DEHISSER OUTPUT PRODUCES HUM WHEN THE CHANNEL SELECT SWITCH SETTING IS OTHER THAN STEREO
This is an easy one:
1) One of your phono cartridge grounds is poor, but you would probably hear this when listening to stereo.
2) The most common reason is that you are in the USA or JAPAN and have the power switch of the DEHISSER set to 240 VAC. Set it back to 120 VAC.