BA11 power filter

Tom Hunter ccth6600 at
Wed Jun 2 06:35:59 CDT 2021

Those filter caps also cause other problems. I had now two cases in
succession where an old computer tripped the Residual Current Device (RCD)
in my switchboard. Both used Sprague 0.1 uF filter caps in "bathtub"
configuration in a metal can. Each cap is connected on one side to ground
and the other side is connected to active and neutral respectively. These
leak more than the 30 mA tolerated by my RCD which is compulsory in all
mains switchboards in Western Australia. I think that the capacitors are
not actually faulty but 0.1 uF is simply too large. The filter design
predates RCDs. There are "proper" mains filters which don't exhibit this
problem with RCDs, but of course they mechanically won't fit where the old
filter caps were.

Best regards
Tom Hunter

On Wed, Jun 2, 2021 at 4:17 AM 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.
> Cheers,
> Steve

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