Shugart 851 power problem

From: Dwight Elvey <elvey_at_hal.com>
Date: Wed Apr 26 13:22:31 2000

"Richard Erlacher" <edick_at_idcomm.com> wrote:
> Well, That leaves little room for doubt . . . it's probably a switcher.
> I've never seen a linear supply from ASTEC. The small transformer and small
> capacitors suggest the same conclusion.
>
> Dick

Hi
 I agree with Dick, it sure sounds like a switcher. The
small transformer and the high voltage warning on the
heat sink are good indicators. I still think it is
a problem in the supply some place. I once had a similar
problem in a switcher for a monitor. I never did track
it down to the exact part that was causing trouble.
My excuse was that I didn't have the schematic and besides,
I did a hack to get it to work. I found that I could get
it to power up if I disconnected the filament of the tube
until it brought the power up. I put an extra push button
on it and my younger brother inherited it later. All in all,
it had about 6 years life instead of 1.5. It wasn't a
recommended fix but it got me by. I'm surely not that
proud, only indicating the options that one could take.
 In switchers, the filter capacitors on the output side
run real close to their limits as far as power dissipation.
This isn't as much a problem on linears. Try tacking
an additional capacitor on the output filter of the 24 volt.
If it is what I think it is, the voltage is coming up
too fast, triggering a current shutdown. The reason it
restarts after cycling is that there is a little charge
left on the load caps and that reduces the power on surge.
 Look for the rectifiers on the output of the transformer,
that is where the capacitors are.
Dwight
Received on Wed Apr 26 2000 - 13:22:31 BST