820-00239 Has 20V but current is cycling

[bjf]

This came in as a no-spill but it looks like it was exposed to some type of very minimal moisture. Very subtle but I had to replace R3509 and R3539. Cleaned it in the ultrasonic. No change. I can't find any shorts. Every 5 or 6 seconds it will try to turn on and a small amount of current will go through for a split second. Stays at 20V the whole time. PPBUS is at 12.6V but moves between 12.59 and 12.61V which makes me think something is pulling it down slightly. Not sure where to go next.

 

[2informaticos]

Check which secondary voltages pulse; all S4/3/0 states.
Also check all rails in page 78, Power FETs.

Try to start in SMC bypass too.

R3509/39 open could cause an issue on 5V_S4 power rail.
Compare diode mode to ground on PPBUS_G3H and L3500.

 

[bjf]

Had a short on 3V3_S4, just had to hunting for it. Finally found it on C4202. Good to go now. Thanks for the help.