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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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Switch mode power supply failure mechanism?
I have a use for one of these all new unused in 1990s, not stored in a
shed or garage over the years. A few dozen ,made in 1992 nice compact 70W Electronic Halogen Transformers (so called including labels as primary and secondary on the external label). Outputs the unrectified HF to drive the 12V lamps. They used to work 15 years or so ago. Now I've tried 3 and all the same , should be 12V HF ac and if attached to high speed diode and a cap and no load then only about 10 to 12V dc. No way will they power even a 20 W lamp (supplies are rated 20W to 70W) tried 40W in case load dependent. Scoping, with no load and a complex oscillation signal. Then as little as 27R load very poor mark space ratio of output and fraction of a volt as DC. These units are potted with that white hard-rubber-like stuff but I have one that I excavated out a lot of the fill and that is the same fault. No green corossion stains or oil films seen associated with any components still in the fill (only removed enough to check what components were in there. Philips EHC 070 S/40 , no controller IC , 2 STL57 driver transistors and the small componentry as SMD on a small daughter board. Stable 350V on the rectified mains caps, polyester, no electrolytics at all in these units. Transformer is good up to 1Gohm on insulation test. Fuse and thermal fuse in the exposed one are good andother stuff cold test ok |
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