Capacitors on M8722 Board

Rob Jarratt robert.jarratt at
Sat Sep 4 04:34:51 CDT 2021

I have a PDP 11/24 which I hope to power up soon. The last time it had power
I noticed a bit of a burning smell which I failed to track down. However, I
did notice one capacitor on a memory board has a strange appearance, almost
as if there is some corrosion under the surface, it doesn't seem to be
bulging though, except along the top, but if it is bulging it is very bumpy.
I have a picture of it here:
It is marked 47uF 30V, but it is also marked 20v as can be seen in the


I lifted it to measure its capacitance and ESR. It measures about 80uF and
the ESR seems OK. I am unsure whether to replace it, and if so what voltage
rating should I replace it with? I don't understand if this is a 30V rated
capacitor or 20V. It seems that Unibus has some 20V signals, so I guess 20V
might be right? I don't think 30V or 20V parts exist though, so I would need
to get 35V or 25V. Maybe the voltage rating isn't too critical?


Any advice?





More information about the cctalk mailing list