Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Repair ATX power supply

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


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Default Repair ATX power supply

Skeleton Man wrote:

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


Generally they're cheaper to replace than to repair.

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Default Repair ATX power supply

On Sat, 24 Nov 2007 16:55:23 -0500, "Skeleton Man"
wrote:

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


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?

Since you have not said what was wrong with the supply, I'm sure that
will work well for you too.
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Default Repair ATX power supply

Generally they're cheaper to replace than to repair.

I have already replaced it, but I want to repair the old one if I can as an
exercise.. for all I know it could be a 5c resistor or something that needs
to be replaced..

Chris


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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




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"Skeleton Man" wrote in message
...
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



These are rarely repaired, however often the problem is bad electrolytic
capacitors. Occasionally one will grenade in such a way as to cook virtually
every semiconductor in it, but often they just refuse to start up. Check the
standby power supply, if that's ok then try to figure out why the main one
won't start up.


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Default Repair ATX power supply

These are rarely repaired, however often the problem is bad electrolytic
capacitors. Occasionally one will grenade in such a way as to cook

virtually
every semiconductor in it, but often they just refuse to start up. Check

the
standby power supply, if that's ok then try to figure out why the main one
won't start up.


I think you may be onto something.. I took a closer look and several
capacitors appear to be bulging a bit at the top.. one does have brown
stuff around the edge, but I figured this was rust or crap that collects
like dust, etc.. (but none of the others have this).. all the capacitors
in question are 1000uF - 2200uF, 10 - 16V.. the small ones look fine..

As for standby power, yes I get +5VSB, it just refuses to turn on and
deliver the rest.. is this common ? I'm guessing there is a seperate
circuit for standby voltage ?

Chris


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"Skeleton Man" schreef in bericht
...
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



Long time ago I wrote the following scheme:


begin
I repaired several PC supplies for a hobby, but if your time is valuable,
buy a new PS.

First of all read the sci.repair.faq. Especially the parts on safety and
SMPS.

Most of the times the fault is found between the mains connection and the
transformer(s).

1. In the most simple cases only the fuse is blown. After replacing
this fuse, connect the PS to the mains using the serial lightbulb trick.
- If the bulb burns brightly, you know that the old fuse had a good
reason to quit, so the case is not simple anymore. The first thing
you have to do now is to find the short circuit. The most suspected
components are the mains rectifier, the filter capacitors and one or
more of the power transistors. Use eyes, nose and an ohmmeter
to find scene of the crime. Remove and check the suspected
components. Replace defective components except for the power
transistors at this time. It makes no sense to continue until you fixed
the short circuit
- If the fuse is good but the PS still dead, you can start to check the
voltages.
2. Check the voltage between pin 3 and pin 9 of the ATX-mainboard connector.
This should be 5V.
- If not you have to check the voltages on the mains side. Otherwise it
will be wise to check the voltages on the mains side as well (3-5).
Then continue reading up to point 12, not to miss some explanation.
Continue at 12.
3. The AC-pins of the mains rectifier should show the mains AC voltage.
- If not you may have an interrupted trace or mains filter.
4. Between plus and minus of this rectifier you should find about 310V DC
or 325V DC depending on your mains voltage. I call it the primary power
voltage.
- If not you may have a faulty mains rectifier.
- If the voltage is much lower (analog meter) or jumping around (digital
meter), the large filter capacitors (p.e. 470 muF, 200V) are also
suspected.
5. Both filter capacitors mentioned above are in serial. The midpoint should
be at half the primary power voltage.
- If not, the mains rectifier, the filter capacitors and the parallel
resistors (parallel to the capacitors) may be defective. Another suspect
is a third capacitor (p.e. 1muF, 250V) that leads from the midpoint to a
transformer.

Explanation:
ATX-PS's usually has three power transistors at the mains side. One
connected to a small transformer, the other two connected to a larger
transformer. You can recognize the pair of transistors best by finding the
emitter of one of them connected to the collector of the other.
First you have to deal with the one transistor and the small transformer.
(Go to 8 if you removed this transistor already.)

