CRT Scope trace scale affected by intensity - PM3394

[EHT]

Hi All,
my PM3394 'combiscope' has a fault which I wonder may be commonly recognizable in CRT scopes. I've checked quite a few things which are all as expected so I'd be interested if anyone can recognize this type of behaviour.

First, I noticed that during the Screen Calibration procedure (manual - sect 7.6), at the maximum value (63) the y-gain is not quite enough to get the markers at the right scale. Whilst trying to get the dark levels right (sect 7.2) I've now found there is an easily reproducible fault with the CRT circuitry:

If i turn up the trace intensity and text intensity fairly high, the X, Y gain of the whole CRT image becomes unstable. Both X and Y scale reduce slowly until the image is around 75% of the full screen, then it will suddenly reset to full scale and repeat with a period of 2-5s depending on the intensity level. Maybe a clue that I can hear a click when this reset happens. This affects both the scope trace, in A or D mode and the on-screen text. As it is affecting both X and Y i was thinking it was a supply problem but it seems not...

I've checked a few things to see if i can see signs of this instability but so far drawn a blank. Checking all of the points below, there is no sign of the instability:

- Main PSU circuit around T1001 is stable - checked 58V, 12V o/p and test points MV2, 6, 7.
- EHT driver circuit, test point MV9, MV11 (200V p-p), no fluctuation in voltage or frequency
- CRT 'modulator' Diagram 5, V4042, V4041 is 77Vpp 165kHz, 
- YPREOUT0, 1 (pin 21, 22 D1001)
- Y Plate output X2103, X2104.

(p.s. won't be back to debugging it 'til Monday)

[George Edmonds]

CRT gone soft?

George G6HIG

[David Hess]

If the vertical and horizontal deflection are both decreasing, then the CRT acceleration voltage is increasing, which leads to the electrons not spending as much time between the deflection plates.  Oscilloscope CRTs use a regulated CRT acceleration voltage for good deflection accuracy so something is wrong with the regulation.  The snapping is high voltage breakdown somewhere.

[EHT]

Thanks very much David and George. I thought it might be the high voltage supply. I've attached the schematic. I tested the primary side at test point MV11 and couldn't see the instability; it was 200V p-p. I think since I did this measurement that I have been able to make it fail in a more extreme way so I'll check again.

It seems odd that the voltage is increasing when it is failing, but maybe it is only increasing to the level that it should be , and I've turned up the X, Y gain adjustments to compensate - i.e. its is normally too low voltage? I may be able to measure the centre-tap T1002 pin 3. Also I guess i could loop some wire around the CRT HT supply lead as a transformer and see if I can detect the sudden snap-back pulse there (I appreciate it should be DC if working correctly).

I don't know how to identify failure of the CRT itself ("soft/gassy"). If there is something wrong with the voltage multiplier circuit, given it is partially working, I suppose that's one of the caps or the sealed multiplier unit. That would be a pain as I doubt it was a standard part.

[shakalnokturn]

It does seem funny, not impossible though, that HV would rise while turning brightness up. That would be a HV regulation problem.

The other option (I haven't been through the schematics to check) is that the higher brightness is loading a the PS and causing one of the voltages needed for the deflection amps to sag.

While you're in there, if you have the equipment and your scope has the MathPlus option I'd be very grateful if you could upload a ROM dump that I could experiment with.

[David Hess]

I tested the primary side at test point MV11 and couldn't see the instability; it was 200V p-p.

Ok, I think I know what is going on, and only because of some experiments I did concerning CRT post acceleration voltage on my Tektronix 7603.

What I said about the deflection decreasing when the CRT acceleration voltage increases is true in general but the CRT post deflection acceleration voltage which is produced by the high voltage multiplier may work the opposite way in some CRTs.  A couple years ago I ran some experiments on my 7603 where I deliberately *lowered* the post deflection acceleration voltage and it did decrease deflection; this is related to scan expansion.

Your test results for MV11 show that the acceleration voltage is regulating correctly.  So what is going on is that the voltage at the output of the high voltage multiplier is sagging with increased beam current.  That and the snapping indicate a problem with the high voltage multiplier itself; close inspection will likely reveal a break in its insulation and scorch marks where a discharge is occurring.  Before removing it,  inspect it in the complete dark while operating; there is a good chance you will be able to see where the failure is from corona discharge and the snapping.

