[Yaesu] FT1000-MP Sub-VFO RX

[ARS NZ3O (Byron) FM29fx]

I have a 1996-era FT-1000MP that just started to
misbehave.  Not a good time of year to do that.

The sub-VFO, when set to 7, 14 or 21 MHz seems
to have no receive.  Instead, you hear a strange oscillator
sound, or the click of the front panel keys, or whatever,
and the oscillator sound changes if you change bands
on the main VFO.  Actually, it kinda goes away.  But when
it does, there isn't any signal there, either.

Transmit is fine, only receive is bad and only on three bands.

I have conducted the following steps:

1. Remove 3v battery and turn emeory battery off.
2. Reset RAM.
3. Retest receive.
3a. Transmit on main. Transmit on sub.  RX on main.  RX on sub.
3b. Repeat 3a on every band 10m-160m.

Attempted direct entry (SUB/CE) of modes and everything.

Sub-VFO has optional Collins 500Hz filter.  Nothing else to it.
Main VFO has factory crystals and INHAD everything else.
Main receiver also has INRAD 70MHZ IF Amp option added.

I don't have a service manual, just the user manual and the
tech ref guide.  Can't find anything new to try.

Thoughts?  Post to list and direct please, as I'm in digest mode.

73, Byron