Converting the PRC-349 for amateur band use?

The Clansman PRC-349, a huge 'hand-held' 1/4W low-band FM transceiver, is the British forces version of the Racal BCC349. These are regarded as useless for amateur purposes as the 10MHz band they cover, from 37 to 46.975MHz, in 25kHz steps, doesnt cross any of the amateur bands.

But - Racals own advertising of the BCC349 clearly states that it can be supplied covering ANY 10MHz segment between 30MHz and 76MHz.  Since the frequency control element is the most expensive (other than the RF Power device), and Racal wished to sell to as many markets as possible, its very unlikely that a major amount of work was required to change the band. Racal would surely have used a standard set of modules, with minor component changes.

So, knowing the operation of these sets, there are a number of necessary steps to convert them to amateur use -

1. Change the Rx and Tx frequency references
2. Disable or otherwise overcome the 150Hz tone squelch control
3. Re-align the filtering/oscillators
4. Modify LOUD and WHISPER settings to have the same mic gain on Tx

Sounds easy? Well, possibly, or possibly not. Studying the frequency synthesis system in use however throws up an interesting aspect - the VCOs do not feed directly to the divider chains! Instead, each (Tx and Rx VCO) is down converted with a fixed reference crystal oscillator, and the resulting roughly 2.5 to 12.5MHz band is fed to the dividers. It makes for a reasonable assumption then, that so long as the reference crystal frequencies are selected to mix the required VCO frequencies down to this range, and the VCOs are modified to cover the desired frequencies, that it should be possible to make the synthesiser work in a segment that covers an amateur band.

However, this wouldnt be all. The VCOs would need modifying certainly to oscillate as required, but also the Tx PA tuning, Band Pass filtering, and Rx front end filtering/tuning will all require modification to the desired frequencies.

It just so happens though that I have a copy of the Racal BCC349 technical manual, and all the necessary circuit diagrams! If we aim for moving up in frequency, to perhaps the 6m band (at least to start with, 4m would be better but lets go easy at first!), then I can see from the diagrams there isnt really too much to modify, and since we are going up, modifications to tuning coils is by removing turns, which is far easier than adding them to resonate lower! It might be that some parts will even align to 6m as is, or perhaps with some simple changes to fixed capacitors.

The likely difficult, or perhaps easy but expensive, part will be replacing the reference oscillator crystals. Custom crystals are in the order of £35 a pair! But, it might be possible to find stock channel pairs from older PMR equipment that will work - at least for testing.

In fact, probably the hardest part of a conversion will be dealing with the tone squelch! This might be defeated the way it is in the PRC-351, with an internal 160Hz oscillator, the difficulty here is that there is almost NO free space inside the PRC-349 AT ALL! So where such an oscillator would be fitted ive no idea!

What I can do though, if find out of my stock of spares a full set of working modules, and arrange a bread-board set-up, so that the various sections can be experimented  on to see if the adjustments are in fact possible - I just need to get hold of a suitable pair of 3rd overtone crystals for the reference oscillators now!