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Old 02-15-2013, 02:58 AM   #3
everell
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Join Date: Jan 2009
City & State: Williamsburg, Virginia
My Country: USA
Line Voltage: 120VAC 60Hz
I'm a: Hobbyist Tech
Posts: 1,222
Default Re: Repairing Bestec TFX0250D5W

After replacing the bad caps, the power supply tester LEDs just blinked, the fan just gave a tick. So I measured the output diodes thinking that maybe one was bad. No, they all checked good. So I started tracing the output circuit to see what was in this power supply. To my surprise, there are NO load resistors on the output rails. So I set up a jig with a 12 ohm resistor for the 12 volt rail, a 5 ohm resistor for the 5 volt rail, and a 3.3 ohm resistor for the 3.3 volt rail. The power supply now came up working as expected.

The reason for writing this post..............make sure the power supply you are working on has load resistance on the output rails - or you may be chasing a ghost.

For anyone else working on this particular model Bestec, here are some drawings of the 5vsb circuit and the output circuit. Also a roadmap of the output capacitors. Perhaps this will be helpful to someone.
Attached Files
File Type: pdf Bestec TFX0250D5W 5vsb circuit.pdf (37.8 KB, 183 views)
File Type: pdf Bestec TFX0250 secondary side output.pdf (46.0 KB, 130 views)
File Type: pdf Bestec TFX0250D5W roadmap.pdf (25.8 KB, 140 views)
File Type: pdf Bestec TFX0250 roadmap.pdf (22.9 KB, 102 views)
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