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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Had a JTS radio mic receiver on the bench today. "No power", said the job
ticket. With 12 volts going in, the output from the four-legged LDO 8 volt regulator, was almost nothing. A quick stab around with the ohm-meter revealed about 1.7 ohms across the output. Nothing obviously short. Nothing getting hot because the regulator was in a full foldback condition. Loads of surface mount 4558's in there, as well as a good selection of more exotic ICs, and the 1.7 ohms could be measured at any of them. I had a quick word with the shop that it came from, and the guy there was of the opinion that it would not be worth pursuing even with the manufacturer, as it was well out of warranty. "He'll just have to buy a new one" he said. That made me feel bad, as I felt that I had perhaps not pursued it far enough. On the basis that the job wasn't going anywhere anyway, and time had already been spent, I decided to get brutal with it, to see if I could make the short show its face. I turned the power supply down to about 4 volts, and linked across the regulator. I then turned the supply back on and settled down to wait. As it turned out, it wasn't for very long ... A cloud of smoke and sparks shot out of a tiny little surface mount solid tantalum 1uF cap. There are hundreds of these - well, tens anyway! - all over the board. It was but a few seconds work with the iron to whip this cap off the board. The short disappeared with it, so I took my bridging link off the regulator, and let it go back to working normally with a full 12 volt input. This time, the output of the regulator was 7.96 volts, and the power LED lit. A quick tune of the signal generator up to 863 MHz, with a bit of wire in the output to act as an antenna, and the RF and AF LEDs lit. As a final check, I hooked it into an amplifier, and got audio from the generator. Sometimes it pays to persevere ... :-) Arfa |
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