KPA500 hum

[VE3NFK]

Hi

I see this has been mentioned before - but my KPA500 was pretty quiet when I first got it (plugged in - but not 'ON')  

Now - sometimes - it has a pretty loud hum while 'OFF'  - I turned the power switch at back on / off a couple of times - and it quieted down..  after a couple of minutes it started humming again at what I consider is an objectionable level, stopped for a minute - now humming again.

Just listening to 20 meters, nothing else going on..

Any suggestions?  ( I have turned the back power switch 'OFF' for the moment - but that is quite inconvenient)

thanks  73   John VE3NFK