[Premium-Rx] Racal RA-6778C Mods so far

[Ben Dover]

Greetings, Troops!

As you remember from the last episode of this drama... I
wanted to add a REAL S-meter to my RA-6778C, and also deal
with adding a speaker to it (I don't consider that THING in
the front panel to be a speaker!).

Re. the S-meter...   the solution was staring me in the face
all the time, but I didn't see it; an external S-meter hooked
to the AGC output already on the rear panel!

Among the junk... err... "excess electronics"...  THAT sounds
a lot better...  in my garage, I found the remains of an old
Boonton field strength meter/receiver (picked up at a long
ago hamfest for two bucks) that had a very impressive meter
movement on it, directly calibrated in microvolts. That, with
the addition of a simple Op amp meter driver & mounted on a
rack panel, solves THAT.


Re. the speaker output... this one required some digging.

I finally found out just WHY the speaker in the rig sounded
so horrible, and why connecting a decent speaker in it's place
did nothing to improve things. Somebody along the line did an
ill advised, and incomprehensible, modification to the radio.

Audio output to the speaker showed heavy distortion and a lot
of clipping at anything above very low volume levels.

Dropping the front panel to get at the LM380N chip that serves
as the output amplifier to the speaker, it was found that some
unknown soul had added a 68 ohm resistor to the board, hooked
in series with the speaker! To get ANYTHING out of the speaker
at all you had to drive the hell out of the LM380N... right 
into the range of input level clipping!

Once the offending resistor was found and removed, the rig
began producing clean, powerful speaker level audio that is
more than sufficient to drive my test bench speaker (10" Rat
Shack hi fi speaker in a steel cabinet) to a level that is
quite comfortable for listening with acceptable distortion
percentages. It's so good now that the audio out of the 
front panel speaker acceptable, tho painful to listen to
for extended periods.

As it is tho I'll STILL need to drill the rear panel of the
radio; my plan to add a 1/4" phone jack in the hole of one
of the IF output BNC connectors doesn't work. Even in a
radio this big there isn't enough inside clearance for said
jack between the rear panel and the internal shielding.

I'm going to look at the problem a bit more before shooting
holes in rear panel aluminum, but what looks reasonable at
the moment is a 3 terminal barrier strip (similar to the
strip already there for other purposes) can be added to the
rig; two terminals for the new speaker connection, and one
which, when jumpered, will activate the speaker on the
front panel.


73's,


Tom, W9LBB