Curing hum modulation on oscillators

[w3jn]

So, you have an old buzzard receiver that's almost unusable on its top band because every signal has a hum on it?  Or the BFO is sounding kinda rough?  This is caused by hum modulation (it's usually a combination of FM and AM) of the local oscillator and/or the BFO.

Sometimes this is caused by tubes with internal electrical leakage.  A quick swap for a known good tube will tell for sure.

I've cured several radios afflicted with this (including a HRO-60, a NC-183, and a Hammarlund SP-400) by examining the ground return method for the filament lead on the oscillator tubes.  Usually one pin is wired to the filament string and the other is grounded.  Often there is a run of wire from that filament pin on the tube socket thru some other pins and then to ground.  You can cure the hum modulation problem by DIRECTLY grounding that pin to the  closest point on the chassis.  This will reduce the 60 hz AC current flowing thru the other pins of the tube that go to ground and greatly reduce, if not eliminate, the hum modulation.

Finally, some receivers have twisted pairs of wires going to the filament string.  In this case, try bypassing both filament leads to ground witha  .1 uF disc capacitor.  Sometimes this helps and sometimes not.

73 John