[Elecraft] KPA500 Hum

[Don Wilhelm]

Yes, the "resonating board" may explain the difference between various 
perceptions of KPA500 owners about the hum level.  To check for that, it 
is a simple matter of lifting the KPA500 up with your hands - if the hum 
level diminishes, then the surface it is sitting on is acting as an 
acoustical amplifier.

There have been other items mentioned in this thread - the transformer 
contacting the Z bracket is one valid problem caused by improper assembly.

The transformer itself has isolating foam washers that were to be placed 
above and below the transformer core for acoustical isolation.  I can 
imagine if the bolt were tightened enough to compress those foam washers 
completely, then hum might be transmitted to the KPA500 enclosure - try 
loosening the bolt slightly to see if the hum goes away or diminishes 
significantly.

73,
Don W3FPR

On 9/27/2011 5:17 PM, Guy Olinger K2AV wrote:
> Another issue is what the amp is resting upon.
>
> Back when I first got my Alpha 76PA, I had a noticeable hum when
> transmitting.  When I wised up a little, I tried it from different places
> and noted that the volume had a very large variation depending on where
> placed on the upper shelf or simply where elsewhere it was placed.  I cut up
> a little mouse pad that had become ugly into 1.5 inch circles and placed
> them under the feet of the amp.  Hum out in the room simply went away.  You
> can barely hear the note from a violin string that is strung between braces
> on a workbench, but the same bow on the same string on a violin can be heard
> thoughout the concert hall.  The shelf was serving as the violin to the
> Alpha's hum.
>
> I still use the mousepad circles under the amp.  If you put the the mouse
> surface of the pad *on the bottom* then the amp will slide around if pushed
> a little and is much easier to position or get into place.
>
>