sure if I would disconnect secondarys and power up without a load just to see if it will oscillate, others might advise?I have had to replace the HV caps in old stuff, they can be hard to source, but you can find them on ebay from China.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 30, 2014, 09:30:26 pm
content: Might be worth a shot. Would I get anything on the CRT at all if just the VG hi VG lo section were connected. Obviously the other part is dependant on the winding that also goes to the tripler. I don't know much about CRTs and what they need to operate. Even a bit if illumination would be a clue as there is absolutely nothing at the moment.The HV transformer is potted so any modification would be a challenge.Still haven't made a HV probe. Would be interesting if there was actually some potential on the HV section.Farnell do these 500pF disc caps.http://uk.farnell.com/vishay-sprague/615r150gast50/capacitor-ceramic-500pf-15000v5u/dp/1311457And they seem to have some 4700pF 6kVHaven't checked all types but might be able to get them all, or maybe slightly higher capacitance or something. Farnell have a £20 minimum order though.

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2014, 09:48:19 pm
content: The point of that test is to find if there is too much secondary load that stops it from oscillating.Again not sure if I would do it and have that HV unloaded for risk of transformer insulation failing.I don't know how high voltages might go. We don't want to pop any thing more!

author: TonyPh
author_rank: Regular Contributor
timestamp: March 30, 2014, 09:57:53 pm
content: Hmm so it is the load of the CRT that keeps the high voltage in check?

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2014, 12:34:37 am
content: Quote from: TonyPh on March 30, 2014, 09:57:53 pmWe do not know, process of elimination.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 31, 2014, 07:29:36 am
content: If I was to buy all the caps from Mouser (Farnell do most but not all values) it comes to a fair bit of money (£20+) by the time they throw their delivery charge of £12 on top. The diodes are harder to find though.

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2014, 08:47:08 am
content: Try this seller:http://www.ebay.com/itm/5pcs-10KV-4700PF-472-High-Voltage-Ceramic-Capacitor-/161045275152?pt=LH_DefaultDomain_0&hash=item257f0be210HV diodes should be alright if they pass a simple blocking and conduction test as in a previous post.

author: TonyPh
author_rank: Regular Contributor
timestamp: March 31, 2014, 10:16:56 am
content: Thanks. Food for thought.For the multipler I would need 3x 500pF 8kV and 1x 470pF 12.5kV, so to replace all then maybe I'd get a batch of 5 off 470pF 20kV rather than having to buy 5x 10kV and 5x 20kV. The 30 stray pF shouldn't make much difference, should it?Maybe these from the US and a touch cheaper, and probably quicker than China.http://www.ebay.co.uk/itm/10-pcs-20KV-20000V-470PF-High-Voltage-Ceramic-Capacitor-/281102573138?pt=LH_DefaultDomain_0&hash=item417304de52For the grid and cathode I would need 2x 4700pF 4Kv and 2x 0.02uF 1.5kV, so I'm thinking that 2x 10nF in parallel would do for each of the 0.02uF.Still quite an expense. I'd love to fix it but what are the odds that it would work? Could be that pesky HV transformer is faulty and I'm just wasting my time and money.Actually I'm wondering why there are 4kV caps in the grid and cathode section when the output voltage is a mere -1580v. I wonder if I could 'get away' with 3kV caps, in which case I could source from the UK for £1.30.Looking at the EHT section though. Those capacitors are rather large. Would any cap, provided it meets the capacitance and volt rating do ok or are these things exotic in construction and required to be used, or is it that manufacturers are now better at making caps than in the 70s.

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2014, 10:53:39 pm
content: I would dismiss HV multiplier at this stage.Close as you can for pF values and near or better for KV values.Component list will provide info of component type(ceramic, polyester etc), just match that and physical size is a secondary consideration.Maybe more research on HV transformer specs or another post seeking info on R's & L's.Maybe a little test project to connect transformer to, to check its function at a lower voltage. Say 5V input @ 5 KHz that should give you 25% of in scope voltages.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 01, 2014, 05:57:57 am
content: Thanks. There's no info on the transformer specs I've seen thus far but I will see what I can dig up.I'm sure a 5v/5kHz sine wave ish signal shouldn't be that difficult to make.I managed to score myself a working 20Mhz scope off eBay yesterday for £27 so that'll help.

