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Old 22nd May 2010, 11:53 am   #2
KeithsTV's Avatar
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,267
Default Re: A puzzling PSU Fault!

If one diode in the bridge has gone O/C that would convert the rectifier from full wave to half wave. Apart from increased ripple current in the electrolytics there would be dc passing through the transformer. The ripple voltage across the main electrolytic would also increase and be at 50Hz instead of 100Hz. If the transformer is small, as I suspect the manufacturers would fit the smallest transformer they could as it would cost less, it would not be able to withstand the dc and would overheat and over time would gradually self destruct. As there is a series pass transistor and the dc output is significantly lower than the input voltage the output would not be significantly affected by the increased ripple voltage. To the user there would be no observable difference so they would continue to use it oblivious to the damage being caused to the PSU.

Well that's my theory.

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