Sudden death in the family :(

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Sep 11 17:06:00 2003

> I just powered up a friend's PDP-11/53, which had been sitting in my
> living room for, er, quite a while, and... nada. Fans dont start up,
> not the usual racket when one powers up a BA23. The light in the power
> switch turns on, though. I can hear a very soft "tick-tick, tick-tick"
> sound in the power supply, and I do see how the fans get started, but
> very quickly die down. Likewise, I see several LED's very briefly
> turn on, and then die off again.

THis is the classic sound of a SMPSU that's detecting a problem and then
shutting down to protect itself and/or the load.

The fault may not be in the PSU -- it could be a short on one of the
outputs -- say a shorted decoupling capactior on one of the boards or
something. I think I'd start by putting the supply on a dummy load (at
least on the +5V and +12V rails), with all the logic boards pulled. See
if the supply works OK on its own. If not, then you need to troubleshoot
the supply (it's probably a defective capacitor!). If the supply works on
dummy load, then put (say) the CPU and memory boards back and see what
happens. Add boards one at a time until the PSU shuts down, then find the
short on the last board you added.

-tony
Received on Thu Sep 11 2003 - 17:06:00 BST