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CRT scope high voltage supply problem?

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Re: CRT scope high voltage supply problem?

Postby Folderol » Fri Feb 14, 2014 12:31 am

Actually, it's a weird circuit anyway!

Other likely candidates are R131, RV14B, RV20B and to a much lesser extent R129. If any of these have gone high resistance there will be a more positive voltage on the base of Q40, which will conduct more and thus cause Q41 to also conduct more, driving Q42, the oscillator, harder.

Doncha just love DC coupled circuits :)
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Re: CRT scope high voltage supply problem?

Postby ef37a » Fri Feb 14, 2014 8:03 am

Folderol wrote:Actually, it's a weird circuit anyway!

Other likely candidates are R131, RV14B, RV20B and to a much lesser extent R129. If any of these have gone high resistance there will be a more positive voltage on the base of Q40, which will conduct more and thus cause Q41 to also conduct more, driving Q42, the oscillator, harder.

Doncha just love DC coupled circuits :)

Weird indeed Will!
Generally such generator circuits have an obvious feedback path from the HT side. This is usually a high value resistor, often 1 to 3.3meg and they used to go high/O/C in TV PSUs with devastating results.
I can see no such feedback here and the whole thing seems to run open loop on a wing and a prayer!

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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Fri Feb 14, 2014 8:03 am

Many thanks for your help with this, Will.
I replaced C72 - no joy.
R131 did indeed measure high, over 5 meg, but replacing it caused no change to the base voltage on Q40 which remains at -13v. The other resistors check out OK.
Might be time to start replacing transistors?

(From my basic understanding of transistors, Q41s emitter should not be more negative than its base, it should be more positive.
And likewise, Q42s base should not be more negative than its emitter, it should be more positive, non?)
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Fri Feb 14, 2014 8:07 am

ef37a wrote:I can see no such feedback here and the whole thing seems to run open loop on a wing and a prayer!
Dave, would the feedback you're talking about be the connection via R131 (2.7Mohm) to the base of Q40?
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Re: CRT scope high voltage supply problem?

Postby ef37a » Fri Feb 14, 2014 8:14 am

Adam Inglis wrote:
ef37a wrote:I can see no such feedback here and the whole thing seems to run open loop on a wing and a prayer!
Dave, would the feedback you're talking about be the connection via R131 (2.7Mohm) to the base of Q40?

It could indeed!!! DOH! HOW did I miss that? There is a V set pre-set as well! If it turns out to be that resistor and the pre-set R is cheap, open frame thing I would replace it with at least a new enclosed type or in fact go for a multi-turn Cermet device, only a pound or so.

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Re: CRT scope high voltage supply problem?

Postby Folderol » Fri Feb 14, 2014 6:15 pm

I must say it is a very badly drawn schematic. It took me quite some time to work out what was what.


From my basic understanding of transistors, Q41s emitter should not be more negative than its base, it should be more positive.

Now that's rather interesting.
Q41 emitter should be at 0V and it's base should be about -0.6V. If the base is positive it means that Q40 and the resistor chain feeding it is actually trying to regulate the drive. I wonder if Q41 has gone S/C collector - emitter.



And likewise, Q42s base should not be more negative than its emitter, it should be more positive, non?

Q42 is an inductively coupled oscillator circuit, and as such the average base voltage is very likely to be negative with respect to the emitter.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Mon Feb 17, 2014 9:17 am

Well, this is embarrassing..
In replacing those transistors, I realise I incorrectly identified their pinouts when I took those voltage measurements.
So now they are replaced, and after tracing and double checking the board and the schematic, the voltages are
Q40 collector 10v, base -24v, emitter -13v
Q41 collector -15v, base 10v, emitter 0 v

in other words, little change. High voltage side of things much the same.
The large negative voltage on the base of Q40 looks wrong to me - I wonder if C90 could be leaky pulling the base towards the -1000v. I'm still looking for high voltage caps in these values - they're getting thin on the ground.

Actually the base-emitter voltage difference on both these transistors looks wrong.
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Re: CRT scope high voltage supply problem?

Postby Folderol » Mon Feb 17, 2014 6:34 pm

I'm relatively happy about the Q40 voltages. These seem 'correct' assuming the circuit is trying to pull back the voltage, but is prevented from doing so due to some issue with Q41. I'm puzzled by its collector voltage. If the transistor is in cutoff (as the base voltage suggests) the collector should be well below Q42 emitter voltage - R122 is trying to pull it down to -50V.

My next suspect is C71 (especially if it's an old wax paper type). If it is leaky it could be supplying uncontrolled current to Q42 base.

