George,
Your PM3 has what the OTs called tuneable hum. The cause and solution for
this problem was well known in the 30's and even the 50's (see Don Mix's
"Novice Special" in QST about 1955 or 56). It was rediscovered when the
direct conversion RXs became popular in the 70s.
RF from the oscillator (or the oscillating detector in the case of a
regenerative receiver) gets into the power supply and is modulated by the
rectifiers. This modulated RF is then detected by the receiver. Naturally
this effect is frequency dependent and tuning the detector will make it
better or worse, hence the term tuneable hum.
Solutions include electrostatic shielding between the primary and secondary
of the power supply, RF bypassing at the rectifiers, and RF chokes on the AC
lines and the DC lines of the power supply. Usually the RF bypassing is all
that you need.
Bypass the input and output of each diode rectifier to ground with a 0.01
mfd cap. I have also see articles where a bypass capacitor was put across
each rectifier. The object is to suppress the modulation of the RF.
About 10 years ago I acquired a Heath GR81 2-tube regen. The tuneable hum
was awful. I could not understand how anyone ever used a regen as a regular
receiver in the 30s and 40s. So I bought two 0.01 mfd/1000V disk caps from
RS and put them from the cathode and anode of the 35W4 to chassis ground.
The improvement was INCREDIBLE. So if any of you find an old blooper at a
flea market, that should be your first mod after you get it home.
I have read of tuneable hum cases occurring even when the RX was powered
from a battery! The solution in that case is usually to bypass the
rectifiers in the other power supplies in the shack.
73,
Tom Hamblin, VA3TH, VE3HIE, sine HN
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