820-3481 PP3V3_S5 4.2V random turns on

[No-Clean]

This board came dead, clean board

Found a short on PP12V_G3H_P3V42 (C7806) but even after fixing that I had no power
Replaced Q7950, Q7970, U7950 and U7970 board now turns on sometimes but LED 1 is off, 2-3-4 are on when iMac is assembled but sometimes it wont turn on.

When board boots PP3V3_S5 is 4.2V with LED 1 off after I found this out I replaced U7600 but same thing
When board cycles and does not turn on PP12V_S5 is around 10-11V this is with bench power supply but with iMac PS does the same thing.

 

[2informaticos]

Which schematic are you using?

 

[No-Clean]

820-3299

 

[2informaticos]

As per my notes:
"Se puede trabajar con esquema/boardview de 820-00291.
Tambien sirve como guia el esquema de 820-3482.
820-3299 usa mismo GPU."

"PP3V3_S5 is 4.2V"
Did you check it at L7610 output pin, or other 3V3_S5 point?
L7610 pin 2 is not proper test point.

Test the board without CMOS cell; should turn on by itself.

 

[No-Clean]

Ok working with 820-00291 schematics now, PP3V3_S5 is 4.2V sometimes at the beggining it even goes up to 4.5V tested at R0301 (Next to LED 1) all info given so far was given without CMOS cell installed.

 

[2informaticos]

Check the feedback network, R7610/30/31.
You can remove C7632, just in case.

 

[No-Clean]

R7610/30/31 good

With C7632 removed board does not boot at all

 

[2informaticos]

Mentioned resistors have correct value?
R7630/31 can only be tested out of board properly.
You can play with them to adjust the output voltage.
Vout=0.6x(1+Ra/Rb), as per schematic...