PDP-12 Restoration at the RICM

David Gesswein djg at pdp8online.com
Sun May 17 18:48:31 CDT 2015

On Sun, May 17, 2015 at 09:30:59AM -0400, Michael Thompson wrote:
> We received lots of comments on reforming the capacitors in the power
> supply and strong recommendations from experts to just replace them. Since
> new caps are a different physical size, and would cost about $250 we
> decided to continue with reforming the originals. We measured the voltage
> ripple on the backplanes near the power connectors.
For intermittent use I have found most are still useable. I replaced two main
filter capacitors in my straight 8, none in MARCH's straight 8.
My 8/I which is similar age as a 12 has the original capacitors also.
I took some data on the capacitor reforming and final parameters.

>    - -30.0V = -31.2V, 800 mV PTP ripple
Is there a regulator on this that removes the ripple when driving
the memory? That seems really high. The straight 8 limit is 50 mV after
the regulator. Does the 12 manual give ripple limits? 
If it has the Faston 1/4 connectors I have found they can get resistive 
which can cause voltage drop or increase in ripple.

> The 800mV of ripple on the -30 is a sign that the caps in the power supply
> were not working well, and is probably too noisy for the core to work. We
> tried to measure the capacitance of the power supply capacitors using an
> ancient, but very nice, GenRad capacitor meter, but unfortunately the caps
> were too big to measure.
The ripple is frequently ESR and not capacitance.

> After running the system for about four hours the ripple on the -30V was
> down to 180mV, so the capacitors are getting better. More run time will
> hopefully reduce the ripple to an acceptable level. If not, we will have to
> replace them.
Interesting. I didn't think ripple would improve.

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