Witlessness and woe

If I've learned anything this evening, it's how to waste three hours work on one simple detail. The new radio was laid out three nights ago, and it looked elegant, compact and concise in its prototype stripboard form. The resonators, 1K pots and 100p COG capacitors arrived today, so it all looked good for a first build. Thursday evenings are free of Sea Cadet obligations, and nothing was planned by the family. I settled down in my untidy position at my desk in the extension, and carefully loaded the stripboard. My youngest son came by to wish me goodnight at around 2100, and I was just hooking-up the tuning pot. The tuning LED glowed and dimmed, the volts were where they were meant to be, but no output from the SA602. I'd put a 100p cap on pin 7 (the oscillator emitter), to take the signal to a keyed buffer for the transmitter. I was patiently listening with my old Realistic DX-392, looking for signs of life. I searched the board for open joints, solder whiskers, anything. I changed the ceramic resonator for another type, I removed the bandsetting padder, I changed the LED coupling capacitor, nothing.

Then it hit me, like a huge, soggy mattress. I am so used to using chips upside-down on a piece of copperclad, that I'd laid the board out with the two chips back to front. The entire layout is scrap, and I face a fresh piece of quadrille paper to start afresh. Ho-hum.

It's wonderful to watch your ideas take shape, and weird to watch the shape change as the ideas mature into workable products. The latest radio is no exception. I went through a panic stage four days ago, and the direct-conversion receiver / cw rig turned into a Pixie II, and back again. Do a search for the Pixie II, it's a fascinating exercise in minimalism. The PA transistor doubles as the receive mixer; keying the PA emitter shorts the receive path and puts the transistor into full drive. On key-up, recovered audio is passed to an LM386, and into a pair of 32-ohm headphones. Usual power source is a 9V PP3, and the housing can be anything from an Altoids tin to al fresco.

I did say it turned back again, and it remains a variant of the Rev. George Dobbs G3RJV's famous 'Sudden' receiver. I'm using Micrometals iron toroids in the receive preselector, and a 3.58MHz ceramic resonator as the frequency controlling element. I need tighter control than George's VFO original, because it's also transmitting. I've settled (for the time being) on a 30kHz band, from 3.55 to 3.58. This includes the QRS 'sandpit' at 3.555 and the QRP centre of activity at 3.560. The radio has a 700Hz transmit offset, given by arranging for a pair of resistors to be switched in and out of the frequency control voltage 'totem-pole', which feeds the LED varactor. Not only am I using a LED as a varactor, but I find that forward bias gives more linearity. This method is not original, but has been used with success in the radio home-brewing world for some years. The other feature of the forward bias is that the LED can be brought out to the front panel, adding interest as the radio is tuned. The LED dims and brightens, glowing well at the lowest frequency.

I'm currently waiting for parts, and I have two evenings ahead of me where I'm committed elsewhere, so it'll be just ideas until the latter part of the next weekend. With a following wind, the prototype may be running inside a week, and taking reports from Southern England. We'll see.

A New Station - M6BMO

My youngest son, Billy, recently qualified as a Foundation licensee. He studied with the Worthing and District Amateur Radio Club, to which we are both very grateful for their help and support. Now Bill is an M6, he needs a radio, and there lies a problem. I can't afford a 'black box' rig for him, and as a Foundation ham he can't use my homebrews. He can, however, use a radio made from a commercially available kit. Some of Tim Walford G3PCJ's kits ( www.users.globalnet.co.uk/~walfor ) are suitable, and maybe I'll buy Bill one for Christmas. Until then, I'll simply have to become a vendor myself!

What I have in mind is a simple kit based on Billy's immediate desires, and which may prove useful to others in the same boat (impoverished, Foundation licence). He wants to use 20m, and cw. This fills my heart with gladness; 20m is my favourite band, the rig is technically unchallenging and can be made in a variety of ways. First thoughts are with either a Pixie II clone or a Sudden / VXO / Pebblecrusher mix. What ever happens, I hope to have a prototype going quickly, and a kit advertised soon after. Then, Bill can build a kit bought with his pocket-money (I'm hoping it'll be that cheap), and get going.