Repair ATX power supply

[Skeleton Man]

I did remove both of them from the circuit for testing..

And you get '0' on a diode test between any combination of pins ?

That's correct.

They're stuffed in that case.

Can you buy replacements ? (original or newer models)

Would bad caps have caused them to fry ?

Chris

 

[JANA]

These power supplies are replaced, and not repaired. The time and cost is
not worth it.

--

JANA
_____


Hi All,

I was wondering does anyone here have a good guide for how to go about
repairing an ATX computer power supply ?

I've never done anything like this before, so any help is appreciated..

Chris

 

[JANA]

If you have a way to operate the supply without it connected, and you can
have the schematics, and necessary test gear, go ahead and start
troubleshooting. It can be as little as a simple component, to an array of
parts!

Take care when working on these switching supplies. The drive voltage and
drive current can be lethal!!!

--

JANA
_____

Way I do it is to go to the computer store, (or Staples, or Office
Depot, or Best Buy, or...) and say: got a power supply?

I have replaced the power supply, and now I want to repair the old one.. I
shouldn't have to throw out a $50 peice of equipment because a 25c part is
broken.. (spending a day or two finding the problem doesn't worry me)

The problem is that it still supplies +5V standby, but refuses to turn on (I
have a load attached and I do have the correct wires for PS_ON and ground).

No fuses are blown, and nothing appears or smells obviously burnt.. I was
told to check for open/high resistors near the large filter caps on the high
side, so I did and they both read the correct ~220Kohms.. there's two
diodes on the high side and both those are ok also.. what else should I test
?

Chris

 

[Sam Goldwasser]

That's correct.


Can you buy replacements ? (original or newer models)

Would bad caps have caused them to fry ?

Could be.

Throw it away. At this point, not only will it cost more to replace
all the bad parts than buying a new spare supply, you're likely to
never repair it as many other things can blow, and if you fail to find even
a single one and replace it, they may all blow again instantly.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Eeyore]

That's correct.


Can you buy replacements ? (original or newer models)
http://cgi.ebay.co.uk/Transistor-2S...ryZ36332QQrdZ1QQssPageNameZWD1VQQcmdZViewItem



Would bad caps have caused them to fry ?

Maybe.

Graham

 

[Meat Plow]

I just tried the supply again (had been sitting a while since it first
stopped working) and this time the fuse blew..

The rectifier appears ok, but the two power transistors (2SC2625) are giving
zero resistance between any of the pins.. does this mean they're fried ?

Not to be mean but if you you don't know something basic like what these
reading mean it's not likely you're going to repair this device by
yourself.

 

[Franc Zabkar]

That's correct.


Can you buy replacements ? (original or newer models)

Would bad caps have caused them to fry ?

Chris

I find it strange that your chopper (?) transistors/MOSFETs are
shorted C-to-E or D-to-S, yet the fuses are intact. Maybe you have an
open NTC resistor ???

- Franc Zabkar

 

[Skeleton Man]

Throw it away. At this point, not only will it cost more to replace

all the bad parts than buying a new spare supply, you're likely to
never repair it as many other things can blow, and if you fail to find even
a single one and replace it, they may all blow again instantly.

It's more an exercise in troubleshooting and repair than anything.. If I
wanted a spare I would buy one..

Also, if I can repair a $30 power supply, the same basic skills should apply
to a $200 power supply.. you wouldn't just throw it away and buy a new one
then.. (doing component level repairs is an an extra skill I'd like to teach
myself)


Chris

 

[James Sweet]

That's correct.


Can you buy replacements ? (original or newer models)

Would bad caps have caused them to fry ?

Chris

Yes, you can buy replacements, what are the numbers on them? Make sure you
check all the other semiconductors, this sort of repair is challenging, if
another defective part remains, everything you replaced can blow again
before you know what happened.

 

[James Sweet]

I find it strange that your chopper (?) transistors/MOSFETs are
shorted C-to-E or D-to-S, yet the fuses are intact. Maybe you have an
open NTC resistor ???

He said the fuse blew.

 

[John Tserkezis]

Also, if I can repair a $30 power supply, the same basic skills should apply
to a $200 power supply.. you wouldn't just throw it away and buy a new one
then.. (doing component level repairs is an an extra skill I'd like to teach
myself)

Doesn't work like that in real life.

