RACAL
The 67-Series of Gems
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This page is under construction. I have compared the two models, RA6790/GM and the RA6793A for all the various subjective qualities. I don't see
(hear) any differences, but the work goes on. I'm using the Watkins-Johnson  WJ-8718A and WJ-8718A/MFP for comparison. I must say that
comparing these 4 receivers is more like being poolside at the Playboy Mansion than work.
Each trial is conducted over several bands corresponding to the antenna selected. All 4 receivers are fed from my Multicoupler with isolated source
inputs and equal signal level. There is no preselector or preconditioner in use. All audio signals feed the same input mixer and feed the overall
system rack amplifier/speaker system which is a studio-grade playback/monitor system.
1
March 2012:   Setup   
Here's the test rack.
The Multicoupler/Distribution Amplifier is the unit near
the top with 2 green lights and the rest all red. The 2
green channels are feeding the 2 Racal radios, at the
time.
The W-J radios are controlled from there as well.
The unit above that is the system's Preselector. Note
that all preselection functions are off (6 red lights at the
top).
None of the external Demodulators are in use.

2
April-May 2012:   Test   I suppose I've invested more
than 100 hours of hard work (read; casual fun) trying to
form opinions for the utility of these four radios. I've
chased LF and VLF non-directional beacons, time
signals and submarine comms. I've listened extensively
to Mil-Comms in both SSB and various digital modes,
tropical band broadcasters, Hams in AM, SSB and CW,
on 160 through 10-meters, and International
broadcasters from 6 to 17 MHz. I've used MultiPSK to
decipher and identify many different digital comms, from
various sources, and, for entertainment value, I have
monitored the CB band, which seems to have crept into
the 12 and 10-meter Amateur bands. Finally, I dialed up
WREF, 850 KHz out of Danbury, Connecticut and
enjoyed hours of studio-quality oldies music.

Evaluation (below)
--- VHF/UHF Amp and Distribution


--- Preselector

--- Distribution/Multicoupler


--- W-J Demodulator


--- WJ-8718A



--- WJ-8718A/MFP


--- AM Demodulator


--- Audio mixer/amplifier

--- Racal 6790/GM



--- Racal 6793A


--- Line filter/monitor

--- Line controller
Counter
A comparison of the two models ... and my favorite Watkins-Johnson Receivers
(The Bitter Blue Pill)
May 24, 2012
The other day, I turned on my RA6790/GM only to see the display show all
zeros, no back lighting, no processor functions and all the keys dead.

Short story: Yank the radio from the rack, remove the covers and take
voltage readings. Well, the +15 regulator is HOT and there is no +15 volt
rail.

Ah ha !!! Ask any experienced Racal owner and (s)he will tell you that
there is a bad tantalum (bypass/decoupling) capacitor in there
somewhere. Okay, that's nice, but there's a catrillion tants in the silly radio.
Best to unplug all the interconnects from all the PC Bds and monitor the
rail as you re-plug the boards. In my case, the A8 input cap was shorted.
Unsolder it, replace it, re-plug everything, connect an antenna and
speaker and ... viola ! ... back in business.

The only negative to this story is the looming threat of anticipated future
tantalum failures. Oh well, it's gone 20+ years, so we'll see what the future
has in store for us. So I guess, it's "
one day at a time" in the tantalum
world.
RACAL Sidebar:
A tale of tantalum tantrum, trauma, and technical tension
3
June 2012:   Evaluation
Well, it's been a few months since I put this page up and I have nothing to report ... sort of. If this was a blind test, I'd really have nothing to report. I
did try to discern even the slightest edge one receiver might have over another. If I could report any subjective impressions, it seemed that the two
Racals were slightly quieter under very specific weak signal or DX conditions. That is an unqualified "maybe," but nonetheless did seem to be the
case on occasion. So maybe the MDS of Racal edges out the W-Js, but that statement would have to be qualified on the test bench. Dynamic range
is a non-issue. A commercial AM broadcaster 2 miles from here didn't bother anything I was doing. Sensitivity on all four units was a wash. I'd give the
edge to W-J for selectivity, but only because the range of suitable options was better for AM broadcast and SW, not the actual filter quality (see chart
below).
I'll add to this report as required, but from what I've experienced so far, don't expect to learn anything. For now, I'll go back to my chore of analyzing,
evaluating and critiquing these radios. It's hard work, but someone has to do it.
Racal 6790/GM
3.4K
7.3K
8K
17K
Racal 6793A
.4K
1.6K
3.2K
7.7K
19K
WJ-8718A and WJ-8718/MFP
.3K
1K
3.2K
6K
16K
Available IF Filters