The Racal TRA-3900 is a very interesting radio. In recent years quite a large number of these have turned up at various British miltary surplus auctions and evey now and again they pop up on ebay.

They were developed by Racal in the late 80’s and first couple of years of the 90’s as part of Racal’s bid for a replacement of the British armys ageing Clansman radios (I have a PRC-320 and PRC-351/2). They share many of the same internal modules as the Racal (now Thales) Panther radios and the Candian PCR-522. How many were made I don’t know, but it would seem many hundreds as there was a need to test all sorts of different configurations and setups in all kinds of vehicles and other scenarios.

At the time it was a very advanced radio. It covers 30 to 108Mhz in 25Khz steps. It has built in encryption for both speech and data. The encrypted voice signal sounds like white noise on a conventional receiver. In addition to the encryption the radio features 100 hop per second frequency hopping to make jamming and tracking harder. It’s worth considering that the Canadian military are still using the almost identical PRC-522 today and have just completed an update to their radios to extend their data capabilities. If the British army had chosen these radios to replace Clansman it’s quite possible that they would still be in use today. They are extremely well made.

For those of us that now own these radios getting any information on them is next to impossible, I have never seen any form of manual. It appears that the only way to programme one for anything other than clear speech is to use a fill gun to load the encryption keys etc into the radio. There doesn’t seem to be a way to manually enter the needed KEK’s (Key Encryption Keys). However it is perfectly possible to enter simplex clear speech frequencies into the radio manually, so you can use them on the 50Mhz and 70Mhz amateur radio bands. BUT the radios require the military spec 150Hz subtone to open the squelch. You can adjust the frequency of the receive subtone to 154Hz so you can use radios with 154.1Hz CTCSS provided the CTCSS tone on the transmitting radio has good deviation. Re-tuning is done with the left (looking from the front of the radio) of the 3 potentiometers on the IF module.

Another niggle with these radios is the need to initialise the radio whenever the power supply has been disconnected for more than about 10 seconds. When you turn the radio on, assuming all is OK it should run through it’s startup tests. If you press the enter key the radio should display “Testing”.

At the end of the test routine the radio should then show “Radio OK”. Next the radio will display “ZALL TAMPER” because the radio has been zeroed (Zero All Tamper). This is a warning radios encryption keys have been erased. Press the “Return” key to clear the warning. Next you will get an “INIT REQUIRED” message. To Initialise the radio turn the Comsec switch to INIT and turn the Volume control to the “*” position and press the enter key. Wait a few seconds before first turning the comsec switch to CLR. You should then see a “INIT DONE” message.

To programme a clear speech channel set the COMSEC knob to CLR. Then turn the volume knob to set. Use the up and down arrows to step through the options until you get to the NET CI option and press enter.

Next use the down arrow to go down to the CLR FREQUENCY option and enter the frequency you want. Remember to enter a “0” first if the frequency is lower than 100 Mhz.

Repeat this for each channel you want to programme, simply turn the channel (NET) control to each channel in turn and enter your desired frequency. Assuming the HUB/backup battery is OK, Frequencies are remembered even when power is removed from the radio and will still be there after the radio is initialised.

If you obtain one of these radios it’s likely that the hold up battery (HUB battery) for the radios memories will be flat. It is a 3V AA sized lithium battery soldered to the motherboard underneath the MOD/SYNTH module. It’s quite easy to change, the radio will throw up all sorts of error messages if the battery is flat. I have two of these radios and both seem to work perfectly, however if you perform a full BIT (Built In Test) routine they both return a “VCM 2085” error. I’m not sure what causes this, it might be because the internal real time clock is not set correctly – The Panther radios throw up a similar error if their memory battery goes flat or if the power supply module is disconnected from the chassis/motherboard and you have not set the clock.

If you have one of these Racal radios I would urge you to replace the 47uF tantalum capacitor in DC-DC converter on the back of the main power supply board (not all have a Tantalum cap, some use a low ESR Electrolytic). This is inside a small screen can on the rear of the power supply board. They have a tendency to go short circuit, explode and catch fire. This will burn out the input choke to the converter and can cause extensive damage to the PCB that may be difficult to repair!  Other common problems with these radios is failing high voltage decoupling capacitors in the power feeds to the tuning circuits of the RF unit. The capacitors start to pull down the 85v dc rail that is used to tune various circuits throughout the radio until the radio will no longer work on certain frequencies or frequency bands. You can access the 85v rail on the bottom of the main motherboard to measure it’s voltage. If you find this is getting pulled down below 80v on certain bands or frequencies then it would be good to look for bad caps on the RF board. The 85V power supply itself is located on the SYN/MOD board.

The radio is of a modular construction. The CIM and VCM modules are the brains of the radio and contain most of the processing, cryptographic and control circuits. The IF unit contains the receive IF amplifier, discriminator/demodulator and squelch processing circuits. The RFH unit contains the receive front end and bandpass filters. These are tuned by varactor diodes. The Syn/Mod module contains the reference TCXO and frequency synthesizer VCO. It also has an interesting high power, push-pull TX VCO that outputs a very high RF level (1w ???) that is fed to the TX Power Amplifier that is in the very rear of the radio underneath the power supply board. Also on the Syn/Mod board is the 85V power supply.