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Old 10-19-2021, 07:13 PM   #16
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Default Re: Repairing Bestec TFX0250D5W

Originally Posted by lewisp69 View Post
BTW, tried to do some testing while waiting for the MOVs to come in and blew the 5v stand-by control chip, U2 (TNY276PN). Ordered some of these too. I had replaced the bridge rectifiers diodes and fuse and the chip blew as soon as I plugged the PS into the outlet. I hope it was already damaged and I don't end up blowing up the new chip.
Yes, sounds like it was damaged already from before. Of course, before putting in new TNY276 chip, check its surrounding components for faults. Otherwise, this could repeat.

Also, to actually test the PSU in a safer way (or at least the 5VSB circuit and the main PSU without any load), you can put a incandescent (or halogen) light bulb (40-100 Watts typical) in series with the PSU's input, like this:
The above method should prevent the new stndby IC from blowing up, as well as the fuse and anything else on the primary side.

First, just hook the PSU with the series bulb and see if the 5VSB comes up correctly. If it does, you can try shorting the PS_ON (typically green wire) to ground to turn on the PSU - but do so without any load (or very light load, such as a PC fan or two or an old useless HDD... or 12V car bulbs.)

The incandescent (dim) bulb "trick" works with many PSUs (switching and linear)... though there are a few exceptions, like for example, switching PSUs with APFC circuit. In the case of the Bestec in this thread - it's an older design SMPS without APFC circuit, so dim bulb trick will be OK with it.

Last edited by momaka; 10-19-2021 at 07:15 PM..
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