Help with ICL power supply fault (Farnell SMPS)

[John S]

Hi,

I have an ICL PC2 CP/M box (like this: http://www.old-computers.com/museum/computer.asp?c=752) which has a faulty PSU, I am hoping someone here can advise me.

The PSU is a Farnell N100/F4190 SMPS, looks like a high quality unit with nice screw terminals for mains in and DC out. 

When fired up with 240V AC and a dummy load the output voltages are:

12V output=1.1V
5V output=2.3V
-12V output=-5.9V

The other worrying thing is it is also drawing 100W power (measured with a one of those plug in mains meters from Maplin), whilst the dummy load is around 20W (split between +12V and +5V).

I've checked all the capacitors with multimeter and ESR meter, and all seem fine, and none are bulging or obviously overheating, DC resistance on each of the output connections to ground is around 60 ohm. 

I couldn't find a short so I don't know where all the power is going to, I suspect there is some protection circuit kicking in?

I don't have a circuit diagram, so my first plea does any one have one please?

The circuit has few ICs, and mostly transistors. There is a CA339E quad comparator chip, and a IL201 opto isolator (I'll try and check these), but no nice single IC controller.

My current strategy is to apply +5V to the comparator chip and apply voltages to the output to try and see some feedback, and also apply 50V DC to the input in the hope this is enough to start up the oscillator (I think this is a small group of transistors, resistors and small capacitors near a small torial transformer, which then feeds to the main conversion transformer).

Any suggestions gratefully received,

Thanks,
John
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