Racal TRA3755 DSP Transceiver

A few weeks ago, my wife and I were out for lunch with a couple of friends when as is normal between like-minded chaps, the topic of conversation turned to Racal equipment. My friend asked if I was up to looking at a couple of TRA3755s that he had bought at an auction.

I have to admit that the TRA3755 is not a piece of kit that I'm familiar with ... other than seeing the occasional bare front panel being sold on a well known auction site. For a start, it's a transceiver, and apart from a brief encounter with a TRA967 many years ago, all the Racal kit that I have dealt with over the years have been 'separates'. What peaked my interest though was the fact that the TA3755 is fully DSP-based, i.e. both the receive and transmit paths make heavy use of Digital Signal Processing'. This is something that most mid to high end Amateur Radio equipment employs these days, but the TRA3755 dates from the mid 1990s, so I was keen to see how well it performed.

As I said, I'm more familiar with Racal's stand-alone receivers and transmitters, with the latter often referred to as drive units. At first sight, the TRA3755 styling is along the same lines as the 37xx series ... same knobs and buttons. But there the similarity ends. Whereas the 37xx series employed a mix of custom 7-segment and dot-matrix back-lit displays, three in total; The TRA3755 employs a single off-the shelf double-row back-lit alphanumeric LCD panel. The other difference is that the TRA3755 is not necessarily mains-operated. Essentially it is a 28V system with an optional built-in mains power-supply. Both the units I was handed were 28V.

So who is the TRA3755 aimed at? In what scenarios is the TA3755 likely to be found? Here, the manual is somewhat vague, and I quote 'The Transceiver is a multi-role (static or mobile) equipment and interfaces with ancillary equipments depending upon the role for which it is required.'

Modes of operation are listed as Voice LSB & USB (J3E), Data LSB & USB (J3D) AM (A3E), CW (J2A) and FSK (F2B).

An optional ALE (Automatic Link Establishment) module is available as well as options for Rack-Mounted and Desk-Top operation. This is the clever bit, enabling the TRA3755 to be remote controlled over normal telephone lines and providing a whole host of features. When fitted, the ALE is referred to as RACE MK3, with RACE standing for Racal Adaptive Communications Equipment. However this board was missing in one transceiver and beyond repair in the other.




I already knew that neither transceiver was mains powered, but the underside of the 'wide' one hinted that something had gone horribly wrong on the inside. There was evidence of something leaking out through the ventilation slots ... definitely investigate this before attempting to connect a DC supply. The photograph on the left shows what I found on the inside of the bottom cover. This was the result of the 3.5V Lithium battery on the ALE board exploding!






Miraculously, the RAM module appeared to be largely unscathed, and it would likely prove easy to find a replacement if it was damaged. I actually thought it might be possible to save the ALE board. That was until I examined the board in greater detail. Sadly, many of the pins on the SMD chips 'down-wind' of the explosion were exceedingly corroded. There was also the fact that a PROM had clearly been removed from the board. Since one of the features available via the ALE/RACE was 'encryption' it is very likely that the software for this feature was originally on the PROM ... and we can't let that sort of thing getting into the wrong hands!



Since the manual clearly stated that the ALE was an option, I figured it worth powering up the transceiver to find out how it coped without the ALE ... did its firmware allow for the board to be removed? When run from a DC supply (28V), the maximum current is close to 20A, maybe slightly more. The 25A fuse on the rear panel is the clue here. The DC input is via a 4-way connector which looks similar to those found on commercial mobile transceivers. However, for some reason it is not specifically identified in any of the items list in the manual, and even if it were, no manufacturer's part numbers are given, only Racal part numbers. Looking at the shape and size, I think it might be a Molex connector. Obviously I would use a PSU with suitable current limit set so as not to do too much damage, should there be a fault. Without having a suitable DC connector, I simply used croc-clips to connect to the chassis and the rear of the power connector. I set the current limit to less than 500mA. The transceiver failed to power up, yet the current drawn was 410mA. I switched it off.

But I had two TRA3755s. I connected the other one in the same manner and switched it on. Bingo! After the reassuring click of a power relay, the LCD display came to life and there was noise from the loudspeaker. I connected an aerial and the noise level increased. Entering various frequencies from the keypad, I was able to confirm that this second 3755 was indeed working. What I found frustrating though was the total lack of any way of monitoring received signal strength ... who would use a receiver without the added confidence of an S-Meter?

Back to the faulty one. The TRA3755 comprises five parts, or modules ... six, if you include the optional Mains Power Unit, seven if the ALE/RACE option is fitted The component parts can be identified as the Control Board, the Front Panel Assy, The Transmitter-Driver/Receiver Assy. (comprises DSP with analogue TX and RX interfacing), The Power Amp and the Switched LP Filter.

Left: The layout of the standard TRA3755. The dark 'vertical' board in the centre is the Control Board, based on a Hitachi HD6475368 16-bit microcontroller running at 16MHz.

