{Collins} 75A-4 Curious Issue


I have noticed a curious "issue" with every 75A-4 I have played with. Try this:

Turn the RF gain full CCW, AF gain to where you can hear. Note that there is almost no hum/noise in the AM position (even with AF gain full CW). Now switch to CW-SSB. Note the rise in hum!

My first thought was the BFO but pulling V20 has no effect. Also, no cathode-fil shorts anywhere. Furthermore since I have found this in all -A4's it appears to be a design deficiency. I do realize the "condx" under which one hears this hum is not "normal" so I can understand how it could go undetected. My best guess at this point is some sort of pickup in the wiring harness and especially the long lead from the output of the BFO to V11. 

Anyone else experience this or have any ideas?

73, Bob

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