Repair Chinese Power Supply MCH-305D

[mava70]

Hello,

I need an help for repairing MCH-K305D:

I did a bad modification of the power supply that blows out one of the Mosfet 10N60C I put a too bigger resistor in series to the Gate and the part shorts out in all pins. After swapping the part, I power on the PS and all seems good with current knob all way down, but after turning it a little up I got another short and the Mosfte blows out again. The Fuse never blows out.
For these test I have a 60W bulb in series to the power supply to limit current of any short I got, but also first short without the bulb doesn't blow out the fuse but main power house switch cut down.

I removed the Mosfet and I try to do some kind of measurement, without the mosfet I can power on the PS without any problems, led lcd's show's all 000 but seems working, I start doing measurement in the low power section and I don't find any fault 12V is present, U1 OPA vary from 0 to 5V when Voltage knobs turn up and down, I have a similar 0-2.4 going up and down at pin 1 of U4.

On pin 11 and pin 14 of U4 (SG3525) on the oscilloscope there is a pulse and the same pulse reflect after the chopper trasformer T2 D15 at the gate of the broken Mosfet Q22 si similarly present.

300V is present as well on Source of Q22, well rectified by the bridge.

I don't see any modification on the pulse out of U4 by turning up and down the voltage knob. Is this normal? I have the same pulse same frequency and same amplitude, measured after T2 in around 12V.

I suspect some problem after the mosfet what do you think? What worth checking more?

schematic is here

Thanks

Marco
 

[tautech]

Oh dear, you learnt not to change gate resistors didn't you. 
As I see it the current sense on B1 may not be working correctly. Sensing is done at B2 and fed back to U4 pin 8.

Full schematics are in this post:
https://www.eevblog.com/forum/testgear/cheap-variable-switching-power-supply/msg1240531/#msg1240531

[mava70]

Oh dear, you learnt not to change gate resistors didn't you. 

Sure!! But how to learn repairing whitout destroing something?= Thanks 

[mava70]

Then, probabily I find the problem, swapped both mosfet that is bot shorted, checheck component around and  seems working, powered on and tadaaa, I'ts working, I put on a small load and with 80 mA of load at 18V everything seems fine, I measure only an hight temperature in RT1 (50-70 degree Celsius) is this normal?  Oscilloscope give me a ripple (chopping noise) of around 100mV pp, normal?

[tautech]

I'ts working, I put on a small load and with 80 mA of load at 18V everything seems fine, I measure only an hight temperature in RT1 (50-70 degree Celsius) is this normal?

Yes, NTC thermistors need to run hot.

Oscilloscope give me a ripple (chopping noise) of around 100mV pp, normal?

Yes normal, these are not very quiet PSU's so for sensitive circuits you need add additional capacitors to lessen the HF ripple.

Good fix, well done.