Req; Help to fix dead ATX SMPS

[no_go]

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

 

[Franc Zabkar]

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

 

[no_go]

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

 

[exsolderman]

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]

Hi and thanks for your answer.

The circuit has the IC KA7500B instead of the TL494, but from looking
at the datasheets of the ICs I guess pin 14 on TL494 corresponds to pin
14 on KA7500B.
The voltage on this pin is 4,9V, so I guess this indicates a good IC.

marty

 

[Sam Goldwasser]

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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[no_go]

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. 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

 

[Bob]

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