[Premium-Rx] More on Racal 6790/GM R-2174(B)

[Gordon & Lois Duff]

I have continued to investigate this radio and am still looking for help to
understand how it works.  I have traced the wiring for the various
additional boards and put the radio back together.  

In a previous posting I mentioned that the display flashed below a certain
frequency.  The correct frequency is 2 kHz.  Below 2k the tuning works but
the frequency display flashes.

Another operating difference is an apparent internal switch when the
frequency passes thru the 110 kHz range.  This seems to differ based on
traversing up or down.  I will investigate this more later.

At MW and higher the radio's audio seems fine via headphones for AM, SSB,
and CW.

I have compared boards at a high level with 2 other 6790 A versions and they
are apparently the same except for the A2 board.  Based on the signal paths
I don't see why A2 would make any difference.  Need to check this out.

On the extra boards:

1.  The A23 board is connected to the VLF antenna input and to the A25
board; I guess it is a low pass filter.

2.  The A14 board supplies the external 3rd IF output at about 15kHz, and
also the IF output at 455kHz (it may get this via A24 and A25).  It appears
to do product detection of some sort via a CD4049.  It has a dip switch,
too!

3.  The A15 board supplies AM and FM output.

4.  A24 gets the 455kHz IF from A8(J6) and returns it to A4(J1).  It has a
MCL RAY-6 mixer, among other components.  It has a great big box labeled as
a notch filter with a center frequency of 277.777 kHz and a bandwidth of 128
kHZ.

5.  A25 get RF via the A23, 20MHz from A8(J8) and a bunch of stuff from A24.
It also has a MCL RAY-6 mixer.

I am going to try and put this all in a diagram, but at this point I have
two questions:

A.  Does anyone know anymore?
B.  Does anyone care, or should I stop posting?

Gordon, KA2NLM