ATX power supply repairs

[bob]

Of those brave souls who have ventured into trying to fix ATX supplies,
what are you mainly finding wrong with them and how are you testing
them? I know its hardly worth economically doing it. But its the
challenge i guess.

Most of the ones i have scrapped have not shown the typical cap failure.
I have resoldered a few and replaced fans in some.

bob

 

[Franc Zabkar]

On Mon, 23 Nov 2009 16:42:30 -0600, bob <sound@inetnebr.com> put
finger to keyboard and composed:

Of those brave souls who have ventured into trying to fix ATX supplies,
what are you mainly finding wrong with them and how are you testing
them?

This may help you understand the inner workings:
http://pavouk.org/hw/en_atxps.html

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[petrus bitbyter]

"bob" <sound@inetnebr.com> schreef in bericht news:hef33l$mi0$1@aioe.org...

Of those brave souls who have ventured into trying to fix ATX supplies,
what are you mainly finding wrong with them and how are you testing them?
I know its hardly worth economically doing it. But its the challenge i
guess.

Most of the ones i have scrapped have not shown the typical cap failure.
I have resoldered a few and replaced fans in some.

bob

Back in 2002 I wrote the following posting and I did so again in 2007. I did
not many repairs ever since. The components required for the repair became
more expensive then a new supply.

Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W
ATX supply on the net. This can be a great help though a little bit late for
me. Just Google for it.

Be aware that the schematic is an example of an ATX supply. You may find a
lot of differences out there though the general idea will be the same. At
least most of the times.

<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>

petrus bitbyter

 

[who where]

On Mon, 23 Nov 2009 16:42:30 -0600, bob <sound@inetnebr.com> wrote:

Of those brave souls who have ventured into trying to fix ATX supplies,
what are you mainly finding wrong with them and how are you testing
them? I know its hardly worth economically doing it. But its the
challenge i guess.

Most of the ones i have scrapped have not shown the typical cap failure.
I have resoldered a few and replaced fans in some.

Some cap failures. Recently one client had a batch of four identical
230V units die the same day. These were auto-sensing (supply voltage)
types, and during brownouts they think they are on 110V rather than
230V, then the voltage rises and phattt! The pair of 1000V 1A
ultrafast diodes went short on all four (same brownout!).

 

[Franc Zabkar]

On Tue, 24 Nov 2009 01:06:45 +0100, "petrus bitbyter"
<pieterkraltlaatditweg@enditookhccnet.nl> put finger to keyboard and
composed:

Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W
ATX supply on the net.

You may like to know that DTK's original circuit diagram is here:
http://www.dtk.com.tw/ftp/circuit/power/2038.zip

More DTK PSU circuits are here:
http://www.dtk.com.tw/tech/circuitp.html

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[who where]

On Tue, 24 Nov 2009 09:13:54 +0800, who where <noone@home.net> wrote:

On Mon, 23 Nov 2009 16:42:30 -0600, bob <sound@inetnebr.com> wrote:

Of those brave souls who have ventured into trying to fix ATX supplies,
what are you mainly finding wrong with them and how are you testing
them? I know its hardly worth economically doing it. But its the
challenge i guess.

Most of the ones i have scrapped have not shown the typical cap failure.
I have resoldered a few and replaced fans in some.

Some cap failures. Recently one client had a batch of four identical
230V units die the same day. These were auto-sensing (supply voltage)
types, and during brownouts they think they are on 110V rather than
230V, then the voltage rises and phattt! The pair of 1000V 1A
ultrafast diodes went short on all four (same brownout!).

Clarifying that, these were protection diodes.

 

[Robert Macy]

On Nov 24, 10:20 pm, Franc Zabkar <fzab...@iinternode.on.net> wrote:

On Tue, 24 Nov 2009 01:06:45 +0100, "petrus bitbyter"
pieterkraltlaatdit...@enditookhccnet.nl> put finger to keyboard and
composed:

Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W
ATX supply on the net.

You may like to know that DTK's original circuit diagram is here:http://www.dtk.com.tw/ftp/circuit/power/2038.zip

More DTK PSU circuits are here:http://www.dtk.com.tw/tech/circuitp.html

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

neither URLs worked for me

 

[Franc Zabkar]

On Thu, 26 Nov 2009 08:07:30 -0800 (PST), Robert Macy
<macy@california.com> put finger to keyboard and composed:

On Nov 24, 10:20 pm, Franc Zabkar <fzab...@iinternode.on.net> wrote:
On Tue, 24 Nov 2009 01:06:45 +0100, "petrus bitbyter"
pieterkraltlaatdit...@enditookhccnet.nl> put finger to keyboard and
composed:

Meanwhile I found that some Czech, Pavel Ruzicka, put a schematic of a 200W
ATX supply on the net.

You may like to know that DTK's original circuit diagram is here:http://www.dtk.com.tw/ftp/circuit/power/2038.zip

More DTK PSU circuits are here:http://www.dtk.com.tw/tech/circuitp.html

neither URLs worked for me

Sorry, it appears that the links are now dead.

I found them at the Wayback machine, though:
http://web.archive.org/web/20080313004813/http://www.dtk.com.tw/tech/circuitp.html
http://web.archive.org/web/20080313004813/http://www.dtk.com.tw/ftp/circuit/power/2038.zip

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.