please help to repair my ATX SMPS board

Dear sir,
i was writing this to get information for rectify the problems i am facing in my ATX power supply. i Let me tell you from beginning. i have observed my atx power supply can supply 20A in 12 v so that i planned to make a work bench power supply. i found a 0-24v 20 A voltage regulator circuit using LM 338T from internet and i made it.That circuit has 4 LM338T which was parallel to each other so that it can supply 20 A( LM 338T can supply 5A as per the data sheet ) i have taken -12v and +12V line so that i thought i can make a variable power supply line of 0- 24V 20 A :???: when i switched it on my volt meter reading was accurate . it was showing 0-24v variable voltage and within few second my power both transister MJE13007 was blown like bomb :bang: . suddenly i switched it off and checked both power transistor was blown and observed dide 4148 and a resistor attached to that area also blown i. i assumed the voltage regulator circuit i made acted as a load to SMPS and tried to drawn 20A current from -12 to +12 and failed becase at -12 v it can only supply 500 mA .I changed everything i have used MJE13009 instead of 13007 so that it can handle more power . i removed the fuse of circuit and connected a 100W bulb so that if anything failed from primary circuit will make the bulb glow and protect the circuit from damage and we can understand it . when i switched it on the bulb filament was glowed like cinder and off suddenly . i observed nothing was shorted . if it yes the bulb will glow bright light mode . i have checked the out put at 12v and found it was .11v and but the +5v line was present . i checked the green wire supply which is using to short with ground to switch on smps and i got 2.15v in the line . i thout to check the main filter capacitor voltage at primary . In india the supply input line voltage is 230-240v so the smps filter capacitor should have 240*1.4142= DC 339V but it has only 150V Dc so i thought the power transistor line was not switched on and i suspected PWM which was giving signal to switch the primary section from secondary via a opto coupler 817 D. i changed the PWm ic TL494 too and everything was same as before. my mosfet is 2N60 which is fine with my multi meter . can you guide me to solve this ?? :bang::bang::bang:

I think you need to show us the adjustable regulator circuit you used. I suspect what has happened is you have 'pushed' the +12V output into the -12V output and destroyed the -12V rectifier circuits. If the output stages of an ATX PSU are damaged like that, the input stages tend to overload and shut down which could account for the low voltages you are seeing.

Be VERY careful when working on SMPSU, the voltages are high and you must be careful never to bridge or take readings across the AC and DC sides of the transformer.

Brian.

I Am attaching the circuit diagram i got from internet . please find the circuit. i have checked mospec 12c20c for shortage at out put and i have seen its working.

OK, and am I right in thinking you connected the points at the top of C3 to the PSU +12V and the bottom of C3 to the -12V ?

Brian.

ys i connected it across the capacitor c2 . i was attached multi meter too with the circuit to get the voltage output and i got variable supply from 1 v to 24 volt once . after few seconds of testing at 24 v, it explored both power transistor and 4148 and 100 k resistor at primary stage

gireesh

I strongly suspect the -12V output circuit of the ATX PSU is damaged then.
Just because the -12V is only rated at 500mA doesn't mean your regulator will only provide 500mA, what you have essentially done is short the high current +12V output to the low current -12V output. Because of their relative ratings, the -12V output would have been lifted to several volts positive and damaged the output components.

Do this:
Follow the -12V wire back to the PSU, usually it is the blue wire.
On the PSU board you will find an electrolytic capacitor with it's negative side connected to the wire. Replace that capacitor.
The next part depends on the PSU design and it varies from one model to another - usually, there will then be an inductor and another electrolytic capacitor. Change that capacitor also. There will also be a diode with it's cathode (banded) end connected to the transformer and the anode end to those capacitors you just changed. Replace that diode.

There is a good chance it will then work again but don't repeat the same mistake!

Brian.

thank you i will do this ... and will return in same thread with my findings.

i have checked everything at output stage . and no short circuit found capacitor/diode was working . checked opto coupler 817 pin number 1 and 2 i got 2.1 v that means opto coupler is switching on the primary part . and i checked each and every part of primary and nothing brocken failed and shortcircuited . i disconnected the output rectifiers and checked it was working . desolder main transformer and checked it was also showing continuity.

You just cannot combine +12V and -12V supply just like that! They are independent and both are designed to supply and not sink current.

The internal coils for the -12V supply are not designed to supply 20A (thinner wires are used).

You can combine 12V adn 5V on the motherboard itself and modify the feedback circuit to get around 17V. If you remove the feedback, perhaps you will get 21V or so- perhaps 22V.

Search for a circuit diagram for the ATX supply on the internet and see what COULD have gone wrong.

i have purchased a capacitance meter with which i have to check the value of polyster and electrolitic capacitor changed or not due to high current passed through input section of smps. meter purchasd onlile and will reach in few days .