Hello, Keith

Be very careful with your new Racals.
Unless you have the semi-rare optional preamp installed,
the quad FET first mixer on board A-2 is dangerously exposed
to static damage. I stupidly used a type N to SO-239 adapter,
and plugged in a random length of wire in the shack,
to try out my receiver, and that ruined one mixer.
Now I use an old Ameco PCL-P Nuvistor preamp to isolate
the Racal FET's from the antenna.

The A-2 first mixer uses four Siliconix type SD-215DE FET's,
and it requires unsoldering the tin box cover on the A-2 board
to get at them. This type of FET has built-in static protection,
but the protection is only between the gate
and substrate, and it is the channel of the FET's
which is exposed at the Racal antenna connection.
The manual doesn't give the schematic of the mixer -
just shows it as a black box.

It is also wise to have an assortment of spare tantalum
electrolytic capacitors around. I've had one
of the little guys go up in smoke, nearly taking out
the regulated power supply. Used boards
bought on eBay also often have some bad tantalums.

Check out the G.E. Datastor back-up battery on the CPU card
(A6A2). If that battery leaks, you are in trouble,
since the boards aren't conformal coated.
I recommend replacing it with a Varta "Mempac"
2.4V 150mA-H (same physical size battery).

Caution: Do +not+ run the receiver self-test function
unless you need to (due to installing new IF filters, say).
The self-test will automatically determine the bandwidths
of the plug-in crystal or mechanical filters, and store
the result in the battery backed-up RAM
(used by the CPU to set BFO offset frequencies).
If your receiver fails the self-test, even for a minor
fault, like an AGC problem, your receiver can be left
brain-dead. You will have to run the self-test
with that CPU card plugged into
a good receiver, using the same filters as yours,
to recover the needed RAM data.

Gary Wingerd sells the Racal R-2174(P)/URR
"Preliminary Technical Repair Instruction Manual".
Although that manual is based on the use of Racal factory test
fixtures, it has many alignment instructions and signal
levels which were left out of the basic manual.
The basic manual is very skimpy, mostly intended
for board-swapping level repairs.

I like my Racal. The audio quality is excellent,
and the synchronous a.m. detector with 6.9 or 18 KHz
bandwidth makes for hi-fi a.m. listening.

73,
Ed Knobloch



Keith Densmore wrote:
Greettings to All,
Does anybody know if anyone does repairs and/or board swaps for the Racal
RA6790.
Two followed me home from the last hamfest.
Thanks,
Keith, ve3ts