Hello Menahem,
Is it on both channels?
I would suspect the 54IC3 / TC9177 as a source. It's located after the input source switch.
Probably hard to get - I never ran across that one. Internet tells it's a Toshiba. Looks like a digitally controlled pot only.
Eventually 54IC1 (right) or IC2 (left)/ 4558 that's actually creating the attentuation.
And I never had a BC9500 - now it's the first on my wishlist meanwhile...
I suppose you have the SM - if not, let me know.
Gunther