Computer Power Supply-Dead

Thread Starter

FiveShorted

Joined Mar 19, 2017
6
Working on a power supply. Just can't toss it without completely destroying it first. :mad: This PSU has almost no use and is out of warranty of course. Upon close-magnified scrutiny of the components nothing indicates failure, i.e. bulging caps, burned FETS, broken wires, fuse, etc.

Primary Symptom: When green wire is grounded at main terminal none of the computer peripherals receive 12V, 5V and/or 3.3V.

The 400V cap is only showing 168V(dc) (waiting on cap tester now). Thinking this cap should be somewhere around 350-375V. This is the first discrepancy I find in circuit.

Question: If cap is good what in the circuit would prevent it from charging to an estimated 350-375V?

I am getting the 5vstdby as annotated on "Bottom" (bottom left;orange).

I have added a few pictures indicating general observations from probing.





 

ronv

Joined Nov 12, 2008
3,770
Working on a power supply. Just can't toss it without completely destroying it first. :mad: This PSU has almost no use and is out of warranty of course. Upon close-magnified scrutiny of the components nothing indicates failure, i.e. bulging caps, burned FETS, broken wires, fuse, etc.

Primary Symptom: When green wire is grounded at main terminal none of the computer peripherals receive 12V, 5V and/or 3.3V.

The 400V cap is only showing 168V(dc) (waiting on cap tester now). Thinking this cap should be somewhere around 350-375V. This is the first discrepancy I find in circuit.

Question: If cap is good what in the circuit would prevent it from charging to an estimated 350-375V?

I am getting the 5vstdby as annotated on "Bottom" (bottom left;orange).

I have added a few pictures indicating general observations from probing.





I would say the 168 volts is probably okay. 1.4 X 120 volts AC in.
 

Alec_t

Joined Sep 17, 2013
14,280
What is your mains voltage (brown and blue inputs suggest 240V?).
Have you checked the input rectifier bridge feeding the big cap?
The big toroidal choke on the left seems to be touching the aluminium plate to its left: not a good sign, since vibration could damage the winding insulation.
 

ian field

Joined Oct 27, 2012
6,536
What is your mains voltage (brown and blue inputs suggest 240V?).
Have you checked the input rectifier bridge feeding the big cap?
The big toroidal choke on the left seems to be touching the aluminium plate to its left: not a good sign, since vibration could damage the winding insulation.
It looks like it has a PFC front end - so the bridge rectifier doesn't feed the "big cap". The rectifier has a low capacitance metalised foil cap which is mostly about keeping noise from getting back into the mains.

A voltage doubling rectifier is Impractical with PFC, and the PFC unit is essentially a boost converter anyway - it can probably accept the 90 - 275V range.

If the PFC fails to drive; the choke will pass DC relatively unhindered. 220V will probably run the half-bridge main switcher and you might not notice the difference. 110V probably won't start the main switcher.

Once or twice I've found incinerated PFC chokes - I just link out the choke and remove the flyback MOSFET. Its OK for my own use and the difference between scrapping it for parts and a free PSU.
 

Dodgydave

Joined Jun 22, 2012
11,284
If you're getting 5Vstby, then the start up push pull is working, this should feed the smpsu pwm chip and when the green is shorted out to black it kicks up and syncs the push pull to produce the 12/5/3 v out,

What is the model of the atx, are you on 110V or 230V input?

There is another pcb on the bottom picture what's that?

Here are some circuits of atxs....

http://danyk.cz/s_atx_en.html
 
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Thread Starter

FiveShorted

Joined Mar 19, 2017
6
I would say the 168 volts is probably okay. 1.4 X 120 volts AC in.
Ron, You math doesn't add up for non-science majors. :) I do have two rectifiers converting to DC just after the second choke and second filtering caps. Both are converting equally the same. Proceeding through the circuit for more troubleshooting as I learn.

Ron, Toroidal choke appears so due to angle of lens,

On 110, but PSU permits 100v-240v in. Actual in 123AC.

Looked briefly at the schematics. Full bridge is two rectifiers? The sub board, I call it, has output lines to all the computer peripherals. Corsair 800watt 2013.
Thanks for the ideas and information everyone.
 
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Dodgydave

Joined Jun 22, 2012
11,284
Best to draw it out on paper and post it, that way we can see what's happening...

Usually 120V input is to two diodes and two capacitors as a bridge rectifier doubler.
 

Dodgydave

Joined Jun 22, 2012
11,284
This is a circuit of the Green smps chip, and internal working. It gets its primary voltage on pin8, then kicks up and uses the slave winding on the pulse transformer on pin6
..it uses a TL 431 as its feedback regulator.


page9.png page3.png
 
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ian field

Joined Oct 27, 2012
6,536
Can't seem to figure out what the EE's purpose of this is? AC Hot and Neutral come together through two SG1s then through a GR8875R C2U01 (7 pin) "chip".
image: https://postimg.org/image/smtd5jtz1/

The 400v cap is only charging to 168 vDC and it should be much higher....like double I am thinking.:(
All the PFC PSUs I've seen had a 450V electrolytic.

The peak from nominal 220V mains is somewhere around 320V and the PFC unit is basically a boost converter.

AFAIK: One size fits all - on 110V mains, the PFC unit simply boosts the rectifier output to whatever the main switcher requires.

The mains rectifier in a PFC PSU doesn't have a reservoir cap which would draw blips of charge current only when the AC peak exceeds the standing capacitor voltage. That's one of the distortions of the mains waveform that PFC eliminates.
 
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