29 May
2007
29 May
'07
8:52 p.m.
Hi,
Personally I've never had any luck repairing switch mode PSUs
either....obviously there's some hole in my understanding of them....
Actually, I have managed to repair a couple; but that was when I did a City
& Guilds course in electronic repair at college (I was bored....) and had a
full schematic and circuit description to work with.
But going in blind? <shudder>
TTFN - Pete.
....Repairing a
switching power supply is one the last
things I would want to do.
Why? 
30 May
30 May
4:05 p.m.
Hi,
Personally I've never had any luck repairing switch mode PSUs
either....obviously there's some hole in my understanding of them....
In which case you need practice in repairing them ;-)
More seriously, there are easy faults with SMPSUs and difficult faults.
Easy faults include things like the mains filter cap[acitors going
short-circuit nad blowing the fuse. Change the capacitors and it'll be
fine. Difficult faults include the all-too-common short-circuit chopper
transistor. It may have been damaged by some other component failing (say
an open-circuit in the snubber network across the chopper transformer
primary), and unless you find and correct that other fualt, your new
chopper will fail at switch-on. And the shorted chopper might have
damaged other components too (like burning out th ecurrent sense resistor
im its emitter circuit, then taking out a couple of small-signal
transistors too).
In the case of this Apple /// supply, I have no idea where this capacitor
is in the circuit (I've not seen the supply, I've not seen a shematic),
but it doesn't sound like a difficult fault.
....Repairing a switching power supply is one the last
things I would want to do.
Why?
Actually, I have managed to repair a couple; but that was when I did a City
& Guilds course in electronic repair at college (I was bored....) and had a
full schematic and circuit description to work with.
But going in blind? <shudder>
Sometimes that's just what yuo have to do. Unless you have a source of
schematics of obscure PSU boards...
-tony
5:39 p.m.
One thing I always remember on power supplies is to not take anything
for granted when it blows out-- regarding the components health. It's
good to have a set of test equipment to check the basics,
Transistor/Mosfet Checker, ESR tester, Milliohm meter, Zener tester
besides just a DMM (a curve tracer is good to have on the bench).
Whenever I'm inside a power supply, I think it's important to take your
time and check it from head to toe--parts aren't free-- and more parts
can blow out if you only replace the first part that's bad.
=Dan
[ My Corner of Cyberspace http://ragooman.home.comcast.net/ ]
Tony Duell wrote:
Hi,
Personally I've never had any luck repairing switch mode PSUs
either....obviously there's some hole in my understanding of them....
In which case you need practice in repairing them ;-)
More seriously, there are easy faults with SMPSUs and difficult faults.
....Repairing a switching power supply is one the last
things I would want to do.
Why? 
31 May
31 May
9:38 a.m.
Dan, that's fine advice. But a capacitor is just a
capacitor. They blow. No reason someone shouldn't
replace one if that'll get their computer back up and
running.
The Tandy 2000 used a funky p/s that was prone to, I
think, one particular cap blowing. What can you do?
You could jury rig another p/s in it's place. But when
mine went (in 1989) I looked at the schematic, and
went to Radio Shack, then went to work. And I had my
computer back.
I guess p/s problems could be some of the WORST
problems to a computer. You could fry a number of
chips (but this is uncommon). You could always effect
the repair, then run it under load for a few days or a
week and see how it's doing. Not everyone keeps or
even knows how to use test equipment, so sometimes you
just have to go with what you know. There's not too
many people these days that can or will work on this
stuff, so what can you do but your best.
--- Dan <ragooman at comcast.net> wrote:
One thing I always remember on power supplies is to
not take anything
for granted when it blows out-- regarding the
components health. It's
good to have a set of test equipment to check the
basics,
Transistor/Mosfet Checker, ESR tester, Milliohm
meter, Zener tester
besides just a DMM (a curve tracer is good to have
on the bench).
Whenever I'm inside a power supply, I think it's
important to take your
time and check it from head to toe--parts aren't
free-- and more parts
can blow out if you only replace the first part
that's bad.
=Dan
[ My Corner of Cyberspace
http://ragooman.home.comcast.net/ ]
Tony Duell wrote:
In which case you need practice in repairing them ;-)
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> Hi,
>
> >>....Repairing a switching power supply is one
the last
> >>things I would want to do.
> >
> > Why?
>
> Personally I've never had any luck repairing
switch mode PSUs
> either....obviously there's some hole in
my
understanding of them....
More seriously, there are easy faults with SMPSUs
and difficult faults.
3:56 p.m.
Dan, that's fine advice. But a capacitor is just a
capacitor. They blow. No reason someone shouldn't
replace one if that'll get their computer back up and
running.
The Tandy 2000 used a funky p/s that was prone to, I
think, one particular cap blowing. What can you do?
You could jury rig another p/s in it's place. But when
mine went (in 1989) I looked at the schematic, and
Incidentally, IIRC, the Tandy 2000 PSU schematic is reprinted in the 2nd
edition of The Art of Elecrtronics (!)
went to Radio Shack, then went to work. And I had my
computer back.