author: tautech
author_rank: Super Contributor
timestamp: April 01, 2014, 06:37:39 am
content: Yep, while you old CRT scopes, you will always need a spare one to fix the other!!!!!BTW what brand/model did you score?If its a good runner, you could put your fixit money towards a new DSO

author: TonyPh
author_rank: Regular Contributor
timestamp: April 01, 2014, 07:20:15 am
content: Ah it's just another Gould, a slightly newer model OS300. I've seen them go for between £50 and £70 or more depending on how well it was marketed and whether there are probes bundled and if delivery was offered. It caught my eye because it was collection only and they usually go for cheaper. I asked if I could send a courier in and he said it was fine. Always a bit risky that I will get a pile of oscilloscope bits delivered though.I have a wein bridge sine wave oscillator circuit 20Hz to 20kHz that I built and a push pull audio amplifier that is good for a few watts that I can hook up together to feed into the HV transformer. I will dig them out and see if I can get it oscillating as sometimes it's a bit of a fiddle as the potentiometer I used seems to drift a touch. I should be able to use the PCBScope to verify the frequency and voltage before attaching to the transformer.What would I be looking for when doing this experiment? Would I be loading down the secondary with something to see what effect it has?

author: tautech
author_rank: Super Contributor
timestamp: April 01, 2014, 08:57:18 am
content: Just try the oscillator alone and scope transformer primary(16 & 17) and secondary. Maybe 50 ohm in series with the primary to protect your oscillator. Say a 1M secondary load for starters.Scope oscillator output OC and then connected to primary to see what happens to waveform under primary windings load.By the info you posted it was ~1 : 50 step-up for the grid/cathode windings.You will want to see ~50x the primary on each of the grid and cathode secondarys and an appropriate step up for the multiplier secondary.We only want to check for transformer action on all secondarys. ie. step-upThis type of transformer is optimized for a certain frequency, IME ~20-30 KHz but I doubt you can drive it that fast with enough current for this test.Watch out for ground loops through your scope ground leads. I know it is a LV low power test, but you may not get a result if it is not set up right.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 02, 2014, 04:59:13 pm
content: So I made this thing of beauty today. The marking is just in case I forget what it's for.20x 10MOhm resistors and a 1MOhm resistor in series. Miliatry grade heatshrink plu 15kV Silicone sleeve plus another 6kV silicone braided sleeve on top, all encased in some tubing. Depending on the range I'm getting around a 220x reduction in voltage.So here's the meaurements that I'm getting right now with the few substituted components still in place.-Vk = -128vVg Hi = -128vCath (on bright up board) = - 126vAnode around 700vSo it's around 10% of the normal output but it is definately stepping up, just not enough.I suppose looking at the plots I got from PCBscope the other day which were only partially oscillating this makes sense.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2014, 06:40:57 pm
content: Well, if you have that other scope coming, you will be able to do some comparative measurements to isolate the fault. :-)

author: TonyPh
author_rank: Regular Contributor
timestamp: April 04, 2014, 02:06:57 pm
content: I received the new oscope. It works!Here's some of the waveforms. Someone on another forum recommended doing some tests with TR814 and TR815 disconnected so these are with them removed and all wiring on primary and secondary attached as normal.This before connecting, an unloaded signal. The signal, by the way, is off a 555 wired up as a 20kHz PWM, with roughly 3.5v peak. I don't know how much power it provides but it's coming off pin 7 so don't think it's that much really.On wires 13, 17 and 16.On wires 12, 14 and 15 (14 is marked as 11 in the schematic).I don't really understand what the four primaries (or are they centre taps) are doing in the circuit but the top half is totally loading down my signal to nothing while the bottom half is not affecting it so much.Now with the scope powered up....This is the oscillation on Position 13. It's a lot slower than I expected. The amplitude is so small I have to use AC coupling to see it. The oscillation is 100Hz so is probably power supply ripple or something.And position 14It's stable like this and only wobbles a very small amount. Nothing is heating up.Swapped in a new (probably old stock) MJE520 at TR813 (to replace the doofah BD435) and there's no change to oscillation speed or amplitude.Looks like power rail ripple, it's at 100Hz and 0.4v. I wonder if the main reservoir cap is doing a good job.I will connect up TR814 and TR815 tomorrow and take a look at the waveform then.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 05, 2014, 08:52:39 am
content: I put TR814 and TR815 on a small breadboard and wired it in place.Now the components are getting warm again, especially TR183 which is providing too much output at the moment.This is the waveform at collector of TR184and at collector of TR815Timebase in both cases was 2uS/div and vertical 2v/div with ground at the bottom. DC coupled. So much faster than it should be and nowhere near the 55v@30kHz it's supposed to be. I've yet to put the original electrlytics back in and the BC212 at TR816 hasn't been put back in, mainly because I can't find itTR815 is getting warmer than TR814 but the warmest is TR813 which even though it's case mounted is hot to the touch, so I'm only running it for short periods, hence the rubbish photos.

