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Secondary supply of ATX MXR-250P

Schematics diagram

Schematics diagram

Description of faults

When I repaired ATX power supply MXR-250P, which is used in Morex and other cases I redraw a piece of circuit - secondary power supply +5VSB. Fault is probably due to overloading C1, which lost capacitance and supply generates about 70 volts on C6. On the input of 7805 appears about 40V. If power supply is good, that we measure about 32V and 19V behind diodes. Sometimes can be destroyed main control circuit TL494 which have maximum voltage 42V.


When secondary supply is not yet destroyed and only generates higher voltage, it's needed to replace C1 and maybe C6 or C4, if they are destroyed and replace them with quality low ESR and 105 grad. celsius types. In many supplies is C1 reversed. Plus is no the ground!
If doesn't starting main supply, it's needed to check if goes voltage to control circuit (pin 12) and maybe to replace TL494.
When secondary supply burns out, we must replace C1, C6, transistors T1 and T2, resistor R1 and maybe R8. Next we must check D1, D2 and D3 diodes. Other components are mostly OK. It's better to test power supply through serial connected lamp bulb which limits current to supply. When will be short circuit, other components are mostly not destroyed. Only bulb lights.


Photo of power supply


On the end I need to say, that some circuit is used in MXR-200P and many other low-end power supplies. Faults are caused by poor design and using of not quality components. 2SC945 it's possible to replace with BC639. KSC5027 is NOT 2SC5027. On his place it's possible to use MJE 13007.