12V rail is not on ?

[bianchi77]

Guys,

Does anyone know why my 12V rail is not on ?
5V standby is on....

The power supply is LC 300ATX, does anyone have the schematic ?
I suspect my 12V rail is short, because of short on my capacitor on motherboard..

it's using to diodes soldered together into heatsink...is it FR302 ?

I saw :
http://danyk.cz/s_atx05a.png
Thanks

[CapLeaker]

Re: 12V rail is not on ?

5VSB is always on. I am guessing that is the only thing that is powered up. No -12V, no +12V, no -5V,no +5V neither the +3.3V. To turn the rest of the PSU on, take a 1k resistor and jump 5VSB to PSon.
Other than that, check that D8 isn't shorted, T4 has continuity between pin 5 and 6, L6 isn't open, and that C26 isn't shorted to GND.

[bianchi77]

Re: 12V rail is not on ?

D8 is bad...from my diode test 0v

[bianchi77]

Re: 12V rail is not on ?

L6 is ok

[bianchi77]

Re: 12V rail is not on ?

Can t find C26? Is the schematic the same between 250w n 300w

[bianchi77]

Re: 12V rail is not on ?

Got it..small 22nF near the chip..is that Pwm switching chip?

[bianchi77]

Re: 12V rail is not on ?

I tried common an ps_on..no response only can hear high pitch noise...

[fzabkar]

Re: 12V rail is not on ?

If there is a short between 12V and Ground, then isolate the relevant components on the secondary side until the short goes away.

[bianchi77]

Re: 12V rail is not on ?

If there is a short between 12V and Ground, then isolate the relevant components on the secondary side until the short goes away.

How?

[fzabkar]

Re: 12V rail is not on ?

CapLeaker has already identified the components that are likely culprits.

In any case you appear to have already found the problem -- shorted D8. After removing D8, test for shorts across the 12V output again. Also confirm that D8 tests shorted out-of-circuit. There should be an electrolytic capacitor filtering the 12V output. Test it also.

Edit:

Here is the circuit diagram for an LC-B250ATX.

"LC-B250ATX ch. Y-B200-ATX ver. 2.9 JNC Computer Co.""
https://cdn.badcaps-static.com/pdfs/...83951b6882.pdf

Some time ago I had a fake 400W PSU which was actually a 200W version of the above circuit:

http://www.electronicspoint.com/thre...atabase.132952

[bianchi77]

Re: 12V rail is not on ?

C27? Is it possible I break other rails?

[fzabkar]

Re: 12V rail is not on ?

Don't switch the PSU on after removing components. Just test them for shorts without powering on.

[bianchi77]

Re: 12V rail is not on ?

Diode in the middle is connected to 12V rail...
and I measure with diode 0.1V both side from the base...

and the left and right diode SBL1040..

I got only 0.4V left and right from the base...

[bianchi77]

Re: 12V rail is not on ?

Are they all faulty ? I measure from new diodes 0.6V

[bianchi77]

Re: 12V rail is not on ?

Some photos :

[bianchi77]

Re: 12V rail is not on ?

Looks like it's different from the schematic, since it's not D8...

[bianchi77]

Re: 12V rail is not on ?

Almost the same with :
"LC-B250ATX ch. Y-B200-ATX ver. 2.9 JNC Computer Co.""


I can see diode label is match...

[CapLeaker]

Re: 12V rail is not on ?

Diode in the middle is connected to 12V rail...
and I measure with diode 0.1V both side from the base...

and the left and right diode SBL1040..

I got only 0.4V left and right from the base...

Umm... that link isn't showing.

With your DMM set to diode mode, probe the left and right pins of the diode to the middle pin. If there is a short, the diode is bad. easy.

The SBL1040 is a 40V, 10A Schottky rectifier that has a voltage drop of 0.6V. Problem is that it is only a diode with 2 legs, it should be one with 3 legs.

Maybe you can upload nice straight shot, high resolution pictures here to badcaps, using the attachment feature.

[bianchi77]

Re: 12V rail is not on ?

Which link ?

[bianchi77]

Re: 12V rail is not on ?

can I replace with :
http://www.aliexpress.com/item/Free-...6-a85bf6bb2cc6