I accidentally found the root of the A.C. buzz problem. It was a bad capacitor, but not the ones of which I was suspicious. There are also a couple of electrolytic capacitors on the board. The 47 MFD one had a bad lead. I accidentally moved it, which restored contact, and the buzz went away temporarily.
So it was off to Radio Shack for a replacement, which is now installed.
But now the volume seems low.......