MAC4100 hummer version

It's a BY2142 ser no.

without any accessories plugged in , there is a barely discernible hum in the audio ( regardless of input selected)

Plugging in an RCA cable to any of the Inputs results in a VERY noticeable AC hum.

Plugging the same into the Tape Out jacks results in the FM audio level jumping up

I aint never seen this on any receiver in my shop

Thanks in advance for any advice

The unit is over 35 years old.....Lots of threads here about restoring these best in class receivers.