Hi,

I am looking for help to fix a dead ATX SMPS.

Background;
I bought a new NorthQ 4001AT 400Watt SMPS. The unit was DOA, and the
seller replaced it immidiately and didn't want the defect one in
return. So I want to fix it - as all components in it are new it may
just be a minor fault and it may serve me for a long time, besides I
would be very satisfied if I could manage to fix it.
The SMPS is dead - when PS_ON (green wire) is connected to earth I hear
a small 'tick' from the SMPS and the fan blades moves half an inch.
There is no voltages on any of the outputs. When doing these tests I
load the +3,3 +5 and +12 Volt outputs with minimum loads (as read on
the box) in the form of power resistors.
When testing there is over 300 V across the two HV capacitors, and half
the voltage between the two. There is 3 power transistors in the
primary circuit 2 pcs J13009 and 1 pcs C5027-R. When testing these with
an DMM all junctions seems fine. There is +5V between pin 5 and 9 on
the ATX power connector.

I would like to ask for help to continue troubleshooting from here. At
least parts of the primary seems to be OK. I have read somewhere that a
possible fault can be the startup resistors of the primary transistors,
and that these resistors usually have quite a high value. But I cannot
find any resistors with large values around these transistors. I have
some experience with electronics, but little with SMPSs. I have
experience with tube amplifiers so I am used to watch myself from B+.

The semiconductor components I have located in the SMPS are;

Primary;
2 x J13009 NPN Power BJT
1 x C5027-R Hi-Volt BJT
diodes

Secondary;
2 x SB3040PT Schottky Barrier Rectifier
4 x component TO-220 3-pin
1 x LM 339N Quad comparator
1 x KA7500B 5V reference
1 x 4-pin DIP component
8 x small 3-pin BJT or similar
lot of diodes

Thank you for helping me.
marty

On 26 Feb 2006 13:39:18 -0800, "no_go" put
finger to keyboard and composed:

Hi,

I am looking for help to fix a dead ATX SMPS.

Background;
I bought a new NorthQ 4001AT 400Watt SMPS. The unit was DOA, and the
seller replaced it immidiately and didn't want the defect one in
return. So I want to fix it - as all components in it are new it may
just be a minor fault and it may serve me for a long time, besides I
would be very satisfied if I could manage to fix it.
The SMPS is dead - when PS_ON (green wire) is connected to earth I hear
a small 'tick' from the SMPS and the fan blades moves half an inch.
There is no voltages on any of the outputs. When doing these tests I
load the +3,3 +5 and +12 Volt outputs with minimum loads (as read on
the box) in the form of power resistors.
When testing there is over 300 V across the two HV capacitors, and half
the voltage between the two. There is 3 power transistors in the
primary circuit 2 pcs J13009 and 1 pcs C5027-R. When testing these with
an DMM all junctions seems fine. There is +5V between pin 5 and 9 on
the ATX power connector.

That's the +5VSB standby voltage.

I would like to ask for help to continue troubleshooting from here. At
least parts of the primary seems to be OK. I have read somewhere that a
possible fault can be the startup resistors of the primary transistors,
and that these resistors usually have quite a high value. But I cannot
find any resistors with large values around these transistors.

In an ATX PSU, there would be one startup resistor for the chopper
transistor in the +5VSB circuit. Yours appears OK.

I have
some experience with electronics, but little with SMPSs. I have
experience with tube amplifiers so I am used to watch myself from B+.

The semiconductor components I have located in the SMPS are;

Primary;
2 x J13009 NPN Power BJT

MJE13009 ???

1 x C5027-R Hi-Volt BJT

Probably a 2SC5027

diodes

Secondary;
2 x SB3040PT Schottky Barrier Rectifier
4 x component TO-220 3-pin
1 x LM 339N Quad comparator

Overvoltage and power good sensing ?

1 x KA7500B 5V reference

PWM controller

1 x 4-pin DIP component

Optocoupler for +5VSB voltage sensing?

8 x small 3-pin BJT or similar
lot of diodes

Thank you for helping me.
marty

This is a typical ATX PSU circuit:
http://www.pavouk.comp.cz/hw/en_atxps.html

The TL494 is equivalent to the KA7500B PWM controller. If pin 4 of the
KA7500B is at +5V, then I would suspect the PSU has shut down due to
an overvoltage fault.

I'd measure the resistance looking into each of the secondary outputs.
A short would point to a diode.

Here is a good repair guide for switchmode PSUs:
http://www.repairfaq.org/REPAIR/F_smpsfaq.html

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

Hello and thank you for your answer.

The resistance looking into the secondary outputs are as follows (with
unit unpowered of course);

+3,3V Output; 0,3 ohms (no change when reversing measuring probes)
+5V Output; 8 ohms (no change when reversing measuring probes)
+12V Output; 604 ohms (when reversing probes I get 100 ohms that rises
slowly to a few kohms)

The voltage at pin 4 of the KA7500B is 3,38 V without the green wire
connected to ground and 4,2 V with the green wire connected to ground.
I guess this indicates there are some bad secondary rectifiers in the
circuit? Should i go ahead and order some new rectifiers or are there
other things to check first?

Thanks again,
marty

no_go wrote:
Hello and thank you for your answer.

The resistance looking into the secondary outputs are as follows (with
unit unpowered of course);

+3,3V Output; 0,3 ohms (no change when reversing measuring probes)
+5V Output; 8 ohms (no change when reversing measuring probes)
+12V Output; 604 ohms (when reversing probes I get 100 ohms that rises
slowly to a few kohms)

The voltage at pin 4 of the KA7500B is 3,38 V without the green wire
connected to ground and 4,2 V with the green wire connected to ground.
I guess this indicates there are some bad secondary rectifiers in the
circuit? Should i go ahead and order some new rectifiers or are there
other things to check first?

Thanks again,
marty
Check Ref Volts Pin14 TL494. If not 5V change IC.

"no_go" writes:

Hello and thank you for your answer.

The resistance looking into the secondary outputs are as follows (with
unit unpowered of course);

+3,3V Output; 0,3 ohms (no change when reversing measuring probes)

This is suspect. Check the rectifiers, caps, and anything else on
this output. Probably a dead short.

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Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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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.

+5V Output; 8 ohms (no change when reversing measuring probes)
+12V Output; 604 ohms (when reversing probes I get 100 ohms that rises
slowly to a few kohms)

The voltage at pin 4 of the KA7500B is 3,38 V without the green wire
connected to ground and 4,2 V with the green wire connected to ground.
I guess this indicates there are some bad secondary rectifiers in the
circuit? Should i go ahead and order some new rectifiers or are there
other things to check first?

Thanks again,
marty

"no_go" wrote in message
ups.com...
Thanks for all help so far.

I have now checked a bit more and found that the 4 TO-220 devices are
STPS2045CTschottky rectifiers connected in parallell two and two.
And one of them on the 3,3 V output were shorted.
I'll have to order a few of them to get a replacement.

You can buy just one if you like....
http://dkc3.digikey.com/PDF/T061/0860.pdf

Is there
anything else I should check before replacing the STPS2045 that is
blown? Can this fault be a secondary fault caused by something else?

Thanks!

Cheers,
marty