[TMC] Need help with several GPR-90RXD puzzles

[John Poulton]

Folks,

I have a couple of questions about the GPR-90RXD that I could use some help 
with.  First is a question about the BFO injection in an "early" 'RXD,
and second is set of questions about later mod's to the 'RXD's.  

I currently have three of these radios in the shack:

1.  This radio appears to be an early GPR-90RXD that matches the Nov 1960 manual
I have on hand.  This receiver couples the BFO not to the first IF as in
the GPR-90, but instead directly to the 6AL5 AM detector (V9), through a
strange little autotransformer, L38.  This arrangement appears to be done
this way to allow for an external BFO input on rear-panel J7, a BNC.
The power supply appears to be identical to the circuit in the GPR-90.
This radio is stamped with S/N 135.

The BFO on this receiver has waaaay too little injection, even though
everything appears to be working correctly.  Does anyone out there have
an "early" GPR-90RXD who'd be willing to measure the voltage that appears
on J7 when the BFO is turned on?   This doesn't require taking anything
apart.. just need to attach a high-impedance scope probe to J7 and take
a reading.  If I knew the correct voltage at J-7, I could figure out for sure
whether the BFO is working correctly or not.

2.  This radio has the usual 5U4 rectifier but, unlike radio #1, 
a plug-in filter capacitor.  Instead of the 6AG5 BFO tube, there is
an octal plug-in unit with a 6BE6 installed.  I'm assuming this is a 
product detector.  This radio has a 6AL5 at V9, like #1.  The external BFO input
appears to be cabled to the plug-in unit.  Incredibly strangely, this radio
is stamped with S/N 135, same as radio #1!

3.  This radio is labelled as a GPR-90RXDS.  It has the same plug-in filter
cap as #2, but no 5U4.  Instead there is a pair of solid-state rectifiers, 
and an additional power resistor, presumably to drop the B+ a bit to match 
the voltage on the original tube rectifier.  This radio also has a 12AT7 at V9, 
instead of the 6AL5, and has the same octal 6BE6 plug-in unit as #2.  This 
radio is serial # 34459.   To make matters more confusing, this radio has been
seriously hacked.. there's a bunch of solid-state circuitry that appears to be
a DC filament supply for the 3.9 MC 1rst IF unit.  Clearly not factory original.

Do any of you have a manual or other info on the later modifications and
variations on the GPR-90RXD that you'd be willing to share?  The best thing
would be to buy or borrow a manual, or reimburse for a good copy.

73, John K4OZY