One of the things that I forgot to mention is the fact that Heath rigs
like (read engineering requirement buried in one of the manuals) to use
RG-62 cable to interconnect them. That cable is 93 ohm coax and they
really like to all be the same length (memory is returning from antique
radio resto). I ran into something like this back many years ago when I
ran this same set of rigs. One more thing to look at is the 6D10 that
is in the 401. They can develop heater to cathode shorts and the same
thing will happen.
I might suggest a Collins test to check whether you really have a
problem or not. Tune up the rig following the book procedure for full
output. What you're using is the unbalanced modulator in Upper sideband
to create output. Then, without touching anything,
disconnect the mic and ground the pin that actuates the transmitter,
watch the plate and grid current to see if any actual drive exists, and
them listen to it on another receiver. You ground the mic connector pin
with something really short to the case. If your carrier is WAY down
and is fairly clean, and its gets hummy, dirty with the mic connected,
it's your mic or cable. If there is only a small amount of hum there,
when the mic is connected, you probably don't have a problem, you just
haven't done this particular type of thing before and you are, like my
Golden Retriever, chasing your tail.

Kim W8ZV


"Fred McKenzie" wrote in message
...
In article
,
wrote:

I¹ve 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.)

David-

When I saw the word, "tunable", the first thing that popped into my
mind
was "parasitic oscillation"!

You might recheck neutralization of the rig's finals. The manual
probably covers it. One thing to check, is to see if the plate
current
dip corresponds to peak power output.

Fred
K4DII