Re: {Collins} 75A-4 Curious Issue

----- Original Message ----- From: "Bob Sullivan" <robert@xxxxxxxxxxx>
To: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Cc: "Collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
Sent: Monday, July 08, 2013 11:50 AM
Subject: {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

The frequency of the hum may be a clue, if 60 hz its probably from heater current or possibly from the bias rectifier. If 120 hz from the power supply. The AM/SSB switch changes three things; the input to the audio amplifer; the configuration of the noise limiter; and it also grounds the screen of the BFO in AM. See if pulling the noise limiter tube makes any difference. Also pull the product detector. You may be able to trace the hum with a scope. If you have encountered this hum in several receivers it may be a hum loop due to a bad ground. That may be hard to find but the usual voodoo of working all the screws that may have grounds made through them is at least worth trying. It may not be a design problem so much as something that shows up with age. I had a mysterious hum in my R-388 that turned out to be a poor ground return for a filament. The return was made through the frame of the headphone jack which was not making good contact due to the paint on the panel. The hum was there all the time even when the rectifier was pulled.

Richard Knoppow
Los Angeles

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