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Volume XLl THE SHORT WAVE MAGAZINE 361

Volume XLl THE SHORT WAVE MAGAZINE 361

A LESSON, A MORAŁ AND A PLEA

A CAUTIONARY TALE FOR CONSTRUCTORS — AND WRITERS

LES MAY, G4HHS

“T CANT get my compressor to work”, said the voice at the

J. other endofthephone. It was John and I knewallaboutthat compressor, having received for the past month a day by day account of his struggles to get together exactly the right components, whilst he waited for the PCB to arrive. “Have you got the ICs the right way round?” I asked knowing the answer would be yes. “ I*ve checked that and the transistors. One 1C and a transistor get hot and the LED is on all the time.” “Mmf”, then a long pause while I tried to think of something constructive to say. “Don’t worry, bring it in tomorrow and wę’ll get it going.” I sounded morę confident than I felt.

John and 1 took the RAE together, spent a year as G8’s and then took the Morse test within a couple of weeks of each other. He doesn’t build much equipment usually preferring to buy black boxcs, having once asked me, “Why should I spend money on making something which might work when I can spend a bit morę and buy something I know will work?” Ali the same I could be surę he wasn’t likely to have madę any very obvious mistake and I knew he had checked out the next few copies of the magazine for corrections to the Circuit even before buying the components.

Over lunch next day I studied the description of how the Circuit was supposed to operate as a prelude to trying to find the fault. Evidently the proof-reading had been skimpy because references to each of the multifunction ICs were confused, but even this promising start lead nowhere. Knowing how the Circuit should operate had given me some idea of what voltagcs to expect at various points in the Circuit and some clues about where to start looking for a fault.

That evening I noted the 1 resistors and the careful soldering but to be on the safe side I searched for solder bridges, misplaced transistors, ICs the wrong way round, wrong value resistors, dry joints and anything else which could be causing a fault. Baffled, I applied power and surę enough the LED showing the unit was in compresssion modę came on, even with no signal applied. A few preliminary prods with the voltmeter showed the transistors were hard on, with practically fuli negative raił voltage on all three leads of the pnp transistor buffering the output of the IC connected as a precision rectifier. “Ha! thelC’’ Ithought. Power off and out came both chips. Still fuli raił voltage. Further prodding around the adjacent components began to suggest I might have missed a solder bridge to the negative raił. Prolonged search proved fruitless. A dud transistor? Could be. But three in the same Circuit?

By now I had been searching for three hours for a fault in a two-IC, three-transistor Circuit. Nothing seemed to make sense and I was thoroughly frustrated. Without any elear idea of where to look next I thought I’d better double check my initial assumptions starting with the lead out pattern of the transistors. The nearest catalogue to hand confirmed that the sketch I had madę earlier and the circuit overlay, were correct. Better check with my copy of Towers just to be surę. All was revealed: there are nine versions of the BC212 in two quite different but superficially identical packages. The component list specified BC2l2’s. Half my catalogues listed BC212’s and the rest the ‘L’ or ‘B’ versions without mentioning there were two sorts of lead out.

It was a bit of a thin tale I had to tell John later that evening when 1 phoned to explain my łąck of luck in getting the Circuit to work. I wasn’t wholly convinced by the explanation I appeared to have stumbled on. Finally we agreed I should buy some new BC212’s and try substituting them.

Next day I got to the shop as the shutters were going up. “Any BC212’s?” The assistant grabbed a tray fuli and asked “What sort? There are two sorts with different connections.” For a moment I felt surę I was the only person in the world who did not know this secret. 1 bought three.

Ten minutes work after tea sorted out everything. John collected his compressor and an hour later phoned to tell me everything was working fine. His faith in homebuilding had been restored. For my part I had learned something new about a very commonplace transistor. And that is a lesson I shall not forget!

The morał of the story? Check first and solder second! It’s the work of only a moment to check a resistor, and checking a transistor or a diodę takes very little longer. Clip the black lead of the met er, switched to its lowest ohms rangę, to the lead you think is the base, then stab each of the other two leads in turn. If, as is usually the case, the black lead of the meter is connected to the positive terminal of the internal battery, then an npn transistor will show a lowish resistance between each of other two leads and base; clipping the red lead to the base should indicate a very high resistance. Reversethe procedurę for pnp transistors. It takes less time to do than to describe and had it been done by John would have immediately revealcd the anomalous base connection and prevented much frustration. Unbranded transistors can often be sorted and used in noncritical applications if this test is applied.

Please, potential authors, next time you write up your latest project, check that the averagc amateur can reproduce your masterpiece from the information you give. Obviously no-one is going to tackle a multimode transceiver without careful thought but the typical smali project should be as bug free as possible. Most of us have limited test equipment, limited reference books and limited knowledgc. If you used a particular transistor or type of capacitor because it was in the spares box then give a generic alternative if possible. When a particular component is essential then giving the supplier can be very useful.

It’s not being condescending to cxplain carefully something you take for granted. Somewhere, someone is starting their First project.

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“. .. give me a ‘Roger \ . . .