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?
A capacitor should, in theory, test as open-circuit. It has no DC path
through 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