Hi Tom.

Very clean SX-28A specimen !

Better shape than my two samples, actually…

And an original SX-42 (R-42) speaker !

I have the radio, but never found the matching speaker…

I have a PM-23 one, however (late war production, with no embedded “H” in the front grill).

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Agree with you that the “hum” is annoying and should not be there.

What surprised me also is the shape (on your scope) of the 60 Hz signal.

Really looks like a hi-current load passing thru a non-linear resistor and something in the 6SC7 stages seems to be referenced to.

Can be the GND spot for the C44 capacitor, as I mentioned previously.

And your third video tell me exactly the same thing: see how the 60Hz signal is distorted right on the plate (pin 2) when you put the bass sw “out”.

And there is already a small 60Hz that can be seen even when the SW is “in” !!!

It should be flat as a rule there, with the volume control at zero.

My guess is that the + side of C44 is “moving” at 60 Hz, and this should not be.

Can you probe it directly ?

Where this cap is physically grounded ??

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BTW, the schematic I post from the beginning comes from the TM11-874 and it indicates pin 5 for the plate of the 6SC7 related to the SW.

Yours (from the 3^rd video) is identified as pin 2 WHY ??

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Ok, made a couple videos, excuse the camera work and narrating……….

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Tom

W3TA