I’ve finished refurbishing a Heathkit SB-401 transmitter, but have one
last problem to fix: 60 Hz tunable hum audible on the output. (Hum
modulates the audio.) I’m using it as a “slave” to an SB-301
receiver, so it uses the various oscillator signals generated by the
receiver. The receiver performs flawlessly, so I know those signals
are clean. Some comments and questions:

1) Heater-cathode leakage is a possibility. I plan to check this by
swapping tubes, but spares won’t arrive in the mail for a day or two.
However, if one tube has this problem, can it foul up the cathode
emission on all other tubes sharing the same filament transformer,
thus making all tubes generate hum?

2) If I pull the mic amp tubes the hum is still present on the
unmodulated signal. However, I notice that plugging a microphone into
the transmitter can make the hum worse.

3) Are there any problems particular to the SB-401 that may cause
this? (Just trying to tap into the collective wisdom out there in
Vacuumland.)

I’m trying to get this unit ready for Field Day (this coming
Saturday), and I know hum problems can be difficult to fix--Help!

-Dave Drumheller, K3WQ