8 Apr
2008
8 Apr
'08
4:21 a.m.
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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9 Apr
9 Apr
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
2:19 p.m.
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 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?). 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.
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.
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.
-tony
3:05 p.m.
On Wednesday 09 April 2008 17:19, Tony Duell wrote:
I've expected that too, in the past, and it hasn't been the case at least
sometiimes...
(Snip)
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 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. 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?
Scoping tbings in there would sure be informative.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
10 Apr
10 Apr
2:28 p.m.
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?
Scoping tbings in there would sure be informative.
3:16 p.m.
On Thursday 10 April 2008 17:28, Tony Duell wrote:
The prolem is that if this is like most small SMPSUs, then almost all the
'interesting' circuitry (oscillator, etc) is on the mains side of the
isolation barrier. And therefore you can't just connect a 'scope to it,
not unless you like shorting out the mains via one of the rectifier
diodes and the earth (ground) lead of the 'scope.
This is true, though at the time I wrote that I was thinking about scoping
the outputs.
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?
Scoping tbings in there would sure be informative. The right way to do it, of course, is to use an
isolating transformer,
but not everybody has one of those in the workshop. I certainly do not
recomend 'floating the scope;' (removing its earth lead so the 'scope
chassis can sit at half mains voltage, or whatever), even though this is
suggested in one service manual I have if an isolating transformer is not
avaialble. I have this objection to large metal ohjects sitting at
dangerous voltages.
Yes indeed.
One way to achieve isolation is to get a couple of filament transformers that
are of similar voltage ratings and use them both, tying both low-voltage
windings together and using one primary as the output. I'd think that they
wouldn't need to be particularly hefty if you're not going to load the supply
much.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
12 Apr
12 Apr
12:35 p.m.
Scoping tbings in there would sure be informative.
The prolem is that if this is like most small SMPSUs, then almost all t= 'interesting' circuitry (oscillator, etc)
is on the mains side of the
isolation barrier. And therefore you can't just connect a 'scope to it,
not unless you like shorting out the mains via one of the rectifier
diodes and the earth (ground) lead of the 'scope.
This is true, though at the time I wrote that I was thinking about scopi=
ng=20
the outputs. 
1:30 p.m.
Tony Duell wrote:
The prolem is that if this is like most small SMPSUs,
then almost all the
'interesting' circuitry (oscillator, etc) is on the mains side of the
isolation barrier. And therefore you can't just connect a 'scope to it,
not unless you like shorting out the mains via one of the rectifier
diodes and the earth (ground) lead of the 'scope.
If you don't mind losing your second channel, you can always apply one input
to channel A and the ground reference to channel B, then set the scope to
"A-B" mode (ChA subtract ChB). Most scopes can do this, and this is exactly
what it's meant for.
The idea is that your ground reference remains mains earth, while one scope
probe measures the ground level, and the other measures the signal you're
actually interested in.
I suppose you could build an isolator of some form, but that sounds like it
might not be as easy as it seems.
--
Phil. | (\_/) This is Bunny. Copy and paste Bunny
classiccmp at philpem.me.uk | (='.'=) into your signature to help him gain
http://www.philpem.me.uk/ | (")_(") world domination.
13 Apr
13 Apr
9:41 a.m.
If you don't mind losing your second channel, you
can always apply one input
to channel A and the ground reference to channel B, then set the scope to
"A-B" mode (ChA subtract ChB). Most scopes can do this, and this is exactly
what it's meant for.
If you do this, make sure the 'scope can stand a 'common mode voltage' of
a few hundred volts, because that's what it's going to be seeing. I would
think a fair number of 'scopes will not be happy looking at a signal of a
few volts (so the Y attenuators are set to, perhaps, 1V/cm) with a common
mode voltage of the full mains.
-tony
11 Apr
11 Apr
8:33 a.m.
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 :-#)#
--
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, VideoGames)
www.flippers.com
"Old pinballers never die, they just flip out"
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9 comments
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participants (6)
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ard＠p850ug1.demon.co.uk -
classiccmp＠philpem.me.uk -
john_a_s2004＠hotmail.com -
jrr＠flippers.com -
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rtellason＠verizon.net