[Boatanchors] Receiver Hummm --

[GRANT YOUNGMAN]

One other point.  I’m not talking about hum while monitoring MY signal — it appears on received AM signals in general, or other transmissions with carrier (e.g. calibrator).  In case that matters.  Since I have no transmit antenna accessible from inside the house, I’m reduced to being an SWL for a while ..

Thanks
Grant NQ5T





> On Mar 4, 2016, at 9:35 AM, GRANT YOUNGMAN <nq5t at tx.rr.com> wrote:
> 
> 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 <mailto: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 <mailto: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
>