author: tautech
author_rank: Super Contributor
timestamp: April 06, 2014, 12:17:59 pm
content: @ Tony PHYour quote:I don't really understand what the four primaries (or are they centre taps) are doing in the circuit but the top half is totally loading down my signal to nothing while the bottom half is not affecting it so much.There is only 1 primary, and it is center tapped 16, 13, 17.The other is a feedback winding to drive the bases of TR814, 815That primary if not oscillating will draw too much current.As I see the circuit, the bases of TR814, 815 are biased on via R818.Have a look at your going scope with your USB scope to understand what you need to see, even take a few DC voltages with your meter while its going.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 06, 2014, 01:04:23 pm
content: Thanks for that. I think I understand.Anyway over the weekend I built a LOPT tester to see what this transformer gives me. I'm not finding the LEDs very useful but on the scope I get the following.Took all the wires off the transformer and used the ring tester on the primary and the feedback windings.First the primary 16 to 17Then a little wider as there is a second ring wave of a lower amplitude, this is probably the feedback winding reacting?Then zoomed in to get the detailSame sort of thing on the feedback winding but with lower amplitude.I also tried across the centre tap and that was the same as above.Had some lunch and tried again to measure the frequency as the stupid dial wasn't on cal but I can't get it to go nowAnyway the results from before looked good.Is it worth trying the secondaries?Edit: also of note is that when I tried the ring before disconnection it totally died. Just the tiniest of blips.

author: tautech
author_rank: Super Contributor
timestamp: April 06, 2014, 07:43:50 pm
content: On power up the primary has to have a start-up pulse for the feedback finding to kick off the oscillation. That oscillation does not just start, it has to be initiated. Check your going scope to get some measurements, running and start-up, use single on USB scope if it has it.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 07, 2014, 07:13:43 am
content: Thanks again.The USB scope does have single shot mode and I will have another go at using it. Previously it had major issues when I removed a signal, or changed a timebase, or basically anything.I don't have much luck with scopes. This new one seems to have developed a slight issue where the CRT shows a trace for a bit, then goes massively blurry, then shows a trace. I've noticed that the EHT regulator voltage, which is supposed to be 405v +/-20 is a little high at 470v! I wonder if my supply voltage, which this voltage is derived from, is a little higher than it's previous home. At least the thing isn't melting when switched on. Still it worked faultlessly until this cropped up so wonder if the scope has been idle for a few years, the caps have dried out and then when it was used this weekend they warmed up and have gone bad.I really should build/buy an ESR meter if I'm going to use old equipment.

author: Nermash
author_rank: Frequent Contributor
timestamp: April 07, 2014, 07:59:40 am
content: A week ago I also got OS300 and while going through initial checkup and calibration, I have noticed that EHT test point was sitting at about 500 V instead of 405 V. My home supply is measured at 230 V AC, and the transformer range on the CRO was set at 220 V.I moved it to 240 V input range (that is the +20V switch) and readjusted the EHT pot for 405 V.So, you might wanna try to set the input to 240 V also, that could bring it to the normal range on the EHT.BTW, OS300 does not have FBT. It gets 950 V AC from the mains transformer and it goes through voltage doubler for the -1850 V cathode supply.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 07, 2014, 08:49:31 am
content: Thanks Nermash. I will have a look at the switch when I get home from work. Unlike you I didn't do the initial checkup and calibration, just plugged it and went for it. I had meant to go back and check over stuff to get a working baseline but looks like I left it a tad late.Yeah I've been looking at the schematic and noticed it works differently to the OS3000 high voltage section, and the CRT needs a much lower overall voltage, around 2.5k vs 10k.I'm also noticing a hum from what I assume is the mains transformer and associated circuitry. While assessing where the dangerous voltages were on the PCB I measured DC and AC voltage off the 300V filter capacitor's body. I'm not sure if that's normal. The case of the scope seems perilously close to those caps as well.

