Need help with Mercury ATX switching power supply problem.

[rajashekar]

Hi All,

I have a Mercury ATX switching power supply - 450w based on WelTrend WT7520 IC and its not powering on. Upon checking I found that Fuse has blown up and also a Zener diode is gone.. The portion of the board under that zener diode was in burnt condition by doing cold check itself. So, I want to replace that Zener Diode, but I don't know what value diode we should replace with. Please help me. I don't find any circuit diagrams of this particular SMPS anywhere. The Diode reference code on the circuit is "D36" . Please help.

Regards, Raj

 

[alfaplus]

Hi All,

I have a Mercury ATX switching power supply - 450w based on WelTrend WT7520 IC and its not powering on. Upon checking I found that Fuse has blown up and also a Zener diode is gone.. The portion of the board under that zener diode was in burnt condition by doing cold check itself. So, I want to replace that Zener Diode, but I don't know what value diode we should replace with. Please help me. I don't find any circuit diagrams of this particular SMPS anywhere. The Diode reference code on the circuit is "D36" . Please help.

Regards, Raj

Hi, Raj
Seems that the Stby PSU (the one that generates the Vcc supply for the rest of the unit and the 5VSB voltage for the mainboard) is fried.
Look if near that zener there is also a small transistor in TO92 case (C945 or 2N2222) and a bigger one in TO220 case (C5067).
Sometimes there is only the bigger one, no problem, test it and look at its emitter: usually is grouded with a 1W resistor of small value (in the range 0.5 - 2.7 ohms), that usually is interrupted.
Check all these components and every diode and resistor in the circuit, maybe you will see that many resistors have altered their values (eg. a 100 ohm resiststor may have turned to 80 kohm).
If you have time and patience draw yourself a schematic of this stage, even if you cannot see the markings on all the components, by putting only the symbol and the numbering used (R1, D6, Q1, etc) and post it here.
I will be able to help you much more with a partial schematic than without it!
Regards,
Alfaplus