6. Check the voltage on the collector of the transistor.
- If this voltage is zero or very low there may be an interruption
between the collector and the primary power voltage.
- If this voltage is below the primary power voltage or jumping, there
seems to be switching activity. You can check this with an AC voltmeter
on a secondary coil of the transformer. The reading will not be correct,
but if you find an AC-voltage you have to continue checking the
secondary rectifier and regulator.
- If this voltage is the primary power voltage the transistor does not
conduct.
7. Check the voltage on the base of the transistor.
- If this voltage 0.6V the startup resistor may be defective.
Otherwise the transistor may be gone (most likely.)
8. Disconnect the PS from the mains and take the safety precautions to
discharge the capacitors.
9. Remove the suspected transistor and check it with an ohmmeter or a
transistorchecker. Most of the times you will have to provide a new
transistor. (Beware! Even a transistor that looks good under test conditions
may malfuntion in the actual circuit.)
This is also the time to remove, check and replace other fried, exploded or
discolored components near the transistor/transformer combination.
10. Re-power the PS using the serial light bulb.
- If the lamp is burning brightly you have a short circuit in your PS.
Most likely your (new) power transistor is conducting due to too high
a continuous base-current. You have to dive deeper into this part of the
circuit until you find the couse of this problem.
- If the lamp is dim or not burning at all you can re-check the voltage
between pin 3 and pin 9 of the mainbord connector. Finding 5V you can
continue, otherwise you have to restart your investigation of the small
transformer/one power transistor part of the circuit.(Point 6.) It makes
no sense to continue until this part of the PS functions correctly.
11. Disconnect the PS from the mains when you are done so far.

Explanation:
For the next part of the repair procedure you have to provide some load to
the PS. This is simply because of some PS's will not function well without
load. You may use an (old) main board. Someone ever told me he uses
12V car bulbs, one on the +5V and one on the +12V. I prefer a huge and
heavy old harddrive. Those old basalt blocks (we use to strengthen our
dikes) consume a lot of energy. The one I use, provided enough load to all
the PS's I ever repaired.

12. Replace the power transistors you may have removed earlier.
Reconnect the PS to the mains using the serial light bulb. Check the
voltage between pin 3 and pin 9 of the main board connector. Connect pin 14
of the main board connector to pin 13. This will switch on the main part of
the PS, the part with the two power transistors and the large trafo.
- If your load start to work, check the voltages of the several power
connectors. When they have the correct values your PS is on air again.
Check it out by removing the serial light bulb.
- If (even after removing the serial light bulb) some but not all of the
values are correct, you have a problem. You have to investigate the
failing voltages from the secondary coil of the transformer till the
connector. Quite a challenge.
- If the light bulb is burning brightly you have a short circuit. Most
likely your power transistors are gone so you have to check (and almost
sure replace) them and their surrounding components, especially the start
resistors. Pay also special attention to the freewheel diodes (between
the collector and the emitter of the power transistors.) Don't forget to
disconnect the PS and to discharge the filtercapacitors first! When you
are done, restart at 12.
- If if your lamp is dim or dark but your load does not work you may have
defective or blocking power transistors. A fault on the secondary side of
the transformer is another possibility.
13. Search for switching activity on the secondary coils of the transformer
using an AC meter.
- If you don't find AC-voltage you have to check the voltages on the
power transistors.
- If you find an AC-voltage you most likely have a defective rectifier,
filter capacitor or regulator at the secundary side. Disconnect from de
mains, discharge the filter capacitors and try to find the failing
components with an ohmmeter. You will have to remove the rectifiers
from the board prior to testing because of the secundary coils have
only few windings of thick wire so they are the shortest shortcuts as
far as your ohmmeter concerns. Another trick is to use a controllable
power supply. Connect it to the point where the removed rectifier was
connected to its filter capacitor. Beware of the polarity! Power on both
PS's and rise the voltage of your controllable PS to the level of normal
operating of your defective PS. The regulator that sucks to much current
with respect to the light load will be the main suspect.
You have to go deeper into the circuit of this regulator if you want to
repair it. Another challenge.
14. Check the voltages on the power transistors. The collector of one of
them should be at the primary power voltage, the emitter of the other
should be at the common. The remaining collector and emitter are tied
together and should be at half the primary power voltage.
- If you can't find the primary power voltage at a collector you have an
interruption. Maybe a bad soldering or the like.
- If no emitter is connected to common you also have an interruption.
- If the tied collector-emitter is not at half the primary power voltage
you most likely have defective power transistors. (In my experience they
always die together.) Disconnect, discharge and remove, check and
replace the power transistors and their surrounding components. Restart
at 12.
- If the tied collector-emitter is at half the primary power voltage you
can check the base-emitter voltage of the power transistors. If they are
0.6V you may be lucky and find only defective startresistor(s) and/or
other base circuit components. But most of the times a defective base
circuit will kill its transistor which in turn will kill its neighbour.
So you will have to replace the whole bunch.

