[Racal] 6790/GM VLF/LF/HF Version

[Dan Rae]

There is a version of the 6790/GM that was fitted with a different first
mixer board and other differences that seem to have been done to special
order,  to make it good down to 10 kHz.  (Apart from the EPROMS of
course)   The problem is that RCI in their wisdom did not identify this
version in any way on the outside,  you have to look at the first mixer
board A2.   On what they call the LF/MF/HF  version the first mixer
board is numbered A08966-3.  The mixer itself is part number C08924 and
is about twice the size of the regular mixer.  This is the most visually
obvious difference.  There are also major differences in the filtering
of the 1st LO input to the board, involving adding a sort of diplexer
with a monolithic TO5 size 40.455 MHz crystal filter to get rid of noise
from the LO when you get down low in frequency.  Finally the first
roofing filter is narrower than the regular one, 16 kHz.    On the
second mixer board A3, the second roofing filter is also narrower, in my
case it is 8 kHz wide.  This has the useful side effect of helping the
close in intermod performance, quite noticeable in a contest.

So if you see one with a really big 1st mixer, this probably is what you
have.

The point that someone made about the crud that you get below 500 kHz is
probably true of any receiver in an urban environment with many multi KW
BC stations within the area.  Low pass filtering will get rid of that
and would also be essential in any LF convertor design.  Without
filtering, in Santa Monica, near many high powered stations in LA,  I
get  lots of strange signals with two lots of modulation, classic
intermod stuff, and these all disappear when you put a LP filter in
line.

I look forward to seeing the design in QST when it eventually reaches
the left hand coast

73 all
Dan
ac6ao g3ncr