im still confused how your getting 12v on the pbus of a macbook air? lol
Hi ZZZAC,
I was not getting 12V on an Air it was a MBP 13" 2011 K90I. I7 This was an interesting fix for me. Laptech had a fix on R7402. Thread no 1934 I had the same problem as him in his case it was a bad R7402 in my case a bad U7400. Just miss read the Air thought it was a pro problem, any way the point is read both sides of R7402. This is Laptech's thread and it is how i solved mine as i was not getting 12V on both sides of R7402 when i changed the chip and got 12v on both sides of R7402 job done a success. Getting the chime on an I7 is so good!! LOL. Big thanks to Rog.
(Thread no 1934 Hi,
Have just had another repair success. This time it is a 820-2936-B K90i logic board. Fault symptoms were it would power on, the fan would spin but it would not post, no chimes and no display. After checking all the main voltages, i started to look deeper into the CPU side of things because it was cold to the touch. Experience has taught me that if a BGA chip is working, be it CPU, Northbridge, or GPU, it gets very warm to the touch, almost to the point where the heat of the IC starts to hurt your fingers when you touch it. In my case, the Northbridge was getting very hot so I knew to some degree that it was working but the CPU chip was only luke warm. When this chip is functioning properly it gets