I guess p/s problems could be some of the WORST
problems to a computer. You could fry a number of
chips (but this is uncommon). You could always effect
Indeed yes. Hence my other comment to always test a PSU on a dummy load,
not on the actual machine. Certainly after repair, and IMHO if the
machine has not been used for a long time (for suitable values of
'long'). It takes a lot less time to do that than to replace whatever
chips have fried...
Most decently-designed PSUs have crowbar circuits to protect the rest of
the machine if the PSU goes crazy. Note I said 'most', not 'all'. For
some unknown reason the HP9815 and HP9825 (and of course the 9831 too) do
not have corwbars. In the former case, at least, the 5V PSU is a
step-dwn swirtching regulator from a 30V DC line. If that chopper
transistor gails, it takes out a lot of chips!
the repair, then run it under load for a few days or
a
week and see how it's doing. Not everyone keeps or
I think you'd have to be very unlucky (and have a fault develop by chance
after the repair) for a PSU to test fine (and regulate) on dummy load and
then have the regulation fail a dew days later. Haviong the chopper fail
after a period of running is not uncommon, but that, in most SMPSUs,
doesn't cause the outputs to go high.
even knows how to use test equipment, so sometimes
you
just have to go with what you know. There's not too
many people these days that can or will work on this
stuff, so what can you do but your best.
The fact that not many people will do this sort of thing now seems to be
to be a good reason for having to do it yourself ;-)
-tony
1 Jun
1 Jun
11:15 a.m.
New subject: Schematics (was: Apple /// Power Supply)
And thusly were the wise words spake by Tony Duell
Speaking of schematics, the TRS-80 schematic (plus a couple of
Radio Shack pong-style videogame schematics) are reprinted and
discussed in "Troubleshooting Microprocessors & Digital Logic" by
Robert L. Goodman.
Cheers,
Bryan
Dan, that's fine advice. But a capacitor is just a
capacitor. They blow. No reason someone shouldn't
replace one if that'll get their computer back up and
running.
The Tandy 2000 used a funky p/s that was prone to, I
think, one particular cap blowing. What can you do?
You could jury rig another p/s in it's place. But when
mine went (in 1989) I looked at the schematic, and
Incidentally, IIRC, the Tandy 2000 PSU schematic is reprinted in the 2nd
edition of The Art of Elecrtronics (!)
31 May
31 May
3:02 p.m.
One thing I always remember on power supplies is to not take anything
for granted when it blows out-- regarding the components health. It's
good to have a set of test equipment to check the basics,
Transistor/Mosfet Checker, ESR tester, Milliohm meter, Zener tester
The other thing I use _a lot_ is a ringing tester, for detecting shorted
turns in transformers, etc. A single shorted turn in the chopper
transformer will cause all sorts of problems, best to find it before you
blow another expensive chopper transistor.
Wehn I was sorting out the Boschert 2-stage PSU in my PERQ 1 (it blew up
in a big way, all 3 chopper transistors chorted, 723 regualtor chip blown
apart, a couple of resistors burnt, afew small signal transistors gone
too (one was just 3 wires sticking out of the PCB), and even a couple of
tracks melted), I managed to borrow a nice, regulated, current-limited HV
PSU. I could apply 300V to the input of the chopper circuit, knwoing I'd
set the limit low enough to protect the new components if/when things
went wrong. Problem is, such supplies are not cheap, and I've not managed
to get one myself yet.
But don't forget the humble (filament) light bulb. It makes in ideal
current limiter. Connect it in series with the HT+ line from the smains
smoothing capacitors to the chopper. It'll at least prevent burnt PCB
tracks if things go wrong again.
-tony
30 May
30 May
9:01 p.m.
From: ard at p850ug1.demon.co.uk (Tony Duell)
Hi
I've found this as well. You think you got it fixed and pop, there goes
another chopper transistor.
Still, I just recently fixed a switcher. It did help to have a schematic.
It was traced to an open wire on a pulse transformer. I just removed
enough potting material to expose the broken wire and spliced more
wire to it. It was stressed from the day of manufacture.
The supply has been fine since then.
Dwight
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Hi,
Personally I've never had any luck repairing switch mode PSUs
either....obviously there's some hole in my understanding of them....
In which case you need practice in repairing them ;-)
More seriously, there are easy faults with SMPSUs and difficult faults.
Easy faults include things like the mains filter cap[acitors going
short-circuit nad blowing the fuse. Change the capacitors and it'll be
fine. Difficult faults include the all-too-common short-circuit chopper
transistor. It may have been damaged by some other component failing (say
an open-circuit in the snubber network across the chopper transformer
primary), and unless you find and correct that other fualt, your new
chopper will fail at switch-on. And the shorted chopper might have
damaged other components too (like burning out th ecurrent sense resistor
im its emitter circuit, then taking out a couple of small-signal
transistors too).
....Repairing a switching power supply is one the last
things I would want to do.
Why?
6188
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6190
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6 participants
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