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

On Jun 22, 4:07*pm, "Kim Herron" wrote:
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

Thanks, Kim. As recommended by another poster, I think the best
course of action is to use an external antenna and make a few contacts
to get signal reports.

You’re right about the monitor scope. There’s a companion Heakthkit
SB-610 Monitor Scope that I don’t have, but could use right now.

-Dave Drumheller, K3WQ

On Jun 22, 3:37*pm, wrote:
On Jun 22, 4:07*pm, "Kim Herron" wrote:

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

Thanks, Kim. *As recommended by another poster, I think the best
course of action is to use an external antenna and make a few contacts
to get signal reports.

You’re right about the monitor scope. *There’s a companion Heakthkit
SB-610 Monitor Scope that I don’t have, but could use right now.

-Dave Drumheller, K3WQ

You mentioned that when you plug in the mic it changes the hum
level,,This may sound too simple,, BUT,,Always bond all items together
(Rec,xmtr,spkr,etc)and to a good earth ground.They used to call it
GROUND LOOPS, I never figured exactly what a groundloop is ,,I do
understand grounding and I hope you have done that first..OK W4PQW
PS (I think a groundloop is when you deliberately wreck your
airplane at the end of the runway just before you kill yourself..)

wrote:
On Jun 22, 4:07 pm, "Kim Herron" wrote:
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

Thanks, Kim. As recommended by another poster, I think the best
course of action is to use an external antenna and make a few contacts
to get signal reports.

You’re right about the monitor scope. There’s a companion Heakthkit
SB-610 Monitor Scope that I don’t have, but could use right now.

-Dave Drumheller, K3WQ

I'm going to be the killjoy for the easy solution. I had a Galaxy 5
with external VFO that did something similar. I forget all the details
but in one of the modes (trcv or rcvr-xmtr or xmtr-rcvr) I got hum
modulation that was apparent on transmit. I could also hear it on
receive without even activating the xmtr.

A previous owner had shielded the cables inside the VFO and done some
other stuff and apparently struggled quite a bit to solve it. I
couldn't resolve it either. I just accepted not to use that particular
switch position.

Gotta be some weirdo ground-looping issue. I know the older Drakes
(twins) were very persnickety (sometimes) about the interconnecting
cable lengths.

If you do figure yours out be sure to post the solution here. I'd be
very interested.

-Bill..

...
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