5 Feb
2011
5 Feb
'11
4:50 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Rob Jarratt
Sent: 30 January 2011 22:12
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: PSU for MicroVAX 3400 Not Faulty But Not Working
about
-----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 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
I was checking the capacitors in-circuit and understand that this is not
correct.
However, I would have thought that a 0 ohm measurement in-circuit would
indicate a shorted capacitor because there would surely not be any point
designing a circuit where both ends of the capacitor are connected together?
That capacitor also measured differently on a working board so it was
clearly suspect. However when I de-soldered it, it turned out to be open
circuit. The resistance in-circuit was not absolutely 0 though, although it
had seemed negligibly close, so it must have been the surrounding load as
has been suggested to me.
Is there a simple way to find shorted caps in-circuit? Would an absolutely 0
ohm resistance while in-circuit indicate a shorted cap for the reason I
suggest above?
Thanks
Rob