Re: Dang'd old electrolytics again?


Robert Morein <morepub@...>
 

I had a 7904A that would not start up with a full load of plugins.
I removed the plugins, started up the scope, ran it for a couple hours.
I replaced the plugins, started it up, and ran it overnight.
No problems since.

It suggests to me that there was a cap seriously leaking in the power supply, that it managed to heal itself, by blowing off whiskers or reforming, and that it was, incredibly, a "good repair."

This doesn't happen often :).

----- Original Message -----
From: Howard Matthews
To: TekScopes@yahoogroups.com
Sent: Monday, January 12, 2004 9:30 PM
Subject: [TekScopes] Dang'd old electrolytics again?


Risking turning this group from a Tek scopes group to an
electrolytic capacitor group, I have questions.

I bought a "fixer" 454. Initial review showed chan 1 working okay,
but chan 2 not. After it sat a few months in my "shop", I started
working on it. First I checked and set all the voltage supplies.
They were all within spec, and now right on. However, now both chan
1 and chan 2 were visible ONLY using the beam finder. They were
both offscreen high.

So, I started tracing the voltage levels and signals in the chan 1
preamp first. A number of DC levels were off enough to throw the
trace off screen. And there was disturbing noise on top of the
signal. But before I could pin-point the source of the problem(s),
chan 1 starts working! It drifts around a bit, but the bias and
noise problems are gone.

Turning to chan 2 - guess what. After a few hours of trying to get
consistent readings and find the source of the problem, BINGO - chan
2 is working.

I left the scope powered off over night. The next day, both
channels are still working and now quite stable. No noise, no drift
after warmup.

The power supplies had and have NO problems. There are a number of
small bypass electrolytics in the preamp circuits which I did not
get around to checking.

Questions:
1. Is it likely that some electrolytics were "reformed" while I was
looking for the problem.

2. If so, is it likely the electrolytics will fail again soon, if
not used frequently? Might they have poor ESR, or high leakage now?

3. Should I pull the little electrolytics in the preamp and
check/replace them? I'm thinking modern tantalums might be good
replacements.

Anyone have similar experiences? Opinions? Advice?

TNX
-Howard





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