RETRO RECEIVER # 4 - RACAL RA6790/GM

RETRO RECEIVER REVIEW # 4

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RACAL RA6790/GM

by Craig Seager VK2HBT

We all like to criticise our governments, and goodness knows they often deserve it. But DXers have for many years reaped some additional benefits courtesy of the instrumentalities that comprise our federal and state administrations.  You see, bodies such as the Department of Defence, DOC and the several coastal stations around the country used to buy some pretty decent HF radios, at prices well beyond what the average punter could afford.  Whether they were used to monitor vessels in distress, naval frigates or Cold War spooks, these were very well maintained high-end sets from manufacturers such as Rohde & Schwarz, Collins, Harris, Dansk Radio, Cubic – or the British company, Racal.  

 Happily, government bodies eventually “wash out” their disused or replaced equipment via surplus auctions, providing rich pickings for opportunistic enthusiasts, at typically a fraction of the original purchase price. Horror of horrors, many boat anchor type radios have also been dumped into landfill over the years. Yes, Philistines live and work amongst us!
One frequently encountered radio in this genre is the Racal RA6790/GM, very popular in the U.S. (where it was made) and also widely used in Australia. Interestingly, a very different looking radio externally, the Racal RA1792 (pictured left), is a quite similar rig internally – but sold in Britain and Europe. In fact, that is a pretty common thread with Racal radios; those with model numbers starting with a ‘6’ were marketed in the U.S., and those beginning with a ‘1’ in the UK (and most of the rest of the world).

 The 6790/GM is a fairly bare-bones set. Don’t expect to find preamps, notch, passband tuning, memories, clocks or other bells and whistles favoured by SWLs.  This is a receiver designed to sit in a rack on a set frequency, and run hour after hour with an expectation of high MTBF (mean time between failure).  

 Conceived in the early 1980s, the 6790 is a dual conversion design with a first I.F at 40.455 MHz, and the second at 455 kHz. Tuning is achieved using a generously proportioned main encoder knob, or by direct keypad entry (unintuitively, the enter button is pressed first, followed by a leading zero for frequencies 9 MHz and below). Three tuning speeds are offered, and resolution of the LCD frequency display is down to 1 Hz. Available modes are AM, LSB, USB, FM, CW or ISB (if an optional board is fitted), displayed in a separate window, along with signal strength, AGC decay (slow, fast, manual) and bandwidth. All receiver settings are accessible from two banks of membrane-type keys, except for IF and AF gain.

 The rear of the set has an “N” type connector for antenna, 455 kHz IF out, reference out (for TCXO reference frequency of 1, 5 or 10 MHz), multi-function DB25 connector which includes audio line out, and a military type multi-pin remote socket for computer control. The whole assembly is designed to fit in a standard 19” rack, and at approximately 15kg, this is a substantial hunk of metal.

 But the real Racal 6790/GM story is more about what it does than how it is laid out. Selectivity is generally excellent due to high quality mechanical and crystal filters (the mix depends on version), with ample additional slots available. Image rejection is quoted at > 80dB, and RF dynamic range >180dB/Hz. But it is the rock solid stability that sets this receiver apart from many others of the time.

 Purchasing a 6790/GM can be fraught with danger, as these radios are now at an age where components can easily start to fail. In fact, it is difficult to find an example without some sort of fault – and resolution is not always easy, despite the modular assembly. A fall-back is to replace a whole circuit board, rather than try to fumble around testing components in-circuit. Fortuitously, there are plenty of broken sets around that owners sell piece-meal for parts. Look out in particular for bleeding LCD displays, sticky keypads, leaking internal batteries and failed edge lighting (an aftermarket board is available from the U.S.).  Some people even hot-rod their edge lighting by adopting shades of blue, green or whatever other hue takes their fancy!

 Many of the tantalum capacitors that adorn most of the boards can also become suspect over time, due to continual exposure to high temperatures. Happily, the 6790/GM has a B.I.T.E. (Built In Test Equipment) routine that can give clues to some of the problems that can develop, at least down to board level. Some of these sets are sold without any filters, so always check inside! Pricing can be anywhere between A$500-$1000+, depending on condition.

 Answers to many 6790 technical woes can be found from a dedicated Yahoo! Group, which is frequented by a couple of noted experts, including Gary Wingerd (Maryland), a former Racal technician who has rebuilt plenty. There are also ample PDFs of operations and service manuals, available as free downloads.

 There are various versions of this receiver. The 6790 is an earlier variant of the 6790/GM, and doesn’t have a fault LED on the front panel. The R-2174(P) is for military applications, frequently encountered, and has only crystal filters as standard. There are also different options, such as a preselector and VLF extension (simply changing out an EPROM can extend coverage from 500 kHz down to 150 kHz). The 6793 is much rarer, has mechanical pushbuttons instead of “bubble” membranes, and also boasts 100 memories.

 Professional sets are not everyone’s cup of tea, but DXers looking for an accessible, workmanlike milspec receiver can do a lot worse than this one.  
(This article first appeared in the August 2010 issue of the the Australian DX News)

Thanks to Craig for making our acquaintance with another rare radio!



 Missed the other Retro Reviews? Check them all out by clicking below:

Retro Receiver Review # 1 - The DANSK RX4000

 Retro Receiver Review # 2 - The HITACHI KH-3800W
  
Retro Receiver Review # 3 - The SCHAUB LORENZ ITT TOURING T104A RECEIVER

Retro Receiver Review # 4 - The RACAL RA6790/GM

Retro Receiver Review # 5 - The REALISTIC (RADIO SHACK) DX160

Retro Receiver Review # 6 - The KENWOOD QR 666

 Retro Receiver Review # 7 - The RODHE & SCHWARZ EK890

 Retro Receiver Review # 8 - The YAESU FR101S

 Retro Receiver Review # 9 - The SELENA VEGA B206

Retro Receiver Review # 10 - The DRAKE SPR-4

 Retro Receiver Review # 11 - The NATIONAL RF-5000

Retro Receiver Review # 12 - The YAESU FRG-100

Retro Receiver Review # 13 - The MARC NR-82 F1

Retro Receiver Review # 14 - The MURPHY B40

Retro Receiver Review # 15 - The STROMBERG CARLSON AWP-8










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© Rob Wagner, Mount Evelyn DX Report, and contributors 2012-2023

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