Converting PS2 Power supply from 12v to Constant current

ps2 power supply diagram

Hello everyone,
I'm Attempting to convert a Playstation 2 power supply from a single rail 12v supply to a 2A constant current supply to drive a stepper motor. I have been unable to find a circuit diagram for the item but I have been able to deduce a few things. ( If anybody has a copy of the circuit feel free to let me know , )

I have worked out the r105 and r104 set the feedback to the 2.5 volt reference zener which in turn drives the opto coupler. By using a 0.1 Ohm resistor as a current shunt and a LM324 OPAMP as a gain of 13 I have been able to get approximately 2A However the supply is not oscillating nicely has audible squealing and large amounts of noise and ringing on the output.

By changing the resistor values (R105 and R104 )I have been able to change the output voltage without any ringing or noise (no opamp voltage reg only ). So I assume that the problem is more than likely with the opamp. The supply for the opamp is being driven from the output. I have used 0.1uF decouples everywhere. I think that the problem may be that any ripple on the output is being amplified 13 times and fedback which in turn is increasing the noise and ringing.

Any feedback or idea's would be greatly appreciated....

Thanks Dave

convert ps2 to 12v

Forgive me this rudess, but talking about this circuit without its schematic diagram is like talking with a blind about colours ..

Could you supply one? .. schematic diagram, that is ..

Regards,
IanP