"Paul G."

Probably a guess, but I suspect the mystery part is a high value
resistor as Phil stated. Its purpose is for "leaking" any DC static
voltage that could build up in the circuit.

** Correct.

That would probably happen
if you were connecting a speaker up, and you were charged up (could be
anywhere from a few volts to tens of thousands of volts). Walking on a
nylon carpet in a dry house (less than 20% humidity) can generate
enough static electicity to jump a gap of more than a centimeter. In
northern Canada during winter, I've generated 2-3cm sparks this way.
The DC voltage or the transient current may stress or damage
components.

** Note that the unit is a * receiver * therefore it has antenna sockets
on the back.

A rooftop mounted antenna can acquire very high DC voltages on a dry, windy
day - more then 10kV.

The PROBLEM is with the insulation of the transformer in the PSU of the
receiver - the primary side is linked to the AC supply so does no "float"
up with the rest of the unit when the antenna becomes charged.

If no leakage path exists to drain away the charge generated by wind on the
antenna, the resulting 10kV plus voltage difference will stress the
insulation in the AC tranny until is flashes over inside, making a small
spark and associated EMI which may be audible during FM or AM reception.

More importantly however, repeated sparking will cause the AC transformer's
insulation to eventually fail and the whole unit becomes live !!!

A resistor of about 4.7 Mohms to 10 Mohms is sufficient to keep DC voltage
build up on the antenna connection within safe limits and poses no shock
hazard to uses despite one end connecting to the live AC conductor.

BTW:

Such resistors are of a special high voltage type approved for this purpose
and should never be replaced with general purpose ones.


..... Phil