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With it in the condition you mention where it’s generating what sounds like the 700 Hz test tone even though you aren’t in that test mode, it would be best to use a scope and see which stage is oscillating. Barring that, maybe a little audio test amp / signal tracer probing ins and outs of each stage around ICs 8 and 9 might locate it. You can also do some resistance measurements around those ICs following the schematic and see if something isn’t right. Even look for possibility of a smd resistor or capacitor knocked off the board during construction or shipping. Those things are rare but do happen.?

Ron