Harman Kardon Citation 22 No Love?

H/K was the first company to realize the importance of square waves and wide frequency response. H/K i.e. Matti Otala designed their amps, including the Citation 12, to output great square waves at both 20Hz and 20khz. A noble task. Most amps of that era couldn't come close to the wide bandwidth output of the Citation amps. It became so important that most audio magazines of that era posted square wave response photos of the amps that were tested and reviewed.

The output heatsink heating up is due to either bad output transistors, too much D.C. at the outputs, or bad bias heatsink transistor. Usually, there is a transistor mounted to the heatsink that adjusts the bias up or down depending on the temp. of the heatsink. That might have failed. It's also possible the bias trim adjustment may have fail or is improperly set. Find a good repair facility in your area and have it serviced. It's probably due for new capacitors as well given it's age.