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Old 09-15-2022, 05:50 PM   #1
everell
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Join Date: Jan 2009
City & State: Williamsburg, Virginia
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Line Voltage: 120VAC 60Hz
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Posts: 1,229
Default EVGA 500 watt with fan twitch

This is an EVGA 500 watt Model 100-W1-0500. On power up only has a small fan twitch. Usually this is just a bad capacitor on one of the output lines. Not this time. I pulled out my storage scope and looked at the output lines during the first 10 msec after power up. 12 volt line was 11.5 volts. 5 volt line was 4.5 volts. But....the 3.3 volt line was also 4.5 volts. Couldn't find a short between the 5 volt line and the 3.3 volt line. So pulled out my multimeter and tried to find a bad component using resistance measurements. Nothing. So I took a guess that the mag-amp transistor might be bad. It uses a tiny SOT transistor marked M7A. The only cross reference I found was a MMBT2907A. So I changed this transistor and the power supply came back to life. The 12 volt line measured 12 volts and the 5 volt line measured 5 volts. But the 3.3 volt line measured 2.36 volts. Looks like the transistor was bad but the replacement must not be the correct one. Help...can anyone find a proper cross reference for a SOT transistor marked M7A.
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