[EHT]

It does seem funny, not impossible though, that HV would rise while turning brightness up. That would be a HV regulation problem.

The other option (I haven't been through the schematics to check) is that the higher brightness is loading a the PS and causing one of the voltages needed for the deflection amps to sag.

I had suspected both of these but haven't been able to observe this..

While you're in there, if you have the equipment and your scope has the MathPlus option I'd be very grateful if you could upload a ROM dump that I could experiment with.

I'm not sure it has Math+, will check

[EHT]

Ok, I think I know what is going on [...]
the voltage at the output of the high voltage multiplier is sagging with increased beam current.

Great, thank you for the insight David! That does make sense. I'll check on Sunday night or Monday when I'm back.

[David Hess]

Ok, I think I know what is going on [...]
the voltage at the output of the high voltage multiplier is sagging with increased beam current.

Great, thank you for the insight David! That does make sense. I'll check on Sunday night or Monday when I'm back.

It actually does not make any conventional sense, which is why I spent some time experimenting with it.  I remember now what I did.

We had questions about exactly how an oscilloscope CRT with post-deflection acceleration would behave with no post-deflection acceleration.  (1) We had several examples where post-deflection acceleration had failed but reports were not consistent.  I happened to have my Tektronix 7904 sitting next to my 7603 so what I did is operate the 7904 with normal post-deflection acceleration, no post-deflection acceleration, and roughly half post-deflection acceleration by using the 7603 high voltage output to power the 7904 CRT.

The results were that deflection was roughly proportional to post-deflection acceleration, but inversely proportional to cathode voltage or pre-deflection acceleration.  This counter intuitive result is because of how the scan expansion mesh interacts with the electric field produced by the post-deflection acceleration and one of the reasons that CRTs which use post deflection acceleration have a higher deflection sensitivity than would be normally expected.

Anyway, your problem is likely failure of the high voltage multiplier which is annoying but probably manageable.  If you cannot find a replacement, then one can be fabricated with some effort.

(1) Without post-deflection acceleration, sharpness decreases, brightness decreases, and as discussed above, deflection sensitive decreases *if* scan expansion is used which will usually be the case.

[EHT]

I noticed the arcing sound was coming from the anode connector on the tube rather than the tripler. Then I found the cap was a bit loose and came off the tube easily. Eventually I got it correctly reseated properly and this fixed the problem!  Sorry to waste everyone's time looking for a complex explanation!

BTW the HV lead and the CRT both needed discharging even quite a long time after switching off.

[David Hess]

I noticed the arcing sound was coming from the anode connector on the tube rather than the tripler. Then I found the cap was a bit loose and came off the tube easily. Eventually I got it correctly reseated properly and this fixed the problem!  Sorry to waste everyone's time looking for a complex explanation!

All is well that ends well then.  It was not a waste of time since the mystery is revealed and the instrument is repaired.  I would clean the CRT envelope around the anode lead.

BTW the HV lead and the CRT both needed discharging even quite a long time after switching off.

That might vary considerably between oscilloscope CRTs.  My Tektronix CRTs have less then 200 picofarads of capacitance at the anode and discharge takes less than a minute.  Internally in the CRT there is a spiral resistive element on the inside of the envelope from the anode to ground which controls the field intensity.  Residual emission from the cathode also discharges the anode.

Of course full high voltage safety precautions should always be followed.  I leave the anode lead shorted to chassis ground when it is disconnected at the high voltage multiplier.

[HighVoltage]

These Fluke scopes keep their HV charge a long time, even days after they have been turned off.
It is always a good idea to discharge before touching.

Depending on how old you scope is, it might have collected dust and dirt around the tube.
Make sure to clean it all before you put it back together.

[EHT]

These Fluke scopes keep their HV charge a long time, even days after they have been turned off.
It is always a good idea to discharge before touching.

Yes, I did discharge it but still was surprised to see a strong spark given it had been powered off for a long time!

Depending on how old you scope is, it might have collected dust and dirt around the tube.
Make sure to clean it all before you put it back together.

Thanks, I cleaned around the anode connection before replacing it.