Help with ICL power supply fault (Farnell SMPS) - test-drb@ccmp.vtda.org

9 Apr 2008

Hi,

I've started to setch the output circuitry, not 100% accutarely as my meter shows all
the transformer windings as 0 ohm, so I have some points that measure 0 ohm to ground
which may be ground or maybe a transformer output.

Thanks Tony for your reply, quote below.

Tony wrote:
...
 John wrote> 
...
   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 first thing that 'bothers' me is that the 12V output is lower than
the 5V one. If this was a simple regulation fault, I'd expect all outputs
to track.

Do you know which output the crowbar operates on (not which one triggers
it, but which one it shorts out?). 
Now I have some of the output circuit, I have found their is an SCR (2N6400) connected as
follows:

Anode - to +12V output
Cathode - to ground
Gate - to a resistor divider, which inturn is connected via a diode (assumed to be a
zenner) to the +5V output.

With the power off I have tried applying +5V to the +12V output, and this drew 250mA - ie
around 20 ohm. most of this current turned out to be going through the SCR, as when I
removed the SCR the current reduced to around 20mA. I think the SCR has been damaged, it
is showing 80ohm between gate and cathode with the meter either way round (I think this
should be a diode!). I guess the SCR must have taken the brut of the 80W or so of excess
power taken by the PSU when I was testing it earlier. I'll buy a couple of
replacements.

...
  It's not uncommon for that to be
something other than the main output (simply becuase it's easier to pull
down), it's possible there's a crowbar thryistor on the 12V output, and
that that's firing. 
Exactly right - thanks for the tip.

...
 I assume this thing doesn't have external sense
inoputs for the main
output, or if it dows you've conencted them to said output. 
No sense inputs.

...
 I think it's reasonable to asusme the chopper is
working (otherwise you'd
get no outputs at all). Does it seem to be running continuously, or do
you get the 'tweet tweet tweet' of a PSU that's starting, detecting a
fault, shutting down, and repearing?

Are tyhe otuptus steady at those votlages? I find an analogue meter best
for this, you can see the needle twitch if the PSU is starting and
shutting down. 
When I tested it earlier I applied power for around 5 seconds at  a time, outut voltages
were fairly stable.

Next thing I'll look at is the feedback circuit from the +5V output via some resistors
and what might be a FET into an opto-isolator to a circuit on the hot side involving a
compartor, transistors, resistors and diodes. This looks fairly complicated (to me,
probably a piece of cake for Tony!). Pity there isn't a nice IC controller like on the
HP-85 PSU :-)

Regards,
John
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