[Boatanchors] Receiver Hummm --

[Rob Atkinson]

your problem is probably something known as "tunable hum."

It goes like this:

Diode rectifiers in a power supply somewhere switch on and off with
the 60 cycle AC, doing what they are supposed to do.  Your transmit
carrier appears on the power line to the diodes (which are not
isolated by a transformer in the case of a switch mode supply, often
in a computer) and your RF also gets switched on and off at 120
cycles.   This modulates the RF at that hum frequency, or 60 cycles if
it is a half wave rectifier, and that modulated RF is re-radiated.  If
your receiver is close to the line doing the re-radiating, your
receiver when tuned to your transmit frequency will receive the
modulated RF, i.e. the hum.

Ever drive under an AC service drop with your car radio tuned to a
strong AM station and hear hum on it as you pass under the drop but
the hum rapidly diminishes with distance from the drop?   Same thing
going on.  Tune away from the station and the hum vanishes and may not
be audible on a weak station.

Get a portable battery powered receiver and tune it to a real strong
AM broadcast station and see if there is hum on its carrier, or
transmit one yourself at say 20 watts if you can.  Pick a dead band
and empty frequency and do an ID and walk around with the receiver and
see where you get the hum.  The problem could be someone else's device
in another dwelling if you share a distribution transformer secondary
with other people.   In that case, chances are you'll never find it
and you'll have to live with it.

By the way, the fix is simple:  Bypass caps to ground ahead of the
diodes, or around them to route the RF so it doesn't get switched on
and off.  Someone made a power supply and didn't want to spend any
money on those capacitors.

73

Rob
K5UJ

On Fri, Mar 4, 2016 at 8:45 AM, GRANT YOUNGMAN <nq5t at tx.rr.com> wrote:
> I have two receivers which both exhibit the same hum issue to one degree or another.   I’m just not sure where to look.  One is a 51J-4, the other a Hammarlund PRO-310.  The J4 is the worse of the two.
>
> Normally, when not tuned to a carrier there is virtually no hum in the receiver audio.  Both radios have new good power supply caps.  The hum — which is 60 Hz — only appears when a carrier is tuned, and appears to be somewhat proportional to carrier strength.  So I’m guessing something around the detector/AVC or a bias (half wave) supply, although even with AVC off it still occurs. The source is before the audio gain control — so detector/AVC/ANL (?).
>
> Any suggestions where I should poke first?  I’m guessing this might be a fairly common problem, although I haven’t run into it before.
>
> Thanks … Grant NQ5T