IIRC I never had a SC failure on the older sets except the rocker
switches on the tuner & thats not your problem.
If you havnt check the remote-manual switch usually on
the SC chassis. You could clean it too. If you did need an IR
mic any old Zenith one should work.
Another test is rattle a bunch of keys in front of mic. That will
give a response even if out of alignment. The hand unit should
give a good snap & feel solid just like a gun with a sweet trigger.
There is a thin spring in there that holds the tuning rods that
often broke. That all should narrow things down......
73 Zeno
LFOD !