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Old 16th Apr 2014, 2:47 pm   #21
WME_bill
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

OS3000.
1."The waveform (P16 & P17)..." I am not clear whether the waveform is the same from both collectors - which if not could indicate one of the transistors TR814/TR815 is failing. If the amplitude falls under load suggests that the regulation feedback is not operating.
2. "I will have a look at adjusting the EHT adjust pot later today..." You do this to see if the output voltage changes, as that will confirm if the EHT feedback and regulation is working, not to attempt to set up some calibration process.
3. "..unfortunately the heat from the soldering iron does lift the traces..." Exactly, that is why I suggested measuring all the voltages around the transistors first. Then you avoid doing unnecessary damage and creating new faults. I hoped you would remember this from the OS300.
4. Do I understand "traces" means the printed copper "tracks". I keep thinking you are talking about other stray waveforms or tracing other faults somewhere else.
5. "..my transistor checking capability at the moment...". HFE is almost irrelevant when you have major operating condition faults. But measuring the voltage around each transistor indicates if the transistor is sound and likely to be operating correctly. Then if you find voltage B to E is different from 0.6V, and or Collector at zero or same as B, then you have a possible transistor fault, and can isolate it for test. wme_bill
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Old 16th Apr 2014, 3:01 pm   #22
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

1. In my tests I have not noted much of a difference measuring one side to the other. At this time I can easily swap the transistors over to test for differences anyway.

2. I'm unsure how much effect turning the pot would have so wanted to be sure I could detect it. I understand roughly what the pot does in circuit and that adjustment should have an effect on output.

3. To be fair on myself I haven't actually taken anything off this board since way before the OS300 incident. This PCB is made using self adhesive tape I think, it's all curvy lines.

4. Now I'm not sure what I normally call them. I meant tracks.

5. This is where my lack of fault finding experience hits me hard.

Tony

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Old 16th Apr 2014, 8:40 pm   #23
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Right some progress this evening, mind you I only just got home.

EHT pot position makes no differece to Cathode voltage (this with tripler disconnected)

I've also noted the voltage coming to the base of TR816, which is only 1.31v. The base of TR811 which follows is 0.75v and the collector of TR811 is reading ground.

TR811 is supposed to be BC107 but a BC147 is fitted. This particular BC147 is a different package to what I can find in a datasheet. Anyway that's not critical as I do have some BC107 to replace if needed. At the moment it looks like my feedback voltage is low. If it is to feedback through TR816, TR811 and onto the TR812 and TR813 then I'm thinking it needs to be higher.

At the moment the voltage coming into the board at the Ref point is 13v.

Edit: Even more interesting is this 1M resistor. I noticed a speck of white on it and touched it with a pen and could feel the damage. This resistor is one of the extra/edited things when comparing the OS3000 and the OS3000A manual. I didn't even think it was that important and never noticed the white mark before. It has a 2.2M next to it and has the focus control pot further up the line (measures 560kOhm - supposed to be 470kOhm)
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Old 16th Apr 2014, 8:57 pm   #24
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Ok, now I think we are getting somewhere. R811/812 form a potential divider to D811/TR812, check these and make sure they are good. Check and make sure TR811/816 are both good.
Which resistor is that 1M? Is it in the feedback chain?
Bill
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Old 16th Apr 2014, 9:01 pm   #25
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Forgive me I neglected to give it it's name, yes it is R802.
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Old 16th Apr 2014, 9:27 pm   #26
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

The conditions around TR816/TR811 imply to me that the regulator is basically shutting down because there's no current flowing down R802/R804 to the focus control and the large negative cathode voltage. In normal operation, I'd expect the voltage on the base of TR816 to be only just above zero, because the connection to the cathode will draw current out of the 'SET EHT' control and R816.

I'm not totally sure how this circuit is supposed to start up, but I think C815 and C817 may have something to do with it, forcing TR812 and TR813 to come on for an instant to let the oscillator get up to speed and start generating that negative voltage. Since they're electrolytics, they're prime candidates for trouble.

To confirm this, what does the emitter of TR813 do just after switch on? Does it go high for a short while (a few tens of milliseconds, I'd expect) and then drop down?

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Old 16th Apr 2014, 9:32 pm   #27
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

I checked the divider R811/R812 seems to be dividing nicely giving me 18.4v at the anode D811 location.

D811 is verified and operating normally with a voltage drop of 0.7v across it. I've checked it backwards and there's no continuity.

TR812 collector 19.2v, base same as D811 cathode (of course). Collector is however 5v? Where did that come from? I'm a little vague with PNP and how voltages move across the junctions. Should the collctor here be whatever is on the base less a diode drop?

