BA11 power filter

Paul Koning paulkoning at
Tue Jun 1 15:38:37 CDT 2021

> On Jun 1, 2021, at 4:16 PM, Steve Maddison via cctalk <cctalk at> wrote:
> After a few years in storage I'm finally getting to play with my PDP-11
> stuff again. Figured I'd start small so rolled out the 11/23. PSU looks
> fine, pulled all the cards and checked voltages, no problem. Shortly after
> however, some magic smoke was released from the block in the back where AC
> comes in and the power switch is mounted. The only other thing in there is
> a big old line filter. Judging by the smell, I suspect a filter capacitor
> has given up the ghost. I'd check, but the whole thing is soldered shut so
> not exactly easily serviceable. Would I be correct in thinking such filters
> aren't entirely necessary and could just be bypassed? It looks like the
> manufacturer (JMK) is still going so a new replacement might not be out of
> the question. Whether I could find one with the same mounting holes etc. is
> another question I've not looked into yet.

I'm not sure if you're talking about the same sort of device I had trouble with, so FYI: the Pro power supply is a box the size of an old-fashioned Bible, with a CEE power inlet at one end.  That inlet is actually part of a sealed metal box containing a power filter.

Mine decided to let out its magic smoke, in quite spectacular fashion, stinking up the workshop and leaking evil looking black tarry stuff over the insides of the power supply.  But the supply appeared to be intact in other respects.

I couldn't find a matching filter, so instead I found a functionaly similar filter that's physically smaller.  I then created an adapter plate with a hole to accept the new filter, and a flange sized to match the old one.  My sheet metal skills aren't great so it doesn't look all that professional but it's plenty good to get the job done.

FYI, the new filter is this one: -- about half the size of the one it replaces.


More information about the cctalk mailing list