Another far more remote possibility is R122 itself being O/C. On very rare occasions, if there is no resistor to draw away current, there can be enough base-collector leakage to bias a power transistor like this into operation. I assume there actually is -50V on the 'bottom' end of this resistor.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Wed Mar 05, 2014 9:57 am

OK, so I'm in the process of ordering replacement caps for this thing, but I'm having a problem with caps C78 and 79, on the high voltage side of the coil.
Schematic here.
Image here.
What kind of caps are these?
The schematic says they are 0.033uF rated at 1 kV, but the image shows the numbers "0.033/10/1600"?
... and yet, looking at the circuit, surely they must have something close to 4000 volts across them in normal operation??
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Re: CRT scope high voltage supply problem?

Postby ef37a » Wed Mar 05, 2014 10:58 am

Adam Inglis wrote:OK, so I'm in the process of ordering replacement caps for this thing, but I'm having a problem with caps C78 and 79, on the high voltage side of the coil.
Schematic here.
Image here.
What kind of caps are these?
The schematic says they are 0.033uF rated at 1 kV, but the image shows the numbers "0.033/10/1600"?
... and yet, looking at the circuit, surely they must have something close to 4000 volts across them in normal operation??


No because they are in series with C76 C77 and they are both much smaller in value and rated at 4kV so I would estimate that those caps have well under 1kV on them. But! Tis a bugger of a circuit to read!

Oh! ***! the bottom end of the .033s goes to deck!

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Re: CRT scope high voltage supply problem?

Postby Folderol » Wed Mar 05, 2014 7:56 pm

The ones in place for C78 & C79 are actually 1.6kV rated. As Dave says the circuit is very hard to read but they go from ground to the -1kV line.

Personally I very much doubt these two are faulty. Similarly I would be surprised if C76 or C77 were the problem. Remember these voltages are too high. If the caps had gone leaky I would expect them to be too low.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Thu Mar 06, 2014 7:12 am

Ah, thanks, I see now - I thought that was a crossing and not a connection at the top of those caps.
Ok, so it is probably a fault on the low voltage side of things.
I have measured the oscillation at 26kHz, when it should be 22 kHz, but I suppose that is just telling us what we already know.
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Re: CRT scope high voltage supply problem?

Postby Folderol » Tue Jun 17, 2014 11:00 pm

Any developments on this?
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Fri Jul 04, 2014 2:24 am

In a word... No!
I needed a scope to fix a few things around here, so I put that job aside, and bought one of these new-fangled digital scopes - a Rigol DS2070... which did the trick and the jobs got done, but now the NEW scope is on the fritz!
Unbelievable... :madas: (with steam coming out ears)
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Re: CRT scope high voltage supply problem?

Postby Folderol » Sat Jul 05, 2014 5:16 pm

Hmm. You haven't done something to offend the god of oscilloscopes, have you? {Terry Pratchett ref.}
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Mon Jan 14, 2019 11:27 am

So, still knocking down those NY resolutions, I've returned to this, 4 years later.

I replaced the 3 rectifiers on the HV side and lo and behold, my two beams have returned. I can move them vertically on the screen and focus them, however, they do not move across the screen with any timebase setting, so I'm assuming I need to look at my horizontal amplifier circuit.
Still, I'm getting somewhere...!
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Re: CRT scope high voltage supply problem?

Postby Folderol » Mon Jan 14, 2019 5:22 pm

I'd quite forgotten about this one :shock:
Are you just getting two spots, or actual lines?
If it's lines, it's not the amplifier but the timebase - depending on the design it might still be the timebase if the amplifier is integrated in to it.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Tue Jan 15, 2019 12:05 am

Well they look like spots until you focus them sharply, then it appears they are very tight irregular shapes that don't don't change.

There is a horizontal amplifier section built into the mainframe, and there is a timebase "plug-in" that interacts with it. Both of these have a number of trims and different voltage points that I will need to check against the manuals calibration procedure.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Wed Jan 16, 2019 8:08 am

Spots are now lines, after trimming the various DC voltages. And when the timebase decides to work, we get pretty waveforms!

I don't have an exact schematic for the timebase plug in, but I do have one for a similar unit. Looks like I have some research and measuring ahead of me.

My one concern re the EHT circuit:
the -1000 volt line is measuring around -1600 volts. The other voltages look OK, but I can't trim this one out. The beams do seem to be behaving as expected, responding to the vertical amps.
I wonder if in all my part replacements I've altered the frequency of the coil. Is this a worry if the CRT voltage sits that high (er... low)?
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Re: CRT scope high voltage supply problem?

Postby Folderol » Wed Jan 16, 2019 8:22 am

Excessive EHT is not good, in that it can increase the energy at which the electrons hit the phosphor. It also tends to increase the scanning voltage required for a given deflection so might screw up the calibration.

Having said that, I don't know what the recommended range is for that CRT. What is the actual PDA voltage (+3kV one). That will be more important.
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