1 Dec
2004
1 Dec
'04
11:08 a.m.
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.
YEs, quite a common soft-start circuit.
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!
Argh!. You want to find the idiot who built that system and connect his
private parts to selected points on the mains side of that PSU :-)
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.
It's not at all uncommon for the regulation to be taken from the 5V line,
and the 12V lines either to 'tag along' or to have linear regulator
circuits. I nthe latter case, particularly, the 12V lines will only work
if there's a significant load on the 5V line, otherwise the PSU runs at
such a low duty cycle (to keep the 5V line right with no load) that the
12V lines are very low.
I would therefor try a respecable load on the 5V line and see what happens!
-tony