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

wrote in message
...
I’ve finished refurbishing a Heathkit SB-401 transmitter, but have one
last problem to fix: 60 Hz tunable hum audible on the output.


If it is "tunable" hum, heard only locally, why are you worried about
it? That is being generated in the receiver, not the transmitter. Do
distant stations hear the hum? That is the question.

On Jun 22, 9:26*am, "Tio Pedro" wrote:
wrote in message

...
I’ve finished refurbishing a Heathkit SB-401 transmitter, but have one
last problem to fix: 60 Hz tunable hum audible on the output.

If it is "tunable" hum, heard only locally, why are you worried about
it? That is being generated in the receiver, not the transmitter. Do
distant stations hear the hum? *That is the question.

Sorry, I wasn't clear. When I listen to the SB-401 transmitter on a
shortwave receiver, I hear hum modulation. The SB-301 receiver, which
drives the SB-401, works fine.

-Dave Drumheller, K3WQ

wrote:
On Jun 22, 9:26 am, "Tio Pedro" wrote:
wrote in message

...
I’ve finished refurbishing a Heathkit SB-401 transmitter, but have one
last problem to fix: 60 Hz tunable hum audible on the output.

If it is "tunable" hum, heard only locally, why are you worried about
it? That is being generated in the receiver, not the transmitter. Do
distant stations hear the hum? That is the question.

Sorry, I wasn't clear. When I listen to the SB-401 transmitter on a
shortwave receiver, I hear hum modulation. The SB-301 receiver, which
drives the SB-401, works fine.

Be sure that what you are hearing isn't the shortwave receiver being
overloaded by the transmitter. I find it is rare when I can receive
a local tube type transmitter with a local receiver and *not* hear hum.

-Chuck

wrote in message
...
On Jun 22, 9:26 am, "Tio Pedro" wrote:
wrote in message
Sorry, I wasn't clear. When I listen to the SB-401 transmitter on a
shortwave receiver, I hear hum modulation. The SB-301 receiver, which
drives the SB-401, works fine.

-Dave Drumheller, K3WQ

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

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

Hi Dave.

Over the years I too have run into the same situation and what the
fellows here are telling you is most likely correct. I can also tell
you the you don't need any antenna to pick up that transmitter when it's
that close. In fact you'll do better if you don't use an antenna or
something very short to get a better picture of what your xmtr sounds
like. This is one time where an RF envelope monitor scope comes in REAL
handy. If you have hum on the transmitted envelope you'll see it on the
scope.
Kim W8ZV
wrote in message
...
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

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

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

Well, I solved the problem. It was feedback due to close proximity of
the transmitter and receiver, just as Chuck, Tio and Kim suggested.

Tio recommended an on-the-air check, and that’s what I did. I erected
a makeshift 80-meter antenna in the backyard--good practice for Field
Day--and called CQ. A station in New York replied. He reported that
my audio was fine, with no hum modulation.

What a relief! Thanks again for all the suggestions.

-Dave Drumheller, K3WQ