On Fri, Nov 15, 2024 at 1:04?PM rafael suarez via groups.io <rafik69pe=
[email protected]> wrote:
I’m afraid I don’t have a curve tracer. Perhaps there’s another way to
test them?
The TVSes should measure open in both directions, until you reach somewhere
beyond their threshold voltage. This is 220/250V respectively, which is why
they're challenging to test. If you can somehow provide a current limited
(1mA will do) ~300V DC, you can simply measure the voltage drop across them
in both directions. It should be roughly 250/220V respectively.
If you can't, perhaps best to leave them alone.
If they've failed open, your switching transistor will fail, though it may
take it a while. If it comes to that you'll want to change both TVSen and
the transistor. I seem to remember replacing a BU508, which is easy enough
to source.
On another note, I’ve found a few solid SMD capacitors in bad condition on
the large board visible as soon as you open the scope.
That would be the CPU board. Also look at the acquisition board, the front
panel board and whichever other boards you have in the scope. If the
electrolytic caps are the same make and vintage, the odds are that they've
failed or are going to fail.
There are many threads in this forum about how to clean boards that have
suffered electrolyte leakage. The problem is that traces may have rotted
away, and if they haven't yet, they may still rot away if the electrolyte
isn't cleaned and/or neutralized.
I believe in North America the recommendation used to be to wash thoroughly
with; Simple Green, distilled water, IPA and then leave the boards to dry,
or "bake" them at a very low temperature. Getting under components where
electrolyte may have pooled can be difficult.
There are many stories of people who managed to find and repair broken
traces and probably as many, of people who gave up. My TDS684B(? I forget)
has never yet worked, even though I cleaned it and changed the
electrolytics :/.
Good luck,
these are very capable scopes and well worth putting some time into.
