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ATX Power Supply repair

 


An ATX like industrial power supply received for repair. after some basic investigations, it was actually a ATX power supply with 24V power rail. Power supply came with complete dead condition.
 

Structure of this type power supplies,
                                                   Total power supply consists of two separate power supplies. one is used for stand by supply and other is used for power delivery. Main power supply can output +3.3V, +5V, +12V, -12V, +24V. Circuit used for supply stand by power can deliver +5V with maximum 2A. Stand by supply was driven by TNY267 power converter ic. which is all in one(built in power mosfet) package.


Since these PSU's have two separate power supplies, you can separately switch on each other. If stand by supply is not working, sometimes main power supply may work. The PSU i got is mostly similar to the model : API3PCD2-Y01, two switch forward power supply. I cannot publish it here because of copyright reasons. but you can download it from the web.
manually switching the main power supply on is a smart idea when all things may go as expected. but it can be disastrous if SMPS switcher or gate driver has a fault.

According to the circuit, supply needed to run secondary switcher (UC3843) was taken from secondry winding of stand by power supply. therefore we have to use separate power source to power up. 

Remove main switching MOSFETS from the board. test them using multimeter. you can manually power up the switcher now (emitter pin of Q4 is better). use the bench power supply and set to 12V, 600mA range. inspect output waveform of switcher using a oscilloscope. frequency must be in kHz range and peak voltage must be at least 12V are signs of good switcher.

Lets turn to the primary stand by power supply. Connect main power to the PSU. always use a series lamp for safety. Check voltage between source and drain pin of the ic(TNY267). must be 300~320V DC( testing on 230V AC). check voltage value of C54. if it has stable value of nearly 15V, switcher is fine. if it has some unstable value, check related components. if it doesnot have any value (0V), replace the switcher ic.

I was able to repair the power supply by replacing TNY267. however i replaced UC3843 also.  


brown liquid is an insulation material used to protect board from moisture and dust.

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