NCR Tower PSU again

From: Jules Richardson <julesrichardsonuk_at_yahoo.co.uk>
Date: Wed Dec 1 15:16:21 2004

Hmm, getting somewhere; found a dead triac near the input which was
supposed to bypass that 10 ohm resistor that was burning out once the
PSU had started up. Presumably the intention was to allow the PSU to
start a little more gracefully.

Nice bodge story: I just found out why the system wasn't starting up at
all. It had been semi-dismantled since it had died years ago; I'd
reassembled it recently when trying to get it running again.

After much wiring tracing I've just discovered although the cable
between the control panel and the backplane is keyed at both ends, at
one end the connector was on back to front. That hadn't stopped whoever
assembled it at the factory from hammering the keyed connector backwards
into its socket!

I hadn't noticed how bent out of shape the socket was when I reassembled
the machine; I'd simply lined up the key on the connector with the
relevant slot on the socket, then spent ages trying to work out why the
PSU wasn't even trying to start up when the switch was operated. Grrr!


Now, I'm getting a good 5.1V output, but there's a nasty wheezing noise
(not a whine!) from somewhere on the main PSU board. The +12V and -12V
rails are seriously up the spout. With a light load (just a couple of
fans), the fans spin up for a couple of seconds before everything shuts
down for a couple more seconds, then the cycle repeats (the wheezing
sounds like breathing at this point!). With no load the wheezing's
constant and the PSU doesn't shut down, but -12V rail is at -4V and +12V
rail is at +5.3V.

I'm going to load the PSU up with an (unwanted) hard disk in a moment
just in case it's not firing properly because of not enough load - not
sure if I want to risk any boards in there at present though, but of
course it's possible the PSU's not operating properly because there's
some feedback from the absent CPU board not present...

Any widsom appreciated. Sure don't like that wheezing noise - cap or
wound component? Fault or just something not being driven at the right
frequency?

cheers

Jules
Received on Wed Dec 01 2004 - 15:16:21 GMT