Of course, this story does not cover all possible faults of PC-power
supplies, but I only once failed to repair a PS using this scheme.
end


Some Czech, Pavouk, drew a schematic of a common ATX supply and wrote an
explanation of it's inner workings.
http://www.pavouk.org/hw/en_atxps.html
which reading I strongly recommend.

I do not repair them anymore this days as most of the times the power
transistors are gone and new ones are more expensive then a new power
supply.


petrus bitbyter




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Long time ago I wrote the following scheme:

That looks excellent, I'll have a read through and see if I can find the
problem..

A quick question about transformers... mine has 4 of them in total, a small
one near the big capacitors, then two more small ones and a really huge one
(like 4x the size of the rest) near the power transistors.. I was comparing
model numbers with a photo of a simmilar supply, and they both seem to have
the following numbers in their model:

Smallest transformer: 16 (WIN-16LA)
Small transformer: 19 (WIN-19L)
Really big transformer: 35 (WIN-35P)

Are these numbers for one of the voltages or what ?

Here's the pic I was comparing with - different brand, but the layout of
components is almost identical to mine:

http://www.oakpc.com/mimg/eart/images/041221oak0235.JPG

Regards,
Chris


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On Sat, 24 Nov 2007 16:55:23 -0500, "Skeleton Man"
put finger to keyboard and composed:

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


Notes on the Troubleshooting and Repair of
Small Switchmode Power Supplies
http://www.repairfaq.org/sam/smpsfaq.htm
http://www.repairfaq.org/sam/smpsfaq.htm#smpsppfd

- Franc Zabkar
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Long time ago I wrote the following scheme:

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 ?

Chris


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Long time ago I wrote the following scheme:

Further to my prev message, I tried testing power transistors on diode
setting, and nomatter what combination of pins/tab I connect to, it always
reads zero volts..

Chris


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Skeleton Man wrote:

These are rarely repaired, however often the problem is bad electrolytic
capacitors. Occasionally one will grenade in such a way as to cook


virtually

every semiconductor in it, but often they just refuse to start up. Check


the

standby power supply, if that's ok then try to figure out why the main one
won't start up.



I think you may be onto something.. I took a closer look and several
capacitors appear to be bulging a bit at the top.. one does have brown
stuff around the edge, but I figured this was rust or crap that collects
like dust, etc.. (but none of the others have this).. all the capacitors
in question are 1000uF - 2200uF, 10 - 16V.. the small ones look fine..


Since you also said there aren't any burn marks or bad smells, my guess
is the caps are bad. That could certainly account for the symptom.

If it's the right vintage, you may be a victim of this:

http://www.siliconchip.com.au/cms/A_30328/article.html


As for standby power, yes I get +5VSB, it just refuses to turn on and
deliver the rest.. is this common ? I'm guessing there is a seperate
circuit for standby voltage ?

Chris




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Skeleton Man wrote:

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..


Do you have any understanding of basic circuit theory ? If you don't even have
that then forget it.

Graham

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"Skeleton Man" wrote in message
. ..
These are rarely repaired, however often the problem is bad electrolytic
capacitors. Occasionally one will grenade in such a way as to cook

virtually
every semiconductor in it, but often they just refuse to start up. Check

the
standby power supply, if that's ok then try to figure out why the main one
won't start up.


