9 Apr
2008
9 Apr
'08
2:19 p.m.
Hi,
I have an ICL PC2 CP/M box (like this:
http://www.old-computers.com/museum/computer.asp?c=752) which has a
faulty PSU, I am hoping someone here can advise me.
The PSU is a Farnell N100/F4190 SMPS, looks like a high quality unit
with nice screw terminals for mains in and DC out.
When fired up with 240V AC and a dummy load the output voltages are:
12V output=1.1V
5V output=2.3V
-12V output=-5.9V
The first thing that 'bothers' me is that the 12V output is lower than
the 5V one. If this was a simple regulation fault, I'd expect all outputs
to track.
Do you know which output the crowbar operates on (not which one triggers
it, but which one it shorts out?). It's not uncommon for that to be
something other than the main output (simply becuase it's easier to pull
down), it's possible there's a crowbar thryistor on the 12V output, and
that that's firing.
I assume this thing doesn't have external sense inoputs for the main
output, or if it dows you've conencted them to said output.
I think it's reasonable to asusme the chopper is working (otherwise you'd
get no outputs at all). Does it seem to be running continuously, or do
you get the 'tweet tweet tweet' of a PSU that's starting, detecting a
fault, shutting down, and repearing?
Are tyhe otuptus steady at those votlages? I find an analogue meter best
for this, you can see the needle twitch if the PSU is starting and
shutting down.
-tony