Thursday, 15 October 2015

Great news - PF8 Non-fault

I had expected a long, drawn out fault finding task with the PF8 receiver, something I wasnt looking forward to on account of its revolutionary, and probably unique, dual PLL frequency system.

The PF8 uses dual, linked Phase Locked Loops in the receive chain, providing an unusual level of AFC, where both the channel crystal and the IF CIO crystal are pulled to correct errors in the receive signals frequency. However, in the case of this unit, the marked receive frequency should have been 453.200MHz, but was in fact 454.35MHz, and the Rx crystal was marked 49.88888MHz - a value that doesnt result in either receive frequency!

Adjusting L6, the rx crystal tuning, did not pull the receiver to 453.200MHz. So, I decided to do the obvious quick test, and swap the crystal for the rx crystal from the donor set. This is on 49.5638MHz, and should give 456.775MHz as the receive frequency...

...which it does! And, rather than the previous -30dBm minimum detectable signal (with the proviso that this is from an untuned whip antenna!), suddenly the receive MDS is -80dBm, and roughly -75dBm for 12dB SINAD, under the same test. If this was a radio with an antenna socket, im confident i'd have an industry standard -118dBm receive! I set the squelch level RV2 to break at about -70dBm in this set up.

Flipping the sig gen output level to 0dBm (1mW), I went for a walk down the garden, all the time with the 1kHz modulation tone purring loud and clear from the PF8!

So receive works fine, transmit works fine, although could do with the frequency trimming a little, as its about 2.5KHz high. squelch works fine, volume control is fine, both PTTs and mics are fine (deviation limits at about 3.5KHz)

What I need now are a pair of crystals for 70cm. This radio is clearly a U0 model 440-470MHz, rather than T1 405-440MHz as really needed for 70cm, but the service manual shows which parts are different, and I dont think there will be any problem with the conversion.

The donor set is proving very valuable!

In many ways, these were remarkable radios for their time, which it seems led to their demise. Incredible in the level of miniturisation, long before the advent of surface mount technology, they utilised very small components on plug in modules, or hybrid 'thick film' modules, and uniquely a multilayer PCB. Power is derived from just 2.4v (2x sub-C 1.2v NiCds) using an inverter circuit, when most other handhelds were using 9v NiCds or 9 to 12v packs. With the use of Roger G3XBMs skeleton plate antenna, there was no external aerial to get broken (as in the case of the PF1's telescopics) or catch on anything. The dual microphone system allowed it to be used almost like a mobile phone! These two features - the internal antenna and the mic at the base away from the speaker, would not be re-adopted by the cellphone industry for nearly quarter of a century!

The radios main downside is its low transmit power. In an era where handheld radios were putting out 1-3W, and many breaking the 5W barrier, its fixed 500mW was limiting. But this was never a radio intended for 'user-to-user', but for use with a high power base station, with antennas located high and clear, or a network of linked repeaters.  Its compactness, when compared with contemporary units in use by the UKs police forces, meant it was ideal for CID and other covert users, being discretely hidden away in the inside pocket of a jacket or the back pocket of the coppers Farrahs. A feature which almost certainly got it noticed by the producers of The Professionals - its exactly the kind of discrete radio that CI5's commander Cowley would have specified!

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