John,

Have you tried feeding a source into the monitor input and see if the unit
sounds OK? I'm guessing it will. While I don't know the HK-230A, I have
seen receivers that the tape monitor switch feeds the volume control
directly (or nearly so) eliminating the the tone control amps.

I strongly suspect that you have a low level (regulated) power supply
failure that is causing the hum. Looking at the power supplies that feed
the preamp circuits with an oscilloscope is going to be the hot ticket
here.

Tim Schwartz
Bristol Electronics

Wouldn't checking ESR on the power supply caps be the first thing for a unit
of that age? I have found that to generally be the issue on the old stuff I
have worked on.

WT