Friday, 16 May 2008

It doesn't look like much,
and it isn't. It's a 14.318MHz Colpitts crystal oscillator, pulled by an inductor and a large variable capacitor down over a 50kHz range to form an external local oscillator for one of my radios, a Racal Minical.

The Racal is normally rockbound (crystal-controlled), with a choice of six channels, but I wanted more freedom. As you can see by the dial marking, the commonly available 14.31818MHz crystal covers a good portion of the cw segment of the 80m amateur band, once the 10.7MHz IF has been accounted for. The numerous other holes in the front panel (and the Belling-Lee connector) are artifacts of previous uses of the box. It has been a regenerative receiver (not a good one), a 14MHz SSB rig, and a 7MHz superhet receiver. The current manifestation uses the mouse connector (violet, six pins) as power from the Racal and LO signal to the Racal. The mouse cable and connector are small, neat and shielded, and so ideal for this application.

It's not enough
I want even more. I want coverage of Topband, 80m, 60m, 40m and a few spot frequencies in between (for use when I'm in uniform as a Sea Cadet communications instructor). I want LSB and USB phone, PSK31/63 and cw transceive capability at 10W output, VOX, audio filters and it must look good. The Racal on its own looks funky and green, but that little plastic box with the grey-painted copperclad panel will always look nasty. So, I have to look to my junk-box for inspiration. I have an old R1132 dial; this is a wonderful object; Bakelite and celluloid, with a slow-motion drive and a big knob. I have crystals which will mix with an MF VFO to cover the bands required, I have fitting skills sufficient to produce a case and the rest is just electronics. I can take time over this project, and hope to keep a record in this here blog.

Just electronics?
It sounds casual; flippant, even. However, the circuitry involved is extensive, but each module consists of ideas and techniques I've successfully implemented before. I already use the Racal for PSK31, I've had it running on LSB (its native sideband is the upper, being of military origin) and I will be trialling cw transmission using a sidetone oscillator (sinewave) in the near future. The PA already exists (an IRF510 unit in a 2-ounce tobacco tin), and works well. Additional, switchable pi-filters for the three octaves will be needed; and the mixer-VFO will be the only 'new' thing.

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