Help with ICL power supply fault (Farnell SMPS)

John Robertson jrr at flippers.com
Fri Apr 11 10:33:21 CDT 2008


John S wrote:
> 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?
>
> ...
Have you looked at the capacitors in this unit? A ESR meter can help a 
lot. Capacitors age and loose their ability to filter AC ripple and this 
will lead to a switching supply shutting down - and drawing more current 
than it should.

There are a number of ESR meters on the market, from Anatek's BLUE Esr 
(which we happen to sell) and others. I suspect one  of your compatriots 
on your side of the pond has a meter they could lend you...You could 
simply replace all the capacitors with low ESR caps, but metering the 
caps would be cheaper...

John :-#)#

-- 
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Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, VideoGames)
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