Thursday, 14 August 2014

Clansman PRC-351 - removing the 150Hz tone

The Clansman PRC-351 is a military VHF manpack set that was manufactured for the British army in the late 1970s .  It is fully synthesized and covers 30MHz to 76MHz in 25KHz steps, so it is suitable for use on the amateur 6m and 4m bands.
Power output is about 4 watts of FM, however an optional 20w clip on amplifier is also available which increases this to 20w .

With the British Army’s move to the newer Bowman series of radios, many older Clansman series radios are becoming available on the surplus market at reasonable prices.  I picked one up for less than €50 although I then had to spend a further €20 on a 24v/1Ah battery and €15 on a clansman headset.

The complete set is shown below –


As the radio has a BNC socket in addition to its 1.2M whip antenna , I was able to connect it to an external 4m half wave antenna and was able to make a couple of contacts on 4m to Artur EI7GMB and Michael EI3GYB  - distances of 15KM and 40KM respectively – not bad for 4w out.

Unfortunately due to the set’s 25KHz channel spacing it does not cover the Irish 4m calling frequency of 70.2625MHz, so I needed to set up contacts on another 4m radio first then QSY to a 25KHz spaced channel to make a contact.

Both Artur and Michael commented on the loud 150Hz tone present on the transmitted audio of the PRC-351. This is due to the military tone squelch used by US and UK military radios.

google search lead to the PRC-351 Yahoo group which had a note on how to disable the tone squelch.
The process is as follows –

-       Disconnect the battery, antenna and audio accessories.

-       Open the 4 Allen bolts on the side of the set containing the audio sockets

-       Carefully remove the end panel of the radio – it is connected to the main radio body by a fragile ribbon cable

-       remove the connector joining the end panel to the radio.


remove the 4 Allen bolts on the control panel  and slide the radio out of its housing .

Locate R9 on module 13 – this is the preset resistor that adjusts the level of the 150Hz tone.

turn this pot fully anti-clockwise

Re-assemble the set and you are done.  Watch out for the drying agent sachets included in the case to protect it from condensation

After this simple adjustment Artur confirmed that the 150Hz tone was gone and the transmitted audio sounded much better.

More information on this interesting radio is available on the VMARS web site - here

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