[Elecraft] AC hum on tune

[Jim Wiley]

Sam -

If you are running the K3 on battery power, then obviously power supply 
hum isn't the problem.  Some possibilities:


Try changing the orientation of ANY AC operated device near the K3.  If 
possible, remove the item temporarily as a test.  If it is not possible 
to move the external device, then at least try rotating it 90 degrees in 
at least 2 planes (one at a time) and test again.  I have experienced 
instances where a external magnetic field was "modulating" a VCO inside 
a radio, and causing the sort of problem you are experiencing.   The 
problem may be coming from something other than a power supply.  I had 
an instance where the magnetic field from a TV set's  horizontal 
oscillator was affecting the VCO in a Kenwood 9800 VHF set, and causing 
it to transmit spurs up and down the band at 15.75 kHz intervals.   That 
computer power supply could be a possible culprit.   Try turning it off 
(remove the power cord to insure a complete shutdown)  just to see for 
sure.


If you have not already tried this, try running the K3 with no external 
connections at all - other than perhaps the battery source.  That means 
no antenna, no microphone or key paddle, nothing at all.   It goes 
without saying that any other connections should be removed for this 
test - no computer interface, no external speaker, no nothing except for 
the K3 by itself.  I believe you should be able to key up the K3 from 
the front panel using either a "test" mode or manually operated 
transmit.   Set the power to as low as it will go before making this 
test.  1 watt or less of RF will not damage the radio even if the 
antenna is completely disconnected.  Listen on another receiver to see 
if you still hear the hum.  If you do, and if you have eliminated any 
possibility of influence by an external AC field, then the problem is 
internal to the K3, and it needs repair.  If not, then try attaching 
external connections one at a time until the hum comes back, and you 
have found the basic problem.


It is possible that a "ground loop" from one or more external 
connections is causing the problem.


Let us know what you find after making these tests. 


- Jim, KL7CC



Sam Morgan wrote:
> <snip> 
>   

> I just went to battery power and hear the same hum
>
> I have turned on a radio shack swl receiver sitting across the room
> and am using 0.10w output
>
>   
<snip>