[Elecraft] KPA500 Hum

[Ron D'Eau Claire]

Mike, I certainly concur with Eric's observation. What I thought was very
unusual was that you said that the hum would come and go, which I presume
happens when you aren't doing anything at the amp. 

Did you build your amp yourself from the kit? Since you said the hum seems
to have increased after moving the amp from another QTH, you might check to
ensure you had the transformer mounting bolt tight enough and that the
transformer has not moved. If it moved toward the Power Supply board, it can
touch a transformer mounted on that board that might be providing some extra
vibration to make the hum louder. If so, you can just loosen the nut at the
top center of the transformer and shift it slightly forward so it isn't
touching the PC board transformer. It is normally very close but not quite
touching. Then retighten the nut securely. 

Most reports say that the residual hum from the transformer with the KPA500
turned off at the front panel is either inaudible or very, very low. 

What I was describing was the hum you hear in normal operation when the
supply is under full load. It is substantially louder, depending upon the
mains voltage tap used the power output and the transmission mode. 

73, 

Ron AC7AC



-----Original Message-----
Hi Mike,

This doesn't sound unusual.

There is also a normal amount of 'resting' idle low level transformer 
hum, which varies with line voltage and frequency.  Also, when the rear 
switch is on, the transformer primary is active even when the front 
switch is off ( but the output windings are disconnected from the amp.)  
the front switch is more like a stand-by - active switch.

73, Eric   WA6HHQ

---
www.elecraft.com


On 9/26/2011 11:49 AM, Ron D'Eau Claire wrote:
> What you are describing is very unusual, Mike.
>
> Typically, one will hear some hum from the transformer with  the "key
down"
> (full 500 watt carrier). It is most pronounced when using the green
> transformer tap (lowest line voltage input). Normally there is no hum with
> the KPA500 off, or when running the KPA500 at low duty-cycle loads such as
> SSB, even at full peak power output.
>
> I'd drop a line to the elecraft support reflector and see what the
> Ele-guru's say.
>
> 73,
>
> Ron AC7AC
>
> -----Original Message-----
>
> My KPA500 now seems to have the hummms.  It comes and goes whether the
> unit is on or off.  It does go away when you turn off the rear switch.
>
> I noticed someone changed the tap and that appeared to fix the hum.
>
> I am running the amp on 110V.  My actual wall voltage is 119V.
>
> I did not notice this at all while I was at our Montana home over the
> summer.
>
> Mike W0MU
>