Req; Help to fix dead ATX SMPS

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

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

That's the +5VSB standby voltage.

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

MJE13009 ???

Probably a 2SC5027

Overvoltage and power good sensing ?

PWM controller

Optocoupler for +5VSB voltage sensing?

This is a typical ATX PSU circuit:

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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:

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- Franc Zabkar

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Reply to
Franc Zabkar

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

Reply to
no_go

Check Ref Volts Pin14 TL494. If not 5V change IC.

Reply to
exsolderman

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

Reply to
no_go

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

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Reply to
Sam Goldwasser

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

Reply to
no_go

You can buy just one if you like....

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Is there

Reply to
Bob

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