[Premium-Rx] Fw: Racal receiver 1792

[michaelob666 at ntlworld.com]

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From: michaelob666 at ntlworld.com 
Sent: Saturday, March 7, 2015 11:51 AM
To: claudio1 
Subject: Re: Racal receiver 1792

Dear Claudio

Many thanks for your email.  

There are many different views as to the virtues and vices of the 1772 and 1792.

I have a good example of each radio and decent test gear (though now old) by HP, Marconi, Racal and Rohde & Schwarz to make the measurements.

There is no doubt that the 1792 was cheaper to make.  Indeed it was created because the 1772 was very complicated to build and wire up and by about 1980 was no longer competitive to manufacture.  As my reviews state, there are 50 soldered connections to the PCB of the 1772, and more connections to the heatsink-mounted components.  The 1792 uses only two small connectors – for mains power in and DC out.

The frond end circuits are very similar with the first mixer of each radio based on the Rafuse switched double-balanced FET ring.

The sensitivity and noise figure of both my sets are similar.  The Third Order Intercepts are very similar too – around +30dBm at 25 kHz signal spacings.

The IF filters at 455 kHz in the 1792 should be better that at 1.4MHz in the 1772.  Alas, only one side of the filters is switched and there is some RF leakage but it is not much.

I think the synthesiser of the 1772 is a lot better but it is far more complicated employing three PLL loops and 2 transfer loops.  This is doing things the complicated and expensive way.  

The synthesiser in the 1792 uses the patented digiphase system designed by the US company Dana (later Racal-Dana) primarily for test equipment.  It synthesises frequency and phase in a single loop, though I do not understand how it is done.  It is all very clever.

For an amateur who does not need the remote control or 100 memories of the 1792, then the 1772 is much nicer to use.  The spin tuning is really lovely and was very expensively engineered.  I am sure you have seen the big round diecast shaft encoder.  Add a drop of oil to the front ball race and the tuning is even better.

You must do the mod to the PSU recommended by several others.  Remove the very hot big square rectifier on the PCB and mount it high up on the metal card cage near the front end tuner.  You will need to make up a four wire cable from the PCB to the terminals of the 4 pins of the rectifier and allow plenty of extra length so that you can unscrew and lay the back plate down on the bench to gain easy access.  It’s all rather cramped in that area.  Use a little heat sink compound to improve the thermal contact between the rectifier and the card cage.  The area of the PCB under this rectifier was very discoloured on my radio from all the heat generated by the rectifier.  It was one of the few design defects on this excellent radio.

While you are doing this also resolder most of the connections under the PCB because I found some dry joints under the +20V regulator. 

On the RA 1792 beware that there are very many tantalum capacitors,  The early one are not reliable and and are liable to degrade or go open circuit, or even worse, they will explode with a big bang and make a mess inside.

The 1772 will work very well right down to 15 kHz.  For VLF I add a passive VLF preselector made by Plessey to go with their PR155 and 1551 range of radios of the 1960 –70s.

I think your 1792 has a problem.  It should have the same sensitivity as the 1772.  The audio of my 1792 is excellent and very clear and free from distortion.  It should give under 1% THD for a single signal in CW mode.  

The 1792 has better AGC.   It is the best I know.   I can tune to an AM station as SSB and the signal clarity is excellent and clean.  Don’t test it on the BBC’s Radio 4 on 198 kHz because that uses a highly processed switching modulator called Optimod which to me distorts the audio quality.  No one will notice on a cheap transistor radio, but when you listen with a professional radio and top quality headphones from eg Sony or Senheisser you will hear it straight away.

I hope this helps
73s
Michael
G8MOB

From: claudio1 
Sent: Thursday, March 5, 2015 2:32 PM
To: michaelob666 at ntlworld.com 
Subject: Racal receiver 1792

Hi Michael 
I read your review on this receiver. 
I own a Racal 1772  and recently I bought a Racal 1792 Mark 6.
Im a bit dissaponted as the audio quality and clarity is not comparable with my  1772, also the sensitivity I think is better on 1772. 
I sent my 1792 for a full MOT and alignment, he guy replaced a few tantalum capacitors and a choke on bfo 2 then he carried out s full alligment. 
Tomorrow i should get back my 1792.. 
But I  think that racal made a miracle when they made the 1772.. What as  great receiver... 
What do you think? 

Thanks 
CLAUDIO 


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