Help with ICL power supply fault (Farnell SMPS)

[Jules Richardson]

John S wrote:
> 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

Hmm, failed (or failing) rectifier diode on one of the outputs maybe? I've had 
to replace quite a few in recent years on SMPSUs; some fail completely shorted 
(and so are obvious), but others only cause problems when loaded. Thankfully 
they're cheap, and a healthy number of modern alternatives exist if the 
original part can't be obtained.

It's quite possible that any sort of protection (and regulation/feedback) only 
happens on the +5V rail in your PSU, so if something's upset on the other 
rails then weird things can start happening. As a test, you might be able to 
disconnect the +/-12V circuitry completely and then see if the +5V starts 
behaving itself.

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

Not me, I'm afraid. I've got a few machines using Farnell switchers back in 
storage in the UK, but no schematics to hand here. Yours sounds relatively 
simple, though - I suspect you could draw one out in a couple of hours if needed.

cheers

Jules