On Jun 22, 10:53*am, "Tio Pedro" wrote:

Hi Dave

Yes, I understand. But remember you are listening to the
transmitter in the near field, which means a substantial amount
of RF is being carried on your house wiring. *The hum you
are hearing may, or may not, be actually present in the
transmitter's RF output.

Tunable hum is not the result of a problem in the transmitter
signal, it is the result of RF being carried on the power
lines (common mode) causing a sixty cycle variation in the
RF received signal. *The hum appears same as if the carrier
is being modulated by a signal cycle signal.

You need to have a local amateur (within *a few miles)
listen to you signal to determine whether the hum
modulation is real, or is an artifact caused by RF interaction
with the shack's AC wiring.

Peter, K1ZJH

Thanks, Peter. I hadn't thought of that.

One more point of clarification: The SB-401 is driving a 50-Ohm dummy
load next to the bench. The shortwave receiver (actually, my Icom 718
solid-state transceiver) picks up the signal using a short length of
wire draped next to the dummy load. Do you think this arrangement
induces the hum modulation as you suggest?

This is helpful--and also good news. Maybe nothing's really wrong
after all!

-Dave Drumheller, K3WQ