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Old 04-10-2012, 03:01 AM   #1
vanakaru
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Why PSUs die?

I have plagued with various power supply's die on me over the years. Most other problems I can fix usually, but PSU is dead end for me. However I have a feeling that there is just something trivial that gets burnt out that prevents PSU to start(for safety or economical!!! reasons). So I call all of you experienced service people to put some light on this.
Usually I had problems with G4 tower PSUs(there is 5 of these in my scrap box). Then I had G5 PSU that worked one day and suddenly no more the next. But now I got a iMac5.1 (C2D) that just is totally dead in the morning while was shut down properly last night.
Since getting PSU replace is as expensive as value of the whole computer I would hope there is a way to fix dead PSU.

BTW there is no visual signs of bulging caps - the only explained cause I have seen on forums.

Last edited by vanakaru; 04-10-2012 at 03:05 AM.
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Old 04-10-2012, 03:47 AM   #2
SirDice
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Oddly enough stuff dies because it's being turned on and off. When you turn a device on there's always a small power surge. It's this spike that blows things up eventually.

Stuff that's never turned off usually runs for years on end.
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Old 04-10-2012, 04:38 AM   #3
vanakaru
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Thank you for the most obvious!
But do you have an idea what are these things that, as you put: blow up eventually. And how to fix a dead PSU?
My question was deliberately general - just to draw attention and state my frustration. What I am really after is a good tip to fix a PSU. And more specifically what part gets broken most often. To check these first.
Another helpful note would be: how to make sure that the PSU is the one that is dead. Like in iMac what is it that turns PSU on when I press "Power".

Last edited by vanakaru; 04-10-2012 at 04:50 AM.
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Old 04-10-2012, 06:20 AM   #4
SirDice
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Most modern power supplies are switched power supplies. That means the mains power usually goes through a couple of diodes to make it more DC instead of AC. Then there's some logic that 'pulses' that power on and off in a certain frequency. It's usually this circuit that's fried.
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Old 04-10-2012, 08:02 AM   #5
wendell
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Quote:
Originally Posted by vanakaru
Thank you for the most obvious!
But do you have an idea what are these things that, as you put: blow up eventually. And how to fix a dead PSU?
My question was deliberately general - just to draw attention and state my frustration. What I am really after is a good tip to fix a PSU. And more specifically what part gets broken most often. To check these first.
Another helpful note would be: how to make sure that the PSU is the one that is dead. Like in iMac what is it that turns PSU on when I press "Power".

I faced these problems in the years I serviced video equipment. These switched supplies, if I remember correctly, used a power oscillator to up the 60 HZ frequency to 15-20 Khz, which allowed the power transformer to be physically smaller and made for good filtering with smaller capacitors. Usually the capacitors in the oscillator circuitry would change value with heat and age. Even with supplied component repair kits, I never was successful in repairing these modules. Maybe others had better luck, but for me it was more cost-efficient just to replace the supply.
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Old 04-10-2012, 10:54 AM   #6
vanakaru
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I see. But a good advise how to check the health of the PSU would be handy here. I tested mine as much I could without a proper knowledge. I get AC in and I get around 3V(2.6-2.8) on GR and brown wire. So in theory if I could pull down the 3V to 0(as the logic board would do) the supply would start. But I have no idea how to. If PSU would start the problem is somewhere else. Maybe DC-DC board?
BTW I take good care not to get electric shock from bare exposed mains.

Last edited by vanakaru; 04-10-2012 at 10:57 AM.
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Old 04-10-2012, 11:00 AM   #7
SirDice
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Quote:
Originally Posted by vanakaru
So in theory if I could pull down the 3V to 0(as the logic board would do) the supply would start. But I have no idea how to.

A 10K resistor usually does the trick.

http://francisshanahan.com/index.php...own-resistors/

Quote:
If PSU would start the problem is somewhere else.

That depends. It's possible you get the correct voltages but as soon as you put some load on it it drops below the required level.
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Old 04-11-2012, 02:45 PM   #8
vanakaru
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It seems to be PSU that is dead here.
I found this very good sounding article about repairing PSUs for game machines. Could that be a good reference?

http://www.mikesarcade.com/cgi-bin/s...e=psrepair.txt
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Old 04-11-2012, 06:21 PM   #9
wendell
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Quote:
Originally Posted by vanakaru
It seems to be PSU that is dead here. Could that be a good reference?

Yes, IMO this is a good procedure to follow. If the transistors or diodes check 'bad,' replace with the original types numbers. DO NOT substitute with 'general purpose' transistors. And the two power transistors should be replaced at the same time.
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