Sounds like classical symptoms of a bad solder connection. Could be
anywhere after the detector including switching. Since it comes and goes
predictably, the service technique is to either look for where audio is
gone with a signal tracer (R used to have a utility audio amplifier that
would be handy for the purpose. Runs an IC on a 9 volt battery with an
input connector and built in speaker). Or else apply alternating cold
(freeze-mist) and heat to identify first the board or region of the
radio with a problem and then work it in smaller areas until you find
the chip or transistor (and it could be INSIDE a transistor, more likely
in a plastic cased transistor) that has the thermal sensitivity. Cold
should restore it. Heat should kill it. A hair dryer works for heat, or
there are hot air guns made for the purpose. That's how I'd find it,
unless I just pulled the audio board and hit all solder connections with
a soldering iron and fresh solder. Its debatable on which technique is
more cost effective. But the soldering won't fix the bad connection
inside a transistor case.
These techniques work for ANY radio made anywhere in the world. I've
read some controller boards from Tentec are repeatedly fixed by the
soldering iron shotgun blast technique.
73, Jerry, K0CQ
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