820-4924 no power, has all rails

[Jay]

820-4924 appears to have all G3H and S5 rails at the right voltages, green light that responds to SMC reset yet no power. U7600 had a little bit of crud on it so I replaced that, no other issues visually with the board.

I ran out of hints as where to look next. Any pointers appreciated.

 

[dukefawks]

Light turning orange? Check for power pulsing. The usual suspects inspected? U1900/1950, U6100 termination resistors/traces? Check rails for short? PPBUS voltage?

 

[Jay]

Green light turns orange after a few sec and stays that way.

U1900 has 3.3 or 3.4V on pins 1, 2, 5 and 13. 372mV on pin 12 and 60/70mV on pin 4 (this was 330mV when I measure before and before that it was fluctuating all over the place. Not it holds steady at 60/70mV).

U1950 16mV pin 2, 3.4V pin 8.

U6100 3.3V all pins, continuity from all pins to their destinations OK.

PPBUS is 12.6V at F7140.

Would rails show the right voltage if there's a short?

Amp draw on PSU is 0.021/23

 

[dukefawks]

A shorted line will not have voltage on it cause it is shorted to GND. Still need to know if it may be power cycling, check on the 5V rail L7520. Preferably use a scope as many multimeters are too slow to catch the short pulses. Again check all rails for shorts to GND.