[Boatanchors] Receiver Hummm --

[GRANT YOUNGMAN]

Thanks .. that gives me something else to look at that I hadn’t thought of.

The receivers are in a new location in the house with a poor indoor antenna and no direct “station” ground system.  My station is currently torn down (to make a long story short — getting ready to relocate).  I didn’t have this issue previously with either of the radios I mentioned, although they’ve been sitting idle for a while.  Another receiver (a GPR-90) doesn’t show the hum in the same environment.

Guess it’s time to start toggling breakers in the house — see if I can find a possible source.  I suppose the temporary receive wire I currently have strung around the ceiling could be the root of more than a little evil … 

Grant NQ5T




> On Mar 4, 2016, at 9:08 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
> 
> 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