[Amps] Alpha 76A - 120hz hum on 160m only

[Carl]

Is this into a dummy load?
Is the amp grounded?

Carl
KM1H



----- Original Message ----- 
From: "Jim Barber" <audioguy at q.com>
To: <Amps at contesting.com>
Sent: Thursday, October 17, 2013 12:59 AM
Subject: [Amps] Alpha 76A - 120hz hum on 160m only


> After replacing the filter caps and equalizing resistors, I fired up
> the 76A today on 40m and had a couple of nice contacts. The 120hz hum
> problem *appeared* to be gone. The spectrum analyzer showed a nice sharp
> clean carrier, with no "funnies" in the sidebands or for a few mHZ
> either side.
> 
> And then I tuned up on 160m. 120hz hum like crazy on a carrier, hum 
> distortion on SSB. Unusable.
> 
> Tuned up on 75m, clean as a whistle.
> 
> So - it hums like crazy on 160m, but not on the other bands.
> Plate choke and/or bypass caps? It loads up fine on 160, just hums like
> crazy.
> 
> Ideas? (other than exorcism)
> 
> Thanks,
> Jim N7CXI
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