Edit : Chris, re the TR813 start up, not as far as I can tell. My DMM is not very fast. My scope, being analogue can't do a single shot.
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Old 16th Apr 2014, 10:41 pm   #28
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Ah! TR812 is PNP. I missed that. The diagram is small... That's my story and I'm sticking to it!

That means the circuit works the opposite way round to what I thought. TR811 is switched hard on, which switches TR812 on, and thus TR813 on. At the moment the circuit is desperately trying to crank up the supply to the oscillator, but is running out of steam. That's why everything's getting hot. I think the current limit arranged by R814/D811 is coming in to play to limit the damage.

You said that the voltage on TR813 emitter increased when you disconnected all the secondaries. Is that right?
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Old 16th Apr 2014, 10:53 pm   #29
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

No TR813 voltage seems to be around 4v with or without load at the moment. The emitter voltage increases dramatically if I disconnect TR814/5 from circuit.

It was the voltage and waveform at Tr814/5 that increases with lower loading.
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Old 17th Apr 2014, 6:59 am   #30
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Also interesting is that when I disconnect the wire from R802 and measure the voltage coming across the feedback resistors R802 and R804 I get -110ish. The voltage at position 11 it is usually soldered to is 1.3v.

I'm not quite sure what to make of this clash of voltsages.
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Old 17th Apr 2014, 7:53 am   #31
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Quote:
Originally Posted by TonyPhs View Post
No TR813 voltage seems to be around 4v with or without load at the moment. The emitter voltage increases dramatically if I disconnect TR814/5 from circuit.
That's significant - when you say 'without load' is that with all the secondaries disconnected or just the 9kV/tripler one? There could be a fault in any of the three secondary circuits which could have this effect. The only way to tell is to disconnect all of them.

If TR813 emitter stays at 4V even with all the secondaries disconnected, then I think we can safely say the transformer is faulty, because the series regulator is current-limiting and the only possible reason for that is a faulty transformer, given that we know that TR814/815 are clearly working.
Quote:
Also interesting is that when I disconnect the wire from R802 and measure the voltage coming across the feedback resistors R802 and R804 I get -110ish. The voltage at position 11 it is usually soldered to is 1.3v.
I'd expect that: what's going on at point 11 is that current should flow out of it down R802 and R804 to the -1500V cathode supply. When the supply is in regulation, that current should be about the same as the current coming from +15V via the SET EHT control and R816. That should put the base of TR816 at about 0V. Given that R816 is 27k, and let's say the voltage on the 'REF' pin is about 10V with the SET EHT control at half mast, that implies that the current flowing down R802 and R804 should be about (10V/27k) 370uA, which sounds about right to me.

So we should be losing about 370V across each 1 megohm of R802 and R804, and 144V across 390k R803, leaving 600V or so of the -1500V cathode voltage across the focus control. If the focus control drops 600V at 370uA, it should have a value of about 1.7 megohms.

Ah, I've just read your note about the 3000A which puts R804 at 2.2 meg and the focus control at 470k. That adds up nicely!

If you disconnect R802, no current will flow that way, and point 11 will be clamped to about 1.3V as the B-E voltage of TR811 and the forward voltage of D812. That's what you're seeing.

Regardless of all the theory, the -110V you're getting at the end of R802 isn't anywhere near enough, because the power supply is struggling to produce anything at all.

Chris
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Old 17th Apr 2014, 9:00 am   #32
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Yes I confirm 'without load' is with all secondaries disconnected.

Tony
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Old 17th Apr 2014, 9:38 am   #33
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

I think that pretty much nails the transformer, then!
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Old 17th Apr 2014, 3:10 pm   #34
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Thanks Chris.

A week or so I built a LOPT ring tester and loaded it on to the transformer with no connections and got the attached oscilloscope trace. I had a similar trace on a SMPS transformer I had kicking about. It gave me hope it might not be the single thing in the EHT that was near impossible to change.

The feedback winding also had a good ring trace.

I never did the same on the secondary side because the backup scope decided to break around that time.

Tony
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Old 17th Apr 2014, 3:54 pm   #35
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

It may be that there's something in the transformer which is only breaking down at higher voltages, so it looks OK on a ring test but doesn't work in practice. I can't think of any other explanation, I'm afraid.

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Old 17th Apr 2014, 5:27 pm   #36
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

Yes that is true. That's why I took it apart to have a look for any trace of arcing, I couldn't find any.

Looks like I will have to get in touch with Dickie.

