In a SX-28A, the CH2 is as far as it can be from the power transformer:

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The black “core” at the left is the CH2.

You can see the bottom of the power transformer and the filtering choke at right.

The closest transformer to the CH2 is the audio output.

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Tom and/or anybody still tuned in. Still looking for what really failed and how.

For anyone wondering about the details and possible source, I copied a bit of your first Video so I could take a closer look.

I see 60 Hz and strong third harmonics, and no visible high harmonics. It reminds me of core saturation.

Power transformer magnetics and primary is the only source I can think of. Any comments?

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don??? va3drl