IBM 5155 Power supply schematics or info

Tony Duell ard at
Sun Nov 6 17:03:28 CST 2005

> > There are 2 protection circuits. One is a current sensor, there's a
> > current transformer in series with the chopper transformer primary
> > winding. The other is a set of voltage sensors.
> >
> > Whatever you do, don't disable any of the trips. If there really is
> > something wrong, the results would be spectacular!.
> >
> > Do you get any voltage appearing on any of the outputs? Even for that
> > half-second?
> Today I have dedicated another time to this power supply :)
> The SG3524 is correctly powered, it receive about 8V to its power pin and
> give 5V on the reference output.
> The sawtooth signal is present and good, like the oscillator output.
> The inhibit pin is not active.

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).


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