Tony.
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Old 17th Apr 2014, 10:11 pm   #37
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

OS3000. I find I agree completely with Chris cmjones01 analysis of point 11 posted today. But surely, that is the position with no feedback, and is what I would expect.
What happens if we connect the feedback from the cathode / focus control chain via R802.
Surely that will now pull point 11 down and bring the feedback loop through TR816, 815, 812 into action.
If it is stuck at +4v, then clearly there is no feedback. Either no -1.5kv voltage or it is disconnected, or is not getting through.
So it could be faulty transformer, or disconnected by TonyPhs, or resistors R802/804 gone high or opencircuit.
We have already seen that the waveforms from the power oscillator transistor collectors, points 16 and 17 seem possible.
If you suggest that R802 is at -110v, then presumably the cathode supply is still connected. But what about the two resistors R802,804. I have found that under high voltage stress, these frequently go high. This could be why you only get -110v. What is the voltage at the top of the focus control or cathode.
Feedback loops are always difficult to diagnose, for if you open the loop, as Tonyphs has done, then you will get very odd results, as nothing is working as designed.
I note that the SET EHT has no effect. But what is the Cathode voltage Vk, and down the chain R802/804. I wonder if your suggestion of transformer isn't excessively gloomy until we know those other readings.
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Old 17th Apr 2014, 10:31 pm   #38
TonyPhs
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

With p11 floating in the breeze the voltage at the cathode is around -150v or so. It drops across the focus control and again across the R804 and the -110v is on the end of the resistor.

With p11 all wired up the voltage drop is more significant with (from memory) -70v between R804 and R802. The cathode is still -150v in this condition as it always seems to be.

Is the resistor damage significant? It has clearly taken some damage from something.

Last edited by TonyPhs; 17th Apr 2014 at 10:46 pm.
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Old 18th Apr 2014, 10:43 am   #39
WME_bill
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

OS3000 EHT supply.

"With p11 floating in the breeze the voltage at the cathode is around -150v.." So with no feedback loop, the oscillator should be going full blast, getting very hot and producing over the 1.5kv at CRT cathode.
So I have reached the same point as Chris jones01.
Have you tried disconnecting the CRT, by unplugging the base, and then connecting the feedback top end, at R803 to the Vg high terminal on the transformer, and earth the other Vg low. This would identify the Cathode winding as faulty, but if the Grid winding is OK, you will get your 1.5kv and then be able to confirm the rest of the regulator/oscillator.
An alternative is to lift point11, and feed a negative voltage into the base of TR816/R816. Put a pot across the -15V rail, and move it from max negative towards zero. This should force the feedback to vary the supply to the oscillator, and so vary the output.
However, I would still like to know if you have checked the resistors in the feedback chain. Lift the common end of R802, 804 Check them and also R803.
Resistor damage could indeed be significant. R802+R804 should be 2M2. On some models, it is a single resistor, a large high stability carbon film, which is traditionally rated for nearer 1000v. The small carbon film is often 400v/500v. Smaller metal oxide or moulded carbon are rated at 200-250v or less All types willl shift under long term stress. That is why later versions seem to use two resistors, as your does, as this splits the voltage.
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Old 18th Apr 2014, 12:30 pm   #40
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Default Re: Another Oscilloscope issue, this time a Vintage Gould OS3000.

I haven't tried anything involving disconnecting the CRT base connector.

I gave the second option a try.

Lifted P11 and fed a negative voltage from a pot and -15v rail into it. I measured the voltage across the pot as -13.5v but as soon as I attached it to Point 11 the voltage I read dropped to 1.3v. As I turned the pot I could reduce the voltage down below 1.3v but not up beyond it. Turning the pot did have a small effect on the waveform amplitude coming off the TR815 collector but no gain discernible cathode voltage output.

One slight difference I noted is that with P11 connected as normal I was getting 0v on TR811 collector, while with the negative voltage fed in I was getting 19v at the same location.

R804 and R802 measure on my meter as 1000k and 1020k respectively while Focus control measured 567k. This measured with P11 disconnected of course. R802 seems to be ok at meter voltage at least.

R803 seems to be measuring ok as well at 405k (nominally it's supposed to be 390k).

The cathode voltages from yesterday were from memory but I've come back and checked them over again.

With P11 floating in the breeze I get -200 to -220v on the cathode but -150-160v when it is connected. Turning a combination of the EHT and the -15v does seem to have an effect on the output voltage, not a massive effect but it is there. The voltages I'm reading on the cathode seem to change but range from -150v to -220v.

I seem to be getting 55 micro-amps running from through the feedback loop through R802 to P11. When I feed in a negative voltage it takes a lot more, 10s of mA.
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