I think you may be onto something.. I took a closer look and several
capacitors appear to be bulging a bit at the top.. one does have brown
stuff around the edge, but I figured this was rust or crap that collects
like dust, etc.. (but none of the others have this).. all the
capacitors
in question are 1000uF - 2200uF, 10 - 16V.. the small ones look fine..

As for standby power, yes I get +5VSB, it just refuses to turn on and
deliver the rest.. is this common ? I'm guessing there is a seperate
circuit for standby voltage ?

Chris



If the caps are bulging, they're bad. It was a real common problem for a few
years.




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"Skeleton Man" wrote in message
. ..
Long time ago I wrote the following scheme:


Further to my prev message, I tried testing power transistors on diode
setting, and nomatter what combination of pins/tab I connect to, it always
reads zero volts..

Chris



They may have shorted from failing capacitors, but more likely they just
appear shorted because of other things in the circuit. You usually have to
remove them from the circuit for testing.


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Default Repair ATX power supply



Skeleton Man wrote:

These are rarely repaired, however often the problem is bad electrolytic
capacitors. Occasionally one will grenade in such a way as to cook
virtually every semiconductor in it, but often they just refuse to start up.

Check
thestandby power supply, if that's ok then try to figure out why the main

one
won't start up.


I think you may be onto something.. I took a closer look and several
capacitors appear to be bulging a bit at the top.. one does have brown
stuff around the edge, but I figured this was rust or crap that collects
like dust, etc.. (but none of the others have this).. all the capacitors
in question are 1000uF - 2200uF, 10 - 16V.. the small ones look fine..


Replace ALL the larger caps even if they aren't bulging. Looks like you have a
PSU made with 'bad caps'.

http://www.badcaps.net/pages.php?vid=5

NB - the caps on the secondary side MUST be 'low ESR' types made for smps use.

Graham

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Skeleton Man wrote:

Long time ago I wrote the following scheme:


Further to my prev message, I tried testing power transistors on diode
setting, and nomatter what combination of pins/tab I connect to, it always
reads zero volts..


In or out of circuit ?

Graham

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They may have shorted from failing capacitors, but more likely they just
appear shorted because of other things in the circuit. You usually have to
remove them from the circuit for testing.


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

Chris


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Skeleton Man wrote:

They may have shorted from failing capacitors, but more likely they just
appear shorted because of other things in the circuit. You usually have to
remove them from the circuit for testing.


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


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

They're stuffed in that case.

Graham



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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


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

--

JANA
_____


"Skeleton Man" wrote in message
...
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



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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
_____


"Skeleton Man" wrote in message
. ..
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



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"Skeleton Man" writes:

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 ?


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
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Skeleton Man wrote:

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)


http://cgi.ebay.co.uk/Transistor-2SC...QQcmdZViewItem



Would bad caps have caused them to fry ?


Maybe.

Graham



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On Sun, 25 Nov 2007 07:07:47 -0500, "Skeleton Man"
put finger to keyboard and composed:

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


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
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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


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"Skeleton Man" wrote in message
...
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



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.


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"Franc Zabkar" wrote in message
...
On Sun, 25 Nov 2007 07:07:47 -0500, "Skeleton Man"
put finger to keyboard and composed:

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


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.


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Default Repair ATX power supply

Skeleton Man wrote:

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.
--
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"John Tserkezis" wrote in message
u...
Skeleton Man wrote:

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.



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.


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"Skeleton Man" wrote in message
. ..
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..


Sorry, but SMPS's are A REAL ******* 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.


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Default Repair ATX power supply

"Dave" writes:

"Skeleton Man" wrote in message
. ..
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..


Sorry, but SMPS's are A REAL ******* 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?

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Skeleton Man wrote:

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 he

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).
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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.


That is EXACTLY what I am talking about ! (just written better lol)

Chris




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Sorry, but SMPS's are A REAL ******* 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.


I'm a novice but not a newbie.. I have studied basic electronics dealing
with how various components work, etc.. I taught a bit about building
stuff, just nothing about repairing it..

"Gee.. what's this big round thing ? I might stick my finger on it..
ZAP!" - Not quite that novice..

Chris



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