Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans, nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted the
green wire to ground, but still nothing. Can I assume the PS is dead, or is
there something else I should be doing?

Thanks

"tempus fugit" writes:

Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans, nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted the
green wire to ground, but still nothing. Can I assume the PS is dead, or is
there something else I should be doing?

Thanks

That should do it assuming your AC power is present.

--- sam | Sci.Electronics.Repair FAQ Home Page: 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 Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

"Sam Goldwasser" wrote in message
...
"tempus fugit" writes:

Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans,
nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black
(ground)
and turned it on, but nothing happened. I then turned it on and shorted
the
green wire to ground, but still nothing. Can I assume the PS is dead, or
is
there something else I should be doing?

Thanks

That should do it assuming your AC power is present.

--- sam | Sci.Electronics.Repair FAQ Home Page: 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 Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work.
To
contact me, please use the feedback form on the S.E.R FAQ Web sites.


You can get a faulty mains switch or power lead too, the quality isn't too
great !

--
Regards ........ Rheilly Phoull

Thanks

I've tried it with a known good power cord, so that's not it.

Thanks for the tips


"Rheilly Phoull" wrote in message
...

"Sam Goldwasser" wrote in message
...
"tempus fugit" writes:

Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans,
nothing.
So, I figure it's either the power supply or the motherboard (at
least).

I want to test the PS first, before just replacing it. It is an ATX
300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black
(ground)
and turned it on, but nothing happened. I then turned it on and
shorted
the
green wire to ground, but still nothing. Can I assume the PS is dead,
or
is
there something else I should be doing?

Thanks

That should do it assuming your AC power is present.

--- sam | Sci.Electronics.Repair FAQ Home Page:
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 Site Info:
http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work.
To
contact me, please use the feedback form on the S.E.R FAQ Web sites.


You can get a faulty mains switch or power lead too, the quality isn't too
great !

--
Regards ........ Rheilly Phoull

tempus fugit wrote:

Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans, nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted the
green wire to ground, but still nothing. Can I assume the PS is dead, or is
there something else I should be doing?

Thanks



You can not just short the +5 to ground through a resistor on an ATX
supply. There is a line on the ATX connector to the motherboard that
must receive a signal before the PS will supply full power. what you
can do is disconnect the PS from the motherboard and jumper the green
wire to a ground (black) wire. The PS should now come on and you should
be able to read voltages across the outputs.

It could be anything. On the ATX motherboards, there is a constant
voltage applied to the motherboard. when you press the power switch,
the motherboard will do a series of tests. If the tests fail, no power
will be supplied to the motherboard. If the tests pass, then power will
be supplied and the system powers up.

There are many factors that will prevent a system from powering up
nowdays... Some won't power up if there is no signal received from the
CPU fan.

Other things to check:
make sure all memory is seated in the slots correctly
make sure that the motherboard isn't grounded against the case
make sure all connectors are plugged into the motherboard properly
I have seen a hard drive prevent a system from powering up, so check
connections there, too
even a PCI (or AGP) card loose will prevent it from powering up

Sam Goldwasser wrote:

"tempus fugit" writes:


Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans, nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted the
green wire to ground, but still nothing. Can I assume the PS is dead, or is
there something else I should be doing?

Thanks


That should do it assuming your AC power is present.

--- sam | Sci.Electronics.Repair FAQ Home Page: 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 Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.


Not on an ATX supply. The PS won't powerup unless the motherboard
signals the everything is good first. You have to ground the green wire
in the ATX connector.

tempus fugit wrote:

Thanks

I've tried it with a known good power cord, so that's not it.

Thanks for the tips


"Rheilly Phoull" wrote in message
...

"Sam Goldwasser" wrote in message
...

"tempus fugit" writes:


Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans,

nothing.

So, I figure it's either the power supply or the motherboard (at

least).

I want to test the PS first, before just replacing it. It is an ATX

300w

supply. I connected an 8 ohm 10W resistor from red (+5v) to black

(ground)

and turned it on, but nothing happened. I then turned it on and

shorted

the

green wire to ground, but still nothing. Can I assume the PS is dead,

or

is

there something else I should be doing?

Thanks

That should do it assuming your AC power is present.

--- sam | Sci.Electronics.Repair FAQ Home Page:

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 Site Info:

http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work.

To

contact me, please use the feedback form on the S.E.R FAQ Web sites.



