pdp-8/l supply

Brad Parker brad at heeltoe.com
Mon May 5 15:18:57 CDT 2008

Hi all,

I recently obtained an old PDP-8/L in reasonable shape (why does this sound
like an AA meeting? :-)

Anyway, it doesn't work.  So I'm happy debugging.  The supplies on the
back plane are not right so I decided to pull the main supply.

I finally broken down and ordered an ESR meter.  Is it smart to check
all the caps?

I've never used an ESR meter but it seems like a good idea to try it on
all the big caps just to see what sort of shape they are in. I assume I
need to open the little caps which are in parallel to get a good reading
(my experience, limited, is that the little axial electrolytics tend to
open/short much more often than the big cans).

One of the unregulated supplies (-6) coming off the main transformer was
a volt low when loaded and I thought I'd check the caps and the diodes.
Unloaded it shows -33v (but looking at the schematic, that may be
actually correct.  Loaded the +5 was low (like 4.5), unloaded it's
perfect (5.01).

The lights do light, and run sets the run light, stop stops but load
address does not.  Worse, it seems to toggle the MA, which is ringing a
bell about a common fault.  I figured I'd get the supplies all cleaned
up first...


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