Electronics > Repair

ATX PSU 3.3V rail too low - dead dual diode?

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HwAoRrDk:
I'm attempting to troubleshoot a PC ATX power supply that has a faulty 3.3V rail. The output is too low, about 2.7V. All other voltage rails are perfectly fine, by the way.

I found a third-party schematic that appears to match what I have (same controller chip, etc, albeit for a 250W model, not the 300W I have), to trace the circuitry of the 3.3V rail.

The dual-diode package I've circled on the schematic (D13, an SBL1040CT in my case) appears to be completely shorted between centre pin and the other two in either polarity. It measures about 5 ohms on resistance. I'm assuming the fact I'm measuring it in-circuit should not make any difference and give a false positive, right?

How would this diode being shorted still allow the PSU to give some kind of output on the 3.3V rail?

I suppose I should de-solder it from the board and double-check. Also, I guess I should put the 3.3V output on the scope to see if it's oscillating and meter is showing 2.7V as just the average.

P.S. L3 and C22 are not populated on my PSU, with L3 being linked out.

wraper:

--- Quote ---How would this diode being shorted still allow the PSU to give some kind of output on the 3.3V rail?
--- End quote ---
Look on the schematic. There is 4.7 OHM resistor on the output to GND and transformer winding is connected to GND too. Nothing wrong with low resistance across the diode.

wraper:
Rather look what's with D28, D29 and R73.

HwAoRrDk:
I 'scoped the 3.3V output. It is oscillating, between approx. 4V and 1.8V at exactly 100 Hz.

Oh, and I forgot to mention the PG signal never goes high.

wraper:
Did you connect any load to all outputs?

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