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Reviews Categories | Receivers: Commercial/Military/Marine adaptable to ham use | Racal 1792 Help


Reviews Summary for Racal 1792
Racal 1792 Reviews: 2 Average rating: 5.0/5 MSRP: $1,400 used
Description: 150 kHz to 30 MHz, continuously tunable synthesizer in 10 Hz steps over the entire frequency range. Frequency setting either by numerical keypad or by single tuning knob with continuously variable tuning rate from 1 kHz per turn to approximately 20 kHz per turn, depending on the speed of rotation. EAROM memory unit may be programmed with up to 100 channel frequencies and mode which may be recalled by keypad or tuning control.
Product is not in production.
More info: http://www.recelectronics.demon.co.uk/ra1792.htm
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G8MOB Rating: 5/5 Jul 9, 2011 15:47 Send this review to a friend
A good receiver but with a few issues  Time owned: more than 12 months
There have been some mixed views on this receiver. One group feel that it is cheap and cheerful, not a patch on its predecessor, the RA1772, and does not deserve the Racal badge. The other view is that it is in fact a very good receiver as is evidenced by the fact that thousands were sold in large numbers to the UK military and (in the form of the RA6790) to the US military. These are fussy organisations with a whole range of competitive products to select from, and that they took these rigs in large quantities shows they must have been pretty good.

Well, I have had my one for about 20 years and I have been a happy bunny. The source was Telford Electronics in the UK and mine was a good one and rather expensive. It was fitted with a non-backlit display which was not easy to read unless you shone a desk lamp at it and even then the display looked pretty dismal. However, my good mate G3YFK did a wonderful job transplanting a back-lit display from a duff remote controller for the 1792 into my 1792 and WOW the improvement was terrific.

My advice is don't buy one unless it's back-lit.

Some folk complain that the 1792 is not as well built as the 1772. I have both sets here and I disagree with this view. The simple fact is that the 1772 is an excellent HF receiver (see my e.ham review) but the design made it difficult and awkward to build and it became very expensive. As an example, the PSU is very complicated requiring over 50 soldered connections from the wiring harness to the PSU's PCB.

By the late 1970s it was becoming uncompetitive.

In the late 1970s Racal's US factory at Silver Spring, Maryland, was developing a new receiver part based on the front end of the 1772 but with a much simpler PSU that only called for two D connectors (AC input and DC output); it had microproccessor controlled switching that elimininated the 1772's complex wiring harness and using instead ribbon cables with IDC connectors and inter-board coax links of thin coax fitted with decent gold-plated SMB connectors and all the memories that a mP can provide.

A further factor is that professional receivers from the mid/late 1970s were being designed for full remote control operation. This has very little application or interest to the average amateur listener but is vital for professionals.

For a professional the ideal place for a receiver is out in the country well away from man-made QRN, typically in the UK in the middle of Bodmin Moor, Cornwall. However, the ideal place for the professional listener and analyst is a lot nearer to civilization, and for the likes of our Home Office and similar, preferably in London.

Similarly on a ship you want the receiver to be near the aerial but to be operated from the radio shack near the bridge.

The remote controlled receiver is the ideal solution, and microprocessor control lends itself to this. Such control must be digital which in turn calls for simple on-off buttons and the avoidance of the complicated multi-wafer switches of olden days. This in turn leads to big savings in manufacture since simple buttons and automatic solder flow machines can replace the expensive hand soldering of fiddly wafer switches. The result was very few rotary controls - basically the shaft encoder and the local AF and IF gains. This is what we find on the RA1792 and most other professional receivers. The other controls are push buttons or the power on/off switch.

So what's the RA1792 like? The panel looks complicated, not helped by dark grey buttons set against a dark grey panel. If only they had retained the far nicer light battleship grey panel of the old RA17 which in my mind has never been bettered. However, most of the buttons each have only one function and each is positioned directly under the LCD which carries the corresponding information. After a little practice operation becomes very instinctive and simple, a tribute to the software and panel designers. It's one of the nicest radios to operate and was probably designed for semi-skilled operators.

The construction is based on a large diecast chassis which provides great strength, stability and RF screening. All compartments on the chassis have individual screens. The chassis is surrounded by a rigid box comprising the front panel, side panels and a rear panel, and above and below there are top and bottom lids. All this imparts great mechanical rigidity and strength.

The computer control boards are mounted parallel to the front panel and the right hand box plate. What worried me is that the CPU is rather near the main IF/AF PCB on top of the chassis. There is in fact some CPU hash pick on the audio line but this is curable - contact me via my email at the end for some hints.

A new synthesiser was developed using American technology that synthesised phase and frequency in a single loop called "Digiphase". I am damned if I understand how it works but it's pretty good, but not below 100kHz and I think that the 5 loop synthesiser in the 1772 is better.

The outcome was the RA6790 taken by the US military and the the UK's RA1792. They look quite different and there are many other points of difference but there is a major commonality inside. I personally prefer the nice tactile push buttons of the 1792 to the "coffee machine" bubble-type buttons of the 6790 and I find the tuning easier on the 1792.

The shaft encoder is frankly rather cheap and cheerful but it works. It's not as good as that on the RA1772 or the later RA3701 but does a reasonable job, it has a flywheel to make for smooth tuning but needs a drop of oil on the bearings periodically.

