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It seems you explored the most obvious plus some of the most obscure prblems. maybe signal tracing with a scope is the best way to track this down.
Can to apply to the antenna a test signal of a flat field of grey. Ensure you can see the bar clearly on the CRT. Then 'scope back from the CRT stage-by-stage. I'd expect you should see the hum when scoped at the CRT and I would trust at some stage the hum will disappear from the scoped waveform.
At this stage I am as flummoxed as you...
Terry
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