9 Apr
2008
9 Apr
'08
6:57 a.m.
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