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John John is offline
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Default Switching PSU trouble.

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