820-3437 pulsing power rails

[arnonabuurs]

Hi Guys, I have an 820-3437 where all the power rails pulls, even PPBUS goes to 8.13 and back to 8.59. I do have green and orange line. Looks like it tries to start, but not. At U7501 I also have pulses on 5v_S5, S4 and 3.3V lines.Board looks pretty clean.

 

[2informaticos]

CPU VCORE pulses too?
Check for short on S3/0 power rails, beyond the Power FETS.
Did you try SMC bypass mode?
3V42 steady?

 

[arnonabuurs]

CPU VCore = no pulse
3V42 = Steady
Checking for shorts now

Could you remind me how to do SMC bypass please? I google it first (ofcourse...) but only find SMC reset and forgot!

 

[arnonabuurs]

ok, found a short on PP5V_S3, diving into the rabit hole , not too many IC in this circuit though....!

 

[arnonabuurs]

Update: found the short lead to PP5V_S4, replaced C7600 where smoke was coming out, short is gone. I still have a pulse on lines, checked at SMC_PM_G2_EN and also pulsing. U1900 has 32mhz.