Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

 

On Tue, 27 Sep 2016 19:00:14 -0300, you wrote:

As I mentioned above, I decided to take my chances with the ESJA53-12
diodes I found on a loca store...
After I replaced the HV diodes, I noticed a great improvement... but
unfortunately it wasn't the definitive fix...
Now the HV doesn't seem to fail anymore if I run the scope without the
outer casing (cooler).
While this is good news in some sense, it made things more difficult to
troubleshoot.
When I install the outer casing, the time to fail increased from 20~30
minutes to about 1 hour.
I measured the leakage of the original cathode diode (CR1503) and under the
same voltage of about 3600V,
it's 25 times bigger than the 1.1nA that I measured in the new diodes,
but 28nA is still very small leakage and I think that it may just have
given the inverter a little bit more breath
so now it takes longer to fail.
I have not quite decided what to make of this but I have some ideas.
If the diodes were the problem, then I would have expected changing
them to have completely fixed it instead of making it a little better.

What I might try is looking for some other part which is getting hot
causing enough leakage to pull the high voltage supply voltages down.
If you do not have an infrared thermometer to scan with, a cotton swab
dipping in rubbing alcohol could be used to cool suspected parts after
the HV supply starts to collapse to see if it has any effect. Q1486
and Q1484 would be especially good targets for this test.

Freeze spray is what would normally be used for this but I would be a
little leery about using it around high voltages where condensation
might cause problems.

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