24 Jul
2000
24 Jul
'00
3:55 a.m.
My Prime 2455's PSU died last week. A replacement has been
offered and will undoubtedly arrive sooner than a repair
will take place. But I have to deep-six this monster, as
they just don't make them anymore, and buying a used one
from a reseller is likely to be cost prohibitive.
So, I'd like to eventually repair it. It's a monster, and
potentially lethal. From poking around with a voltmeter,
I can see that it chops the AC to DC and doubles the voltage
to about 280 volts, one positive supply, one negative supply.
From there, I'm lost. Has anyone every written up a
general
description of troubleshooting and repair techniques? Is it
even possible for devices that vary in design not only from
manufacturer to manufacturer, but wven model to model?
tia,
-doug quebbeman
24 Jul
24 Jul
6:31 a.m.
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25 Jul
25 Jul
12:24 a.m.
I have both helpful hints and a cautionary tale about
switchmode PSUs...
It's possible to get a clue about the nature of the fault
by a simple voltage measurement. First switch on, and
measure the voltage across the smooting capacitor on the
high-voltage DC side. Should be a few hundred volts, but
if it's zero, then the rectifier or input filter is
open-circuit.
Now switch off, with the voltmeter connected. If the
voltage drops quite rapidly, this indicates that the
chopper transistor is probably OK -- the capacitor is
dicharging throught it and the transformer primary. If the
voltage remains high for some time (see below), then the
chopper transistor is suspect, since nothing is draining
the cap. (unless there's a bleeder resistor).
Good, modern electrolytic capacitors can hold a charge of a
few hundred volts for some time. Days, in fact. I once
picked up a dead switch-mode PSU which had a failed chopper
transistor, and hence a well charged-up capacitor. The
middle finger of my left hand made contact with the
terminals of said capacitor, causing me to exclaim loudly.
Well, run around the office yelling and cursing, in fact.
I ended up with two neat little burn marks on my finger,
clearly labelled "+" and "-" (from the capacitor terminals).
--
John Honniball
Email: John.Honniball(a)uwe.ac.uk
University of the West of England
24 Jul
24 Jul
12:28 p.m.
----- Original Message -----
From: "Douglas Quebbeman" <dhquebbeman(a)theestopinalgroup.com>
To: <classiccmp(a)classiccmp.org>
Sent: Tuesday, July 25, 2000 1:25 AM
Subject: Switching PSU Theory of Operation / Repair Tips & Techniques
So, I'd like to eventually repair it. It's a
monster, and
potentially lethal. From poking around with a voltmeter,
I can see that it chops the AC to DC and doubles the voltage
to about 280 volts, one positive supply, one negative supply.
Ok, so it's a switch mode power supply that produces +280v and -280v
(wrtg) rails right?
Is this used as a rail to feed low voltage supplies, or are they
generated in the main supply.
Or do you REALLY mean that it produces a postive and a negative with
280v across them?
That is what I would expect to see.
From there, I'm lost. Has anyone every written up
a general
description of troubleshooting and repair techniques?
Not that I know of. Others may know of something other than the usual
service manuals.
****PAGING TONY DUELL****
A circuit diagram would be useful here, but psu's aren't THAT
complicated really. Often the type of fault will indicate where the
problem lies.
I take it the 280v rails you mention are still present in it's non
functional condition?
That suggests that the mains rectifier is still functional, they don't
'chop' the ac mains.
Switch mode supplies generally rectify and filter the mains, and use the
resultant 250-350vdc to drive
the rest of it. If you have no low voltage stuff running, and no blown
fuses, it could be as simple as
a startup resistor being open circuit. Look for a high value (~300k -
1Meg) resistor from the + side
of the dc rail to the electronics of the switch mode supply. These go
open circuit at switch on fairly often,
and the supply doesn't get the initial starting pulse it needs.
****WARNING****
These supplies are filtered by one or more largish reservoir capacitors
that can and do store a potentially dangerous amount of power, they are
DANGEROUS even when the unit is off and unplugged until they are
discharged. They may or may not have a bleed resistor across them. I
strongly recommend that you check with a meter after about a half hour
with it switched off to see if they have discharged before you stick
fingers in there or you might get a nasty surprise.
If you identify a likely resistor, you will find that you get ambiguous
readings unless you remove it from the circuit. Open circuit startup
resistors are a common fault in switch mode supplies.
possible for devices that vary in design not only
from
manufacturer to manufacturer, but wven model to model?
Never fixed TV sets have you. :^) Emphatically yes.
Hope this helps.
Cheers
Geoff Roberts
Computer Systems Manager
Saint Mark's College
Port Pirie,
South Australia
geoffrob(a)stmarks.pp.catholic.edu.au
netcafe(a)tell.net.au
ICQ: 1970476
8916
days inactive
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3 comments
4 participants
participants (4)
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ard＠p850ug1.demon.co.uk -
dhquebbeman＠theestopinalgroup.com -
geoffrob＠stmarks.pp.catholic.edu.au -
John.Honniball＠uwe.ac.uk