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

[Jim Barber]

Hi Bill,

It's definitely on the on-air signal. I worked several stations on 160M 
AM, (100W carrier, 400W PEP) into two different antennas and all said 
the hum was very loud. Move to 75M or higher and it disappears.

I know what you mean about hum modulation on the power line and local 
receivers, but this is strong enough that it almost wipes a transmitted 
SSB signal out mixing with it, and CW signal is nasty. (strongly 
distorted to receiving station at distance)

Also - I took a look at the plate choke this afternoon and it doesn't
appear to have been overly hot. There's no discoloration of the enamel
on the wire, or puddles of anything at the base. Now one of the .002 
bypass caps *does* look like it's been warm, though. It has that "wet"
look they get when the outside coating has been heated and cooled.

Investigation in progress. :-)

Thanks,
Jim N7CXI

On 10/17/2013 7:25 PM, Fuqua, Bill L wrote:
>     If you are observing the signal on a spectrum analyzer or receiver inside the ham shack near
> the amplifier by picking up the field near the amp it may not be there at all.
>     If you have an antenna on the spectrum analyzer you may be observing the RF from the
> power line and not the RF on the amplifier's output.  The RF on the power line can be modulated
> by the power supply rectifiers in the amplifier itself as they switch on and off while doing their job
> at 120 Hz.
>     This is a common problem with direct conversion receivers where the local oscillator (BFO) is actually
> on the same frequency being received and the receiver picks up its own oscillator signal which is modulated
> by the power supply diodes. The solution in that case is to put RF bypass capacitors each of the rectifiers.
>     In the case of a power amplifier, it can be ignored because you don't usually listen to your own signal.
>     Get with another ham and see if he can observe the hum of the signal going out the antenna or
> get a high power attenuator so that you can observe the output directly from the amplifier rather
> than sampling the signal via a small antenna on the spectrum analyzer.
>     It may be more noticeable on 160 meters because the power supply line filters may not be doing
> a good job on the low frequency band.
>
>   73
> Bill wa4lav
>
> ________________________________________
> From: Amps [amps-bounces at contesting.com] on behalf of Jim Barber [audioguy at q.com]
> Sent: Thursday, October 17, 2013 12:59 AM
> To: Amps at contesting.com
> 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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