Be careful here!
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Sometimes, the screw passes through the heatsink to a
socket, and the screw is the electrical connection to
the tab. Other times the screw screws into the heatsink,
and must be isolated from the tab.
Be sure what your situation is. The schematic may be a
more clear indication of the need for isolation.
The 577 has one such nylon collar. It is not difficult to overlook or lose this item. I nearly left the collar off while re-assembling my 577. The first two transistors (Q766 and Q788) from the bottom have a mica and do not require the addition of the nylon insulator collar. The next IC above them (U732) has a mica insulator and the nylon collar under the head of the screw. Finally the very top IC (U722) has neither the mica insulator or the nylon collar. The mica insulators are very thin and difficult to see under some circumstances. I double checked the "mechanical parts" section of the manual, just to be sure I was not missing something.