Quote:
Originally Posted by mhennessy
If problem is an overload caused by the HV supply, things get tricky. There are a lot of high voltage diodes and capacitors that could be breaking down. And the transformer itself could be suspect. But I note that it generates a 225V rail that is used elsewhere in the 'scope, so an overload on these could be the problem.
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Thanks, Mark, for the insight. Indeed, I'm pretty sure the HV supply is drawing too much current. The increase is measurable from the very beginning, a steady increase that accelerates towards the end. It still takes some seconds between, say, the usual upper limit and the moment the current limiter comes on (approx. 25% above).
I could not find any fault on the 225V rail so far:
- monitoring the current through the series regulator revealed a very low number, suggesting the 225V current is not more than some mA. And it remained steady.
- Except in the HV supply itself, I only found the 225V in the output stage for the deflection plates. I have absolutely no sign of anomaly on the image on the CRT, so I cannot see what could be wrong there.
I noticed the HV transformer getting quite warm at the end, but not hot. Something like 50-60°C according to my thermometer. Wonder if this is OK.
I certainly will check the capacitors and diodes in the HV supply. Measuring while the Tek is running is tricky, and measuring while switched off may not reveal the problem.
What puzzles me is that although it seems to be a thermal problem, it does not react at all when I give the HV supply an extra cooling...
cheers
Martin