You can get a faulty mains switch or power lead too, the quality isn't too
great !

--
Regards ........ Rheilly Phoull






Geez.. I am sorry.. I should have read the OP closer... looks like my
mind stopped comprehending things at the shorting the red to ground, and
I missed seeing where you also shorted the green. If you shorted the
green and stilll no output from the PS, and your input voltage is good,
then I would assume there is a problem with the PS... could only be a
bad fuse in the supply, or something more involved.

"Daniel L. Belton" writes:

tempus fugit wrote:

Hey all;
A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans, nothing.
So, I figure it's either the power supply or the motherboard (at least).
I want to test the PS first, before just replacing it. It is an ATX
300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted the
green wire to ground, but still nothing. Can I assume the PS is dead, or is
there something else I should be doing?
Thanks

You can not just short the +5 to ground through a resistor on an ATX
supply. There is a line on the ATX connector to the motherboard that
must receive a signal before the PS will supply full power. what you
can do is disconnect the PS from the motherboard and jumper the green
wire to a ground (black) wire. The PS should now come on and you
should be able to read voltages across the outputs.

Isn't that exactly what he said he did?

--- sam | Sci.Electronics.Repair FAQ Home Page: 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 Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

"tempus fugit" schreef in bericht
...
Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans,
nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted
the
green wire to ground, but still nothing. Can I assume the PS is dead, or
is
there something else I should be doing?

Thanks




Hooi,

I ever wrote a reply on repairing PC power supplies. It follows below. IMHO
it contains all you need (and more) to decide wether your power supply has
gone or not.

petrus

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.



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.554 / Virus Database: 346 - Release Date: 21-12-2003

Geezzh!

Thanks a lot - that will come in handy.


"petrus bitbyter" wrote in message
.. .

"tempus fugit" schreef in bericht
...
Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans,
nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black
(ground)
and turned it on, but nothing happened. I then turned it on and shorted
the
green wire to ground, but still nothing. Can I assume the PS is dead, or
is
there something else I should be doing?

Thanks




Hooi,

I ever wrote a reply on repairing PC power supplies. It follows below.
IMHO
it contains all you need (and more) to decide wether your power supply has
gone or not.

petrus

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.



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Sam Goldwasser wrote:
"Daniel L. Belton" writes:


tempus fugit wrote:


Hey all;
A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans, nothing.
So, I figure it's either the power supply or the motherboard (at least).
I want to test the PS first, before just replacing it. It is an ATX
300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted the
green wire to ground, but still nothing. Can I assume the PS is dead, or is
there something else I should be doing?
Thanks


You can not just short the +5 to ground through a resistor on an ATX
supply. There is a line on the ATX connector to the motherboard that
must receive a signal before the PS will supply full power. what you
can do is disconnect the PS from the motherboard and jumper the green
wire to a ground (black) wire. The PS should now come on and you
should be able to read voltages across the outputs.


Isn't that exactly what he said he did?

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Yea.. On the first pass, I missed where he said that he did short the
green to ground..

That was a very good source for information!

tempus fugit wrote:

Geezzh!

Thanks a lot - that will come in handy.


"petrus bitbyter" wrote in message
.. .

"tempus fugit" schreef in bericht
...

Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans,

nothing.

So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black

(ground)

and turned it on, but nothing happened. I then turned it on and shorted

the

green wire to ground, but still nothing. Can I assume the PS is dead, or

is

there something else I should be doing?

Thanks




Hooi,

I ever wrote a reply on repairing PC power supplies. It follows below.
snip

tempus fugit wrote:
Hey all;

A friend handed me her dead computer to see if I could get it up and
running. When you turn it on, nothing happens. No lights, no fans, nothing.
So, I figure it's either the power supply or the motherboard (at least).

I want to test the PS first, before just replacing it. It is an ATX 300w
supply. I connected an 8 ohm 10W resistor from red (+5v) to black (ground)
and turned it on, but nothing happened. I then turned it on and shorted the
green wire to ground, but still nothing. Can I assume the PS is dead, or is
there something else I should be doing?

Thanks

On more than one occasion, I've seen it be simply the 110/220 input
voltage switch.

I think sometimes they get moved partially out of position when the case
is grasped just right [just wrong?] and pulled out from under a desk.

Worth not overlooking.

-Denny