[Premium-Rx] Racal RA1772

[Danny Higgins]

Hi Michael.

One of my first jobs at Racal as a young graduate in 1975 was to sit in the
test department and tune each 1772 from 0 to 30 MHz by hand (no remote
control) and record every spurious signal heard.  I think that the
specification was "no spurious signals that would degrade the sensitivity in
CW or SSB by more than 3dB".  I wrote a program in BASIC that for each
spurious signal would work out the frequency of each VCO in the synthesiser
and work out which combinations of signals or harmonics would produce the
spurious signal.  The wiring harness was all done by hand, so the cross
coupling between different wires varied from one 1772 to another, unlike the
predictable performance of ribbon cables (1792) or motherboards (37xx).  The
methods of reducing spurious signals included ferrite beads, extra
decoupling, different earthing and you could even notch out some of the
higher order ones by adjusting the tension on the screws that held the HF
Loop board to the chassis casting.  Of course, every time you reduced one
spurious signal you had to check the whole range again to make sure that the
changes hadn't made any of the others worse.  No wonder that they cost so
much, but at least the performance was guaranteed.

I'm still working on my controller for the RA3702/MA3752/TA3762, but
progress is a bit slow at the moment.

73

Danny, G3XVR

-----Original Message-----
From: premium-rx-bounces at mailman.qth.net
[mailto:premium-rx-bounces at mailman.qth.net] On Behalf Of Michael O'Beirne
Sent: 09 August 2010 22:20
To: PREMIUM-RX
Subject: Re: [Premium-Rx] Racal RA1772


Gents

In amplification of Pat's comments, tonight, I might mention that I once met

one of the designers of the RA1772 at the RSGB's annual VHF rally at Sandown

Park about 20 years or more ago.  I seem to recollect that the poor chap was

going through a divorce and selling kit (presumably to keep the lawyers 
happy!).

His part was the design of the complex wiring harness.  If you have not seen

one in pristine order, it is, believe me, a thing of much beauty.

At the time I was not aware of the heat issue and did not tackle him on it, 
and I do not suppose at the manufacturing time Racal were either, otherwise 
they could presumably have used PTFE cabling for not much additional cost.

He said Racal nearly went bonkers trying to get the wiring right to avoid 
cross-coupling unwanted signals.  Each set was tuned by hand from zero to 
30MHz to check on the spurii, and "adjustments" made.  This is not the way 
to make a cheap product and keep accountants happy.

The RA1792's wiring is pedestrian and cheap in contrast, but it has to be 
said in fairness that it works, albeit that the microprocessor board should 
have been properly screened and decoupled, as it is in the replacement, the 
RA37xx series.

73s
Michael
G8MOB

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