author: Nermash
author_rank: Frequent Contributor
timestamp: April 07, 2014, 09:19:57 am
content: I haven't noticed any audible hum on my unit. Topside of the main filter cap is close to the top cover, but I think that you probably measured some leakage voltage.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 07, 2014, 10:02:19 am
content: The OS300...It's audible and low frequency. I'ts not too dissimilar from the hum I get off one of my cheap 1.5A regulated power supplies I use sometimes. It was just an observation but I noticed the noise does increase slightly with the intensity dial so is probably just a factor of additional power draw. The other scope is pretty much silent.I measured the voltage off the capacitor body on the area where the vent is, while powered up of course. It wasn't a significant voltage, just that I'm not sure whether it's normally present on a capacitor body as I've never noticed it before, but I don't usually probe mains filter caps. My concern was that with a slight bowing of the scope's case the case might actually touch the cap and short something out.And for the OS3000....So in light of there being no EHT oscillator on the OS300, just a regulator section I'm not sure what to look for.Is it worth hooking up the HV transformer to the connections on the primary side and leave the secondary detached to see if oscillation is initiated, or would that damage anything?

author: TonyPh
author_rank: Regular Contributor
timestamp: April 07, 2014, 05:07:42 pm
content: Quote from: Nermash on April 07, 2014, 07:59:40 amMine was already on the 240v range. I modded the voltage to near 405v and still no proper trace, so it's something else.A diode in the centre of the board, D705, between the large resistors looks a bit worse for wear and there is a lot of dirt around the area and it's right on the highest voltage line. It shows a 0.6v drop in circuit.Has a quick look on the board for a dodgy connection and the only thing I can see is a slightly corroded connection to the clear coated 2 way cable with large separation that goes off to the CRT. Tapping this seems to have no effect so it's probably not that.In at the deep end with these oscilloscopes. I really should have bought a modern digitial scope, but then I wouldn't be having all this amazing fun.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 07, 2014, 06:31:51 pm
content: I've been offered £20 for a working EHT unit from another OS3000 unit. I wonder if it's worth it considering I'd probably continue dropping money on it until I find the fault, or scrap the whole scope out of frsutration.At the very least it would give me a working EHT unit to analyse.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 09, 2014, 08:34:53 pm
content: OS300. I found the output transistor that drives the horizontal plate to be open circuit. Swapped them over and I now get a trace but it's only a short one to the side of the screen. So that's a set of new high voltage output transistors for me ordered.The image is of the partial trace on the second scope by the way.-----------------------OS3000.Am waiting on the repair of the OS300 before progressing. I probably will go ahead with the £20 EHT unit replacement. It's an easy fix and I can always hang on to the dud EHT unit for maybe a bit of fun later.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 10, 2014, 05:42:23 pm
content: OS3000 (old scope).Hmm just reconnected the primary and feedback to the PCB and breadboard, and connected only the secondary for the Vg Hi and VG Lo and the relevant output wires tht go off to the bright up board. Measuring 4v on the collector of the oscillator trnasistor and the output voltage is just the same as before, around -170 volts or so, measured to case ground. So the oscillation is still not kicking in. Will swap over secondaries and see if the multi and other section will work while the Vg Hi & Vg Lo are disconnected.Can't seem to find the horizontal output so it must be in the middle somewhere (haven't looked that hard though) so will wait on the new delivery of new parts for the new scope.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 10, 2014, 10:46:08 pm
content: A little bit interesting.I disconnected all HV secondaries and connected all primary and feedback wiring.Let the thing run. I measured 9v on my DMM and 25kHz speed.So I hooked up the second scope to the output and got this wave.This is in x10 as I get more screen display than x1 in this mode.Now I'm thinking this isn't far off the sort of wave I'm expecting. It's certainly much higher than before. 25v peak. Obviously the calibration of the screen is way off at the moment for timebase but it's so much better than it was with the load.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 14, 2014, 12:54:04 pm
content: I managed to get the newer scope, the OS300 fully working again.The fault was indeed the output transistor. I created an additional fault by destroying a PCB track (I'm good at that) but I fixed that too.So now I can get back to the OS3000.--------------OS3000.The oscillation I posted above, on the unloaded transformer is probably just the transformer oscillating at it's own frequency and doesn't involve any proper feedback from the feedback winding. As far as I can tell feedback also comes through a 2.2M resistor (it's actually 1M and 1.2M in series) and the focus control. I did check both these and they seem to be measuring normally, the 470k focus control pot does measure a tad high but is within the ballpark.As I introduce load to the secondary the amplitude of the oscillation drops accordingly, until I fully load it and then the oscillation goes sporadic and a bit crazy. I also tried some high value resistors and its more or less the same story.I also disconnected the CRT anode and this had no effect.It transpires I might be able to get that EHT unit from a guy, potentially FOC if I can get it from him 200 miles away.I would still like to work out what is wrong with this one though and be sure it is one thing or another.

