IBM 5155 Power supply schematics or info

OK, I've looked at mu schemaitcs. I am asusming this supply is the same as the one you have. The 3524 supply, at least at startup comes from the mains transformer T5 on the mainboard. C21 (470uF) is the smoothing capacitor for that. Now, the 3524 is oscillating, but I assume it's producing no output to the the driver transsitors Q5 and Q6, right? There seem to be 3 possibilites 1) The 3524 is defective 2) It's shut down. The shutdown pin is driven from the SCR CR11 on the 1144-01E PCB. That takes inputs from the current sense circuit and the overvoltage shutdown. Since you say that the shutdown pin is not asserted (do check this at the 'other' side of R11, 6k8), I think this is not the problem weeither. 3) The other way to shut down a 3524 is via the comp pin (compensation). This is actually the output of the error amplifier, of course. In this supply, IC2a (LM339) can clamp this pin to ground. It appears this occurs if the startup voltage is not high enough (the inputs to this chip ateh te reference voltage form the 3524 and a potted-down version of the supply to that chip). -tony