This happens all the time when somebody replaces the filter caps with the
typical modern ones that have the case connected to the negative lead. The
original capacitor probably had two leads, neither one grounded to the case.

You may need to insulate the capacitor case from chassis ground.


This filter cap is an actual new old stock in the old style case
which sits in an octal socket isolated from ground. I can try pulling
the cap and see if the B- line goes up to 500 ohms above ground.