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Old 09-15-2022, 05:50 PM   #1
everell
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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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Old 09-15-2022, 06:22 PM   #2
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Default Re: EVGA 500 watt with fan twitch

MMBT2907A, device marking is M7A. If the 3.3v supply was getting +5v I suspect something on the 3.3 volts line got damaged ie a zener diode used for protection.
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Old 09-16-2022, 07:10 AM   #3
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Default Re: EVGA 500 watt with fan twitch

My replacement MMBT2907A is marked 2F. I have checked the data sheet of several manufacturers and they all agree that the marking is 2F. Perhaps not all MMBT2907A transistors are equal???
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Old 09-16-2022, 08:22 AM   #4
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Default Re: EVGA 500 watt with fan twitch

I finally found MMBT2907A with M7A marking. It is made by Panjit. On Digikey and Mouser it is the cheapest MMBT2907A you can buy. Price is 8 cents apiece. What do you expect from a power supply made in China!
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Old 09-16-2022, 12:08 PM   #5
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Default Re: EVGA 500 watt with fan twitch

Ummmmmmmm high volume Delta power supplies are made in Dongguan, China. Probably the same with Lite On, Acbel, Fortron/Sparkle, and [u]Your Favorite Brand[/o] power supplies. The key factors are design quality, component selection, and employee retention (= keeping well trained people).
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Old 09-16-2022, 08:04 PM   #6
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Default Re: EVGA 500 watt with fan twitch

Quote:
Originally Posted by PeteS in CA View Post
Ummmmmmmm high volume Delta power supplies are made in Dongguan, China. Probably the same with Lite On, Acbel, Fortron/Sparkle, and [u]Your Favorite Brand[/o] power supplies. The key factors are design quality, component selection, and employee retention (= keeping well trained people).
If i remember right, most EVGA PSUs are made by HEC (or at least the lower end ones are), and i'm pretty sure this particular model of PSU is part of EVGA's lowest end power supply series. I think i have this exact model floating around somewhere, and when i compared it to a mid-range Corsair 400w PSU (also made by HEC) the Corsair was somewhat beefier than the EVGA, despite the lower power rating. I should probably post pics of the two to the power supply build pictorial someday.
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Old 09-18-2022, 12:49 PM   #7
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Default Re: EVGA 500 watt with fan twitch

What does this switching power supply look like inside under the hood
Just curious about this
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Old 10-01-2022, 06:41 PM   #8
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Default Re: EVGA 500 watt with fan twitch

I installed a Panjit transistor marked M7A. End result was the same....2.4 volts on the 3.3 volt line. I replaced the 431 regulator chip. No change. I removed the two output caps and tested for esr. Both OK. Totally frustrated, I finally changed the tester I was using. Now everything measures ok. So it looks like I was chasing a ghost!

Conclusions: EVGA 500 watt power supply had a bad mag amp smt transistor. The transistors marked M7A and 2F are apparently interchangeable. The power supply is now in my garage powering an old KT7A motherboard and working fine. And finally, I hope I am not the only one who ocasionally goes chasing a ghost!
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Old 10-01-2022, 06:45 PM   #9
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Default Re: EVGA 500 watt with fan twitch

Found another EVGA 500 watt power supply with same symptoms. Replaced the 3.3 volt mag amp smt transistor. Original was marked M7A. Replaced it with a 2F. Power supply now works OK. Not sure if this is a design problem or just OEM using a POS M7A smt transistor.
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Old 10-02-2022, 04:59 AM   #10
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Default Re: EVGA 500 watt with fan twitch

Quote:
Originally Posted by everell View Post
I finally changed the tester I was using.
u mean u are using a power supply tester? those are notoriously unreliable for testing and checking a power supply. good ol' multimeter works best. its what i use for checking a psu's voltage levels. i never use those power supply testers.
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Old 10-02-2022, 05:31 AM   #11
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Default Re: EVGA 500 watt with fan twitch

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Originally Posted by ChaosLegionnaire View Post
u mean u are using a power supply tester? those are notoriously unreliable for testing and checking a power supply. good ol' multimeter works best. its what i use for checking a psu's voltage levels. i never use those power supply testers.
I was using a home made tester which had LEDs and there was a load resistor on the 3.3 volt line...which was obviously not correct. The cheap power supply testers you are referring to did a much better job than the one I was using. The real test is a dynamic test using a real mother board as a load. Both power supplies work ok using a real mother board. So the problem in both cases was a defective mag amp smt transistor marked M7A.
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