[Racal] 6790 On VLF

[Dan Rae]

Barry et al,
The A1 board that was fitted as an option to some 6790/GMs is actually the
standard A1 board from the UK version , the RA1792.  It does, as you say provide
about 8 dB pre mixer gain, which is needed in the '92 as that radio   has much
less post mixer gain than the 6790.    The LP filtering is after the amp and is
electrically identical to the filter in the 6790/GM.   The link is around the
amp only.     I don't like the design of this amp, apart from the fact that it
uses yet another unobtainable 2N5160,  the noise figure is not very good.  A
Norton amp with a pair of 2N5109s is a much better bet, this is what I have
fitted to my RA6772E, which does help that somewhat insensitive receiver a lot
above 20 MHz or so.  The other thing with the 1792 A1 board is that it does
provide a muting facility and input protection to the radio.   Details and a
schematic I think are, or were, on Al Klase's site.    Neither of my 6790/GMs
would benefit from any extra gain from the numbers, except perhaps for very weak
signal work on 28 MHz in another location that is quieter than this one.

I totally agree with Dallas that the standard mixer does fine down to about 150
kHz with the standard mixer, but it does drop off below that drastically.  I
have both versions here and the big fat mixer version holds up down to 10 kHz.
I didn't bother to record the numbers when I did the tests.  We should
differentiate maybe between LF(MF) and VLF here maybe.

And yes that version of the tuning steps sounds very desirable.  Pity they
couldn't do the automatic speed up that the 1792 has.  That is pretty well
perfect. Some versions also have a reversible tuning disc  with a slower rate.

Dallas's noise reduction scheme sounds perfect, but I wonder how much it would
help here with 66 kV power lines visible from the house :-(

Dan