CRT Varistors blow on a Philips PM3264

[fnpfar]

Hello.

I recently got a Philips PM3264 100MHz oscilloscope. It worked fine for days but before working a couple hours with it one day, white fumes started to come from the back. They were 3x 3kV varistors on the back of the CRT which exploded.

As it is an old oscilloscope I assumed that the varistors were damaged by the time (they are here to absorb high voltage peaks), so I replace them by some new.

I turned on the oscilloscope and it worked fine, but the problem is that before some hours of working, the 3 new varistors have exploded again.

What can I do? I have seen that the scope works without these varistors (they are here for protection), but I dont want to burn the CRT..

Thank you.



the scope

[oldway]

They are 3 x 4700pf 3.3Kv capacitors and not varistors.

[fnpfar]

They are 3 x 4700pf 3.3Kv capacitors and not varistors.

Yes, but I think they act as a varistor absorbing voltage peks. Anyway, what is the source of the problem? Maybe the diode multiplier on the supply? And could I work without these caps?

[CJay]

I'd be very worried about using any circuit that can destroy 3 correctly rated 3KV capacitors.

Did you replace them with 4700pF 3KV capacitors or did you fit some other part?

[fnpfar]

I'd be very worried about using any circuit that can destroy 3 correctly rated 3KV capacitors.
Did you replace them with 4700pF 3KV capacitors or did you fit some other part?

Yes, I replace them with three 4700pF 3KV... that makes me think that the problem isnt here but on the driver of the CRT.

[CJay]

I think it's more possible the CRT has a problem, it may be flashing over or have an internal short.

[bktemp]

I couldn't find the service manual for PM3264, but it is available for PM3262. It looks similar.
There are 3 4.7nF capacitors used as a voltage doubler for generating the negative -2kV for the CRT and another 3 for capacitive coupling for Z-modulation. So all capacitors are connected between -2kV and GND (or other slightly higher voltages).
Does the scope still works when the capacitors start to fail?

Did you check all other voltage rails to see if the regulation of the SMPS is ok?

[oldway]

You must measure the 2KV on C2026 and ajust it with R2048 if necessary.
I have already repaired a lot of PM3264 and I never seen this kind of problem.
If the HV is too high and the trimpot R2048 does not vary the HV, check V2023 (BD232), V2024 (BC549C) and V2026 (BC549B) or replace them.
It seems that the 2KV is too high, above 3.3KV....That could explain the failure of the capacitors.

Be careful with high voltage, that's dangerous.
Use a HV probe, don't try to measure this with your multimeter without HV probe.