31 Jan
2011
31 Jan
'11
1:38 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 31 January 2011 20:02
To: cctalk at classiccmp.org
Subject: Re: PSU for MicroVAX 3400 Not Faulty But Not Working
be
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 open circuit.
through
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?
A capacitor should, in theory, test as open-circuit. It has no DC path it.
Some cpacitors are polarised, and may test lower in the 'wrong'
direction. On most digital metes, the red probe is, indeed, positive on
the
resistance ranges, on most analogue meters the red
probe is -ve.
Hwoever, any capacitor which tests as low as 20Ohms is defective.
I assume you were testign them out of circuit. If they're on the PCB,
there
are all sorts of other things in parallel with them
(like the chips, if
they;re
supply line decoupling capacitors) so you can't
measure just the
resistance
of one capacitor.
-tony
I did not test out of circuit so I will desolder the suspect capacitor and
test it again as it certainly measures differently to the same one on a
working board.
Regards
Rob