30 Jan
2011
30 Jan
'11
2:12 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 06 January 2011 20:31
To: cctalk at classiccmp.org
Subject: Re: PSU for MicroVAX 3400 Not Faulty But Not Working
>
> A while back I mentioned that I have a MicroVAX 3400 with one of the
> H7868 PSUs not wanting to work (green light fails to come on). I have
> just tried the "faulty" PSU in a MicroVAX 3500 and it works just fine.
> I took a PSU from the 3500 and put it in the 3400 and it does not work
> in the 3400 either, only in the 3500. So clearly there is something
> else that is causing the problem and I am looking for suggestions,
> especially as the machine is in an awkward location and hard to
> dismantle speculatively. As a reminder this machine was working fine
> and I had not done anything at all to it prior to its failure, I had not
moved
it, changed any components or anything.
My first thoguht is the obvious one, taht something in the 3400 is shorted
and overloading one of the outputs of the PSU that isn't coming on.
Could it be something as trivial (but not trivial to find!) as a
decoupling
capacitor that's shorted?
-tony
I found the problem was indeed the CPU board. I tested the capacitors with a
multimeter and found one to be shorted as the resistance measured zero.
While measuring other similar capacitors I found the resistance to be about
20ohms on the rest, but one seems to be open circuit. The replacement
capacitor I bought (but have not yet fitted) also seems to be open circuit.
I tested the capacitors on a spare CPU board I have and they too were
generally about 20ohms. What values should I expect for resistance? Have I
got a load of capacitors which are all about to fail?
Regards
Rob