Re: Very odd fault with Dynatron Radiogram
A couple of thoughts that might help you track it down :
Can you find a pair of small-ish speakers that work OK with the unit in your workshop? If so take them back with the unit to the customer and try them there. If it doesn't motorboat with those, then the fault is presumably speaker-related either de-stabilising the amplifier feedback loop or mechanical feedback to some part of the preamplifier.
Do you have a variac? Try it in your workshop with (sensibly) high and low mains voltages. Can you get it to motorboat?
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