The parts alone will cost more than the replacement of the power supply.
Likely even for the $200 power supply.

And whether or not you like it, your time IS worth something, and it's
usually a LOT more than a measly $200 for a power supply.

There may be some conditions that control this, such as this may be a
special-purpose supply that can't be replaced, or off-the-shelf replacements
are not available for some time, and you can fix it faster than the time the
replacement comes in.

And, from what you've described, it doesn't appear you're up to the task
anyway, and 'fixing by correspondence' doesn't work in this case. Much like
trying to get your mother to completely dismantle your car engine, repair and
reassemble going off instruction on usenet.

 

[James Sweet]

Doesn't work like that in real life.

The parts alone will cost more than the replacement of the power supply.
Likely even for the $200 power supply.

And whether or not you like it, your time IS worth something, and it's
usually a LOT more than a measly $200 for a power supply.

Huh? I've repaired a lot of power supplies over the years, and never
encountered one where anywhere near $200 in parts was required.

The value of the power supply is nothing compared to the value of the
education it could provide. When I started repairing things, I learned by
working on equipment that was mostly pretty worthless, some of it I fixed,
some of it I broke worse, the odds steadily improved with practice. You've
gotta start somewhere.

 

[Dave]

It's more an exercise in troubleshooting and repair than anything.. If I
wanted a spare I would buy one..

Sorry, but SMPS's are A REAL BASTARD to diagnose and repair... that's why so
many very competent professionals who repair electronic gear for a living
have suggested that you throw it away. It's like saying "I'd like to learn
math, and I'm going to start with this advanced calculus text." If you want
to learn to fix something, choose a) something fixable and b) something
worth fixing. The way in which switch-mode power supplies operate makes
them by nature dangerous to work on as well, which is less than ideal for a
novice as you really can hurt yourself.

Also, if I can repair a $30 power supply, the same basic skills should
apply
to a $200 power supply.. you wouldn't just throw it away and buy a new
one
then.. (doing component level repairs is an an extra skill I'd like to
teach
myself)

Good luck.

Dave S.

 

[Sam Goldwasser]

Sorry, but SMPS's are A REAL BASTARD to diagnose and repair... that's why so
many very competent professionals who repair electronic gear for a living
have suggested that you throw it away. It's like saying "I'd like to learn
math, and I'm going to start with this advanced calculus text." If you want
to learn to fix something, choose a) something fixable and b) something
worth fixing. The way in which switch-mode power supplies operate makes
them by nature dangerous to work on as well, which is less than ideal for a
novice as you really can hurt yourself.

Another thing to consider is: Would you trust that repaired power supply
to be a spare for your new turbo-charged PC if its power supply dies,
given that you may have barely understood what you did to get it working?

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[larry moe 'n curly]

I just tried the supply again (had been sitting a while since it first
stopped working) and this time the fuse blew..

The rectifier appears ok, but the two power transistors (2SC2625) are giving
zero resistance between any of the pins.. does this mean they're fried ?

Yes. You can probably replace them with about any high voltage
transistors from another PC PSU, even an old AT one, provided they're
they same type (NPN) and are rated for at last as much voltage,
current, and power. But if the new ones come in different packaging
you may have to add electrical insulation, such as a silicone rubber
transistor insulator sheet between the transistor and heatsink or a
flanged nylon washer for the mounting screw, emphasis on "flanged".
Be absolutely certain that the transistors are insulated from the
heatsink or they'll instantly blow out when the power is turned on.
BTW the heatsink for those transistors is often connected directly to
about 350V DC, so don't plug in the power unless the PSU cover is
installed and screwed on.

A spec sheet for the 2SC2625 can be seen here:

www.ortodoxism.ro/datasheets/mospec/2SC2625.pdf

This website:

www.smps.us/computer-power-supply.html

has information about computer PSUs, including schematics for a couple
of them.

I think that your PSU has three transformers: main, standby, and one
to drive the high voltage transistors. The fourth thing that looks
like a transformer (in the lower right of your picture) is actually an
AC line filter (OK, it works as a transformer).