The frequency stability depends on the 5MHz standard fitted. The TCXO is fine for most requirements. The alternative is the Racal-Dana 9442 standard delivering a temperature spec of around 3 parts in 10^9 per degree C which for us mortals is way OTT! Alternatively an external reference on 1MHz or 5MHz can be fed in.

The IF filters work at 455kHz and are either the excellent blue-cased professional mechanical filters from Collins or the equally good crystal filters from ITT or STC (or sometimes a mixture as the IF board will accept both). The software is sufficiently clever to sort out whatever filters are fitted.

The sound quality through the filters is impressive. No ringing even on the 300Hz CW filter. There are separate LSB and USB filters so that there is no need to shift the tuned frequency. These filters have a wider bandwidth than on most ham radios - 2.95kHz at -6dB. Readers may be confused because the filters are marked "3.2kHz" but this refers to the -6dB audio limit and not to the bandwidth. The actual -6dB spectrum is from 250Hz to 3.2khz = 2.95kHz. This may be too wide for the keen DXer on a crowded band but it is as I much prefer. I find listening to the restricted audio from narrow 2.1 and 2.4 kHz filters deeply unpleasant. Furthermore, the SSB filters are asymmetric - ie there is more attenuation on the carrier side, increasing the suppression of the carrier and opposite sideband by about 10db compared with "normal" filters, thus improving the filter shape factor.

These SSB filters are ideal for listening to a noisy AM signal in SSB mode. The recovered audio sounds unusually natural compared to many other receivers. One additional reason is because the IF intermodulation is very low at about -50dB.

The 6kHz AM filter is just right. The AM detector is a synchronous circuit and can sound very good with poor signals, but I think I prefer the AM detector of the RA1772, although the 1772 uses an 8kHz filter which is too wide for weak AM signals.

There is an IF output on 455kHz which could be applied to an external demodulator from say Sherwood or the more sophisticated demodulators from W-J.

A useful add-on tuning aid is the Racal MA1105B bargraph (designed for 455kHz rather than 1.4kHz or 100kHz of their other MA1105s). See my review of the RA3701 for a full description of these devices.

The AGC performance is frankly stunning. About 1dB variation for 100dB change and there are none of the pops or intrusive burbles that I have encountered on other receivers. It's the best AGC I know of.

The memories basically store all the front panel settings per channel. It's dead easy to set the channels and to recall them and they can be made to scan in various permutations. Under computer control the memories are probably almost unlimited.

Acess to the PCBs is easy. All coax links have SMB connectors which makes isolation and testing easy. The set has extensive BITE ("Built in Test Equipment"). On switching on power the radio goes through a large number of self tests and any failure pops up as a number on the one of the LCDs and can be identified via a schedule in the manual. Further testing can be done with a signature analyser such as the HP 5006A. There is a fat wodge of data tables in the manual to help.

So what are the defects?

(1) The sets are now about 25 - 30 years old and time has taken its toll on the very many tantalum capacitors. Many will need replacing. Some owners report buzzes on the audio, probably caused by bad tantalums decouplers on the switching voltage regulator ICs.

(2) The older LCDs are prone to "bleed" and eventually become unuseable. You are most unlikely to find any original replacements, but there are a number of sources of alternatives. Google will reveal them.

(3) The PSU buzzes a bit but not too irritatingly.

(4) Potential buyers must note that there are dozens of variants and filter arrangements. A surprise variant sometimes encountered has a 400Hz b/w filter centered on a frequency displaced 1700Hz into the lower sideband. This is a telex-over-radio filter and the 1792 will probably have come from a ship or an oil rig. Others have differently displaced filters - probably for Piccolo or other data systems. (The UK military and diplomatic wireless service used the excellent Piccolo system for many years).

Resist the temptation to swop boards from dud sets. The hard-wired software is likely to be incompatible. I have a chart prepared by Racal of what goes with what if anyone has a query. In particular, the receivers built for remote control have control boards that may not work in "stand-alone" receivers.

(5) The RF amplifier cannot be switched in and out from the panel. You have to open up the module and readjust two internal links.

(6) The lack of any form of pre-mixer selectivity exposes the set to intermodulation from very large signals, but the interecept point is so high that this will be rare. At 25kHz spacings it is about +31dBm. At a professional station using log periodics or rhombics feeding wideband distribution amps this is a possibility but hardly for the average listener.

Racal made a self-tuning passive preselector for this radio comprising 4 tuned circuits. The tuning information is derived by linking the preselector to the receiver's Local Oscillator output and a clever look-up table in the preselector and motor-driven servo mechanisms turn the capacitors and range switching. But these units are rare and extremely heavy. I believe the main use was on board warships.

Overall this is a good receiver that will deliver very nice sounding audio but it has none of the "frills" of a dedicated amateur receiver such as a noise limiter, noise blanker or rejection tuning.

Contact me on michaelob666@ntlworld.com for any points or queries.
 
IW0HEI Rating: 5/5 Sep 22, 2007 05:44 Send this review to a friend
Outstanding  Time owned: more than 12 months
Manufactured for, and purchased mostly by, government agencies, it's a fantastic receiver.
 


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