Problem found.

Actually two problems.

In addition to the atypical grid voltage readings on the IF tubes leading me
to suspect AVC involvement, R195 (68 ohms) that feeds the cathodes of the IF
tubes (V11 and V12) and is delivered to those tubes from a jumper from V5
(6BA6) was open. Once replaced, I still had very low voltage (1/2 volt)
instead of 1.5 volt on the cathode. What else can be left? The filament? Hey
those things NEVER go bad, right? So I checked the filament voltages and,
while V12 was good (12.8 VAC), the 1st IF (6BZ6) was missing. Back and forth
a hundred times checking the little white wire from V5 that feeds the
filaments too but it was good. I look at the filament string on the
schematic and there's a 0.005 uF capacitor in parallel with the filament of
V11 which was good. Here's the very odd thing: Of course the filament for
V11 uses only one side of the leg of the filament string to develop its
voltage BUT MY VOLTAGE CHART SHOWS IT SHOULD BE 12.8 VAC. In addition, it is
clearly silk-screened on the chassis of V5, "12BA6." But yet the voltage
chart refers to V5 as a "6BA6." Figuring I'd better double check the 6BA6 or
12BA6 or whatever was in there I discovered it was a 12BA6 with its
lettering almost rubbed off. Voila. Exchanging the 12BA6 for a 6BA6
immediately returned my filament voltage on V11 to 6.3 VAC (so I notated the
correction on the manual's voltage chart), cathode voltage returned to a
healthy 1.5 VDC, S-meter easily zeroed and worked and finally
again...hissssssssssssssss.

This transceiver was given to me by a gentleman who said the receiver didn't
work very well. I think I know why.

Thanks to Scott, Richard and Garey for your suggestions. They were all good
ones but I think Richard's crystal ball may have been closest when he
stated, "check the obvious."