[Milsurplus] BC-348 PSU questions

[Ian Wilson]

I am working on a BC-348-O which has had some hammification,
including an AC C-L-C PSU built into the dyno space - classic 5Y3 supply
but using Si diodes instead. The B- was returned correctly to the choke
whose internal resistance generates negative bias for the 41 audio o/p
tube.

I rebuilt the PSU, replacing the diodes and the electrolytics with something
more modern and somewhat higher value.

There are two problems with the result:
  - the B+ is much too high (about 330V - this is with about 45mA load)
  - there is a large amount of hum

Looking at the -ve voltage developed, it has a large amount of hum on
it. However, I don't see how this can be avoided: surely the diode
currents pass through the (300 ohm) resistance in the B- lead? Perhaps
there is a problem with the choke (has about 600 ohm DC resistance; no
idea what the inductance is).

I can increase the capacitor values a lot. However, why does this large
AC component not cause a problem in other BC-348's that I've worked on
with (presumably) similar power supplies? This is the only radio that has
exhibited this problem.

Grateful for any enlightenment.

73, ian K3IMW