The RF Power Amplifier is top-right. There is a lot more to this board than meets the eye. Like most Racal RF amplifiers, it is deliberately designed to be exceptionally robust. Given that a single MRF136 N-Channel Enhancement-Mode MOSFET is capable of delivering 15W at VHF, it should come as no surprise that the first stage of two such devices in Push-Pull, delivers a very modest 200mW, which can be used to drive any of the TA37xx series stand-alone Power Amplifiers. This is followed by a second Push-Pull pair of MRF136s driving a final Push-Pull pair of MRF422 Bipolar devices to a nominal 125W output. The amplifier is bolted to a chunky force-air-cooled heat-sink

Between the Power Amp and the Front-Panel Assy we have the Transmitter-Driver/Receiver module comprising the Frequency Synthesizer PCB, Transceiver Front-End PCB, Transceiver Back-End PCB and 12V Regulator PCB. All four assemblies interface with the control board via a Motherboard/Backplane. A single coax carries the TX and RX signals to and from the Power Amp Assy.

It did not take me very long to ascertain that the the fault which was preventing the unit powering up was on the Control PCB. My initial thought was that a surface-mounting Tantalum capacitor was likely the culprit. I was not disappointed.


This is where a keen eye comes in handy. With the DC applied and the unit switched on, I was looking at the board, when I noticed a thin column of smoke rising from somewhere on the board close to the heavy-duty connector on the side of the PA module. I switched off the power and looked for obvious damage, of which there was nothing glaringly obvious. Intuition led me to the 5V regulator, ML26, and its associated tantalum capacitors and I found that C62 (22uF) was more or less a short-circuit. On its underside there was a small burn mark. C62 was replaced with a radial leaded miniature aluminium type. I reapplied the 28V DC and switched on. I got the reassuring relay click and the TRA3755 powered up normally. Sorted!


Right: Whilst investigating the previous fault, I noticed this. There are four TO220 SUP60N06 N-Channel Power MOSFETs in a row on the Power Amplifier PCB. I noticed that the insulating pad for TR43 was seriously misaligned. Although the Source of TR43 was not shorting to the heatsink I felt that to leave it un-corrected was to tempt fate. To remove the PCB from the chassis requires no less than thirty screws to be removed!


A feature of Racal communications equipment in the 37xx series is the BITE, or Built In Test Equipment. This usually comes in two forms ... continuous and 'on demand'. With continuous BITE, the system is running software which is continuously monitoring key areas, like power supply rails. If a voltage rail drops outside a pre-determined range, a Fault-flag will appear in the the display. If this happens, it is a simple matter to identify the fault using the Menu system. This is certainly the way with the RA3701 and RA3711 etc. and it looks like the TRA3755 BITE operates in the same way. As with the the RA3701 etc. the user can also initiate a series of on-demand tests such as Factory Test, Unit Test or any individual part-test.

With power now supplied by my 40A 28V power supply, and the antenna port terminated in a suitable dummy load, I selected 'Unit Test' for each TRA3755 in turn. How gratifying it was to see and hear the BITE working its way through all the individual tests and for No Faults to be displayed at the end.

When I was repairing the control board in the 'wide' TRA3755, I noticed that it was different to that in the 'narrow' TRA3755. The 'Wide' transceiver has a control board identified as issue 3, whilst the control board in the smaller, or 'narrow' transceiver is identified as issue 5. However, after a protracted period of examining both boards, I have to conclude that although they definitely look different, that difference appears to be limited to the use of smaller (and different) surface mount components.






The photograph on the right is obviously a close-up of the 125W RF Power Amplifier, with the amplification stages across the top. The two RF devices on the left produce a modest 200mW for driving the 2-stage Power Amp. The coax connector in the centre of the board takes the 200mW to a rear panel BNC. This can be used for driving stand-alone amplifiers like the TA3762 or multiples thereof.

The three RF connectors along the bottom edge of the board are (from left to right) SK3 which carries RF to and from the Transceiver/DSP module, SK4 which is the RX input from the Filter PCB, and SK2 which is the High Power output to the Filter PCB.





Above, we have the Filter PCB Assembly. On receive, this assembly is by-passed and the RF is passed directly to the PA board which in turn routes the signal to the transceiver/DSP module. On transmit the output from the 125W PA is fed through one of eight Low-Pass filters. The filter operating frequencies are as follows:


The Module also incorporates a VSWR Detector Circuit.



I keep asking myself the same question ... Who is this transceiver aimed at? When you look at the front panel, ALL the semi-technical buttons are conspicuous by their absence. There is no Bandwidth button. There is no AGC button. This is one seriously dumbed-down piece of kit. The same could be said about your typical man-pack transceiver, where robustness and simplicity are a must, in which case simplicity of operation is expected, ... but this is a table-top transceiver. It obviously isn't intended for battlefield operation. So who is this beautifully made transceiver intended to be operated by? It is no more complicated that one you might find in your own kitchen. My theory is that the TRA3755 was aimed at Embassy or Consulate installations.

Perhaps the missing ALE (RACE) PCB Assembly gives us a clue. At the back of the TRA3755 Service manual there is a large section devoted to the ALE. The first paragraph in the Functional Description says this ...

'The primary functions of the ALE PCB Assembly are to provide Voice, Data and Facsimile (FAX) Communications over a High Frequency radio link. The ALE Assembly evaluates and selects the best available frequency channel over which a link can be established in a configured radio network.'

The ALE board also enables the TRA3755 to be remotely controlled via PC or telephone line with encryption. Definitely NOT battlefield comms. What's my opinion of the TRA3755? I have to admit that I'm a tad disappointed in it. I'm a hands-on type of guy, and the TRA3755 actually takes the operator right out of the equation. However I understand the design concept, and there is no doubt that a TRA3755 with a fully configured ALE/RACE option is a powerful and versatile piece of equipment which does not require the operator to be fully conversant with the vagaries of HF communications.