[Elecraft] KPA500 Hum

[Guy Olinger K2AV]

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.

73, Guy.

On Tue, Sep 27, 2011 at 2:33 PM, Roy Morris <w4wfb at carolina.rr.com> wrote:

> Referring to kit assembly instructions, Rev B, June 22, 2011:
> The following is on page 33.  When placing the transformer on the chassis
> you are told to press it toward the Z-Bracket so it does not hit the rubber
> bumpers installed earlier.  Then
> above figure 49 on the same page it says the bumpers press
> against the side of the transformer near the bottom.  Maybe some are
> installing the transformer so it is resting against the Z-Bracket causing
> vibrations (hum) and not firmly against the rubber bumpers as it should be.
> Also some of the wires may be trapped (pressed) between the transformer and
> the Z-Bracket causing the Z-Bracket to vibrate.  I snugged the transformer
> against the rubber bumpers, and I can see light between the transformer and
> the Z-Bracket.  I have not experienced any hum at all.  Roy  W4WFB
>
>
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