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Thanks, Jean-Paul, Mark, and Tom,

The relaxation oscillator I was referring to is the one that makes the neon
flash which is comprised of R3019-C3019-DS3019.

I was originally writing the above email when it suddenly occurred to me that
after removing the power supply from the scope, I had not connected the dummy
load so, of course, it was ticking! I connected the dummy load and the PS came
to life normally.

I now need to find out what has gone bad in the scope itself. As has been
suggested, it's likely a shorted tantalum and, if I had my guess, it's probably
one of the ones on the main interface board where the sense line connects. At
least now I know better where to look for the problem.

Thanks to all who replied,
Barry - N4BUQ

When there is an overload, the PS shuts down and tries again a bit later. Each
tick is the sound made by the magnetics as current starts to flow then shuts
off. No master oscillator drives the tick frequency. The whole PS is acting as
a relaxation oscillator in response to the overload.

Most likely you have a shorted tant somewhere, but whatever the cause, just
follow the standard debug tree. There’s a terrific writeup for the 7000 series;
it’s been referenced here a lot.

Good luck!

Tom

Sent from an iThing; please forgive the typos and brevity

On Dec 7, 2024, at 15:38, n4buq <n4buq@...> wrote:

?I powered up one of my 7704As which worked fine the last time I used it several
?months ago. This time it worked for about 30 seconds and stopped with the
?power supply going into a rapid tick mode which I presume the frequency of the
?relaxation oscillator.