I'm servicing an old PC switching PSU.
It's odd form factor and very small dimension prevents me to substitute the
inner board and forces me to repair...
It gave 23V (!) on the +12V, +3.81V on the +5V, -5 and -12 were OK.
At first glance, I noticed a resistor with a dark brown colour, 1/4W, R1
across the +12V (red-red-burnt-gold-white).
Not considering the fifth stripe (white) it could be 22/220/2200.
But 22 Ohm will dissipate about 6.55W, too much for 1/4W.
Near R1, there's R2, (brown-black-brown-gold-white), 100 Ohm 1/4W
I replaced all capacitors. Almost all of them were leaking (ESR5), and
one was severely burnt on the bottom.
Now the oddities.
Initially I thought that R1/R2 were 5 band resistors, so I put R1=22 Ohm,
R2=10 Ohm.
The PSU came up with +5V and 11.8V, but the PCB near R1,R2 was very hot.
Then I put the correct (?) value for R2, 100 Ohm.
The voltages were correct again, and R1 suddenly smoked (6.55W on 1/4W).
Increasing R1 (on the +12V) leads to Vcc of the 3844 (PWM) under startup
value.
I put back the original R2, measured as 97.7 Ohm, leaving R1 at 22 Ohm.
Vcc again under minimum value.
R2=100 Ohm, holy smoke from R1.

Odd, because R1=22 Ohm, R2=10 Ohm worked...

So, I'm missing something.
The original resistors were 1/4 W, across 5V and 12V...
I don't think R1 can be 22 Ohm...

Initial condition was this
http://www.supervinx.com/temp/01.JPG
----
http://www.supervinx.com/temp/02.JPG
http://www.supervinx.com/temp/03.JPG
http://www.supervinx.com/temp/04.JPG
http://www.supervinx.com/temp/05.JPG
http://www.supervinx.com/temp/06.JPG

On Mon, 17 Jun 2013 00:25:24 +0000 (UTC), John
wrote:

I'm servicing an old PC switching PSU.
It's odd form factor and very small dimension prevents me to substitute the
inner board and forces me to repair...

I don't have much of a clue on how to fix that power supply. Too much
of the circuitry is under the black goo, and therefore impossible to
identify or easily replace.

If you supply some additional info, I think I find a suitable surplus
equivalent power supply:
What maker and model PC used this power supply?
Any maker or model number on the power supply case?
ITX? Mini ITX? Micro ATX? Mini ATX? Pico ITX? etc?
What are the dimensions of the power supply case?

Maybe:
http://www.mpja.com/Computer-Power-Supplies/products/36/
http://www.alltronics.com/cgi-bin/category/158
http://www.ebay.com/itm/160655942010

If you're totally stuck and can't find anything small enough to fit,
look into the various micro/pico power supply devices, where most of
the power supply is on the power plug, and is fed from an external 12V
regulated power source.
http://www.ebay.com/bhp/picopsu
If that will fit, you could use the space previously occupied by the
power supply case to install a 12V PS.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

John wrote:
I'm servicing an old PC switching PSU.

From the pictures, it looks like R2 (100 ohms) is across one of the
outputs - probably the +5 V output (red wires) - and 0 V / chassis
ground. This gives 0.05 A through it and 0.25 W dissipated. If it
really is a 1/4 W resistor, this is right at the limit, but
believable.

R1 looks like it is across the +12 V output and 0 V / chassis ground.
My guess is that R1 and R2 are there to provide a bare minimum load
on the +5 V and +12 V outputs.

Since R2 seems to have been OK originally, and it measured close to
the 100 ohm value you would get from reading the brown-black-brown-gold
bands and ignoring the white one, it seems reasonable that R1 would work
the same way: red-red-?-gold. You already found out that 22 ohms for R1
doesn't work. 220 ohms for R1 would give 0.055 A through it and 0.65 W
dissipated - too much for a 1/4 W, but maybe believable. 2200 ohms for
R1 would give 0.0055 A through it and 0.065 W dissipated - fine for a
1/4 W.

Have you tried measuring the original R1?

At a guess, I'd put in a 100 ohm, 0.5 watt resistor for R2, and maybe a
220 ohm, 1 watt resistor for R1. If you don't like how hot R1 gets,
maybe 2200 ohm, 0.25 watt for R1. I guess that a 22,000 ohm resistor
for R1 won't draw enough current to really do any good.

Matt Roberds

Il Mon, 17 Jun 2013 03:13:08 +0000, mroberds ha scritto:

John wrote:
I'm servicing an old PC switching PSU.

From the pictures, it looks like R2 (100 ohms) is across one of the
outputs - probably the +5 V output (red wires) - and 0 V / chassis
ground. This gives 0.05 A through it and 0.25 W dissipated. If it
really is a 1/4 W resistor, this is right at the limit, but believable.

R1 looks like it is across the +12 V output and 0 V / chassis ground. My
guess is that R1 and R2 are there to provide a bare minimum load on the
+5 V and +12 V outputs.

Since R2 seems to have been OK originally, and it measured close to the
100 ohm value you would get from reading the brown-black-brown-gold
bands and ignoring the white one, it seems reasonable that R1 would work
the same way: red-red-?-gold. You already found out that 22 ohms for R1
doesn't work. 220 ohms for R1 would give 0.055 A through it and 0.65 W
dissipated - too much for a 1/4 W, but maybe believable. 2200 ohms for
R1 would give 0.0055 A through it and 0.065 W dissipated - fine for a
1/4 W.

Have you tried measuring the original R1?

At a guess, I'd put in a 100 ohm, 0.5 watt resistor for R2, and maybe a
220 ohm, 1 watt resistor for R1. If you don't like how hot R1 gets,
maybe 2200 ohm, 0.25 watt for R1. I guess that a 22,000 ohm resistor
for R1 won't draw enough current to really do any good.

Matt Roberds

R1 measures 22.1 Ohm .



--
http://www.supervinx.com/Retrocomputer