author: tautech
author_rank: Super Contributor
timestamp: April 14, 2014, 01:28:04 pm
content: Transformer cannot oscillate by itself with a DC supply!The part of the circuit you mention is for output regulation (HV).Yep get the other HV unit.With the scope you have repaired, you will at some stage need to check its accuracy after replacing the plate drive transistors.Where they horizontal output?While not critical or urgent, it would be nice to know if it is close.

author: TonyPh
author_rank: Regular Contributor
timestamp: April 14, 2014, 02:31:53 pm
content: I also noticed that the oscillator side in my OS3000 differs from that of the manual. Mine actually resembles the OS3000A EHT oscillator. The only differences are the high value resistor from focus is split into two, there's a resistor running from PNP TR816 emitter to +20v rail and a diode from the EHT set pot to the same location. I'm not sure what difference they make to anything but Gould must have thought them important enough to include in their updated scope.Will be interesting to see if I can get that donor EHT unit. The guy reckons the scope he has is in a very poor state and probably not worth restoring.The transistors were for the horizontal output on the new scope. I have quickly adjusted to suit the internal cal signal as that's probably the most accurate reference I have. I will compare with the 1v cal reference on the OS3000 scope to see if there's a discrepancy.

author: tautech
author_rank: Super Contributor
timestamp: April 14, 2014, 03:02:48 pm
content: Note that a scopes Cal output can not be called accurate, but it will do for now. Think about a better freq standard for horizontal adjustment.Maybe others might suggest a simple and more accurate freq standard.Its all progress!!!!

author: TonyPh
author_rank: Regular Contributor
timestamp: April 26, 2014, 04:45:35 pm
content: Some progress...It's fixed.Replaced the whole EHT unit.I still think the old scope needs some attention in some places but at least it's now showing a trace and doing what it should.

author: tautech
author_rank: Super Contributor
timestamp: April 26, 2014, 07:30:21 pm
content: Well done!Now you have it working, you should document some measurements, especially for the EHT unit.Transformer winding resistances and inductance, waveforms, oscillator frequency in operation, etc. Try to follow any Manual test setup for powered measurements.Look back through this thread and measure all the stuff you asked about. Add it to the manual.Fix your old unit if possible, it is the part of a CRT oscilloscope that commonly fails.Then YOU will be the EEVblog Gould expert :-)Enjoy!Rob

author: oldway
author_rank: Super Contributor
timestamp: April 27, 2014, 04:21:46 am
content: Quote from: oldway on March 27, 2014, 10:25:29 pmThe solution was already on page 3.

author: turbo!
author_rank: Regular Contributor
timestamp: May 14, 2014, 02:12:32 am
content: Quote from: tautech on April 14, 2014, 03:02:48 pmThe AC power line is very accurate. It will be easier that he is in Europe where it's 50Hz as it lines up better with all the settings than 60HzSet to 2mS/div and make the entire line cycle fit edge to edge. Set to 10mS/div, and line up the peak up the middle of one peak on far left, then match so the 5th peak match exactly on the far right edge, do the same but line up the 10th 20ms/div.In the long term, it's more accurate than most clock and even in the short run, the deviation is smaller than what you can line up on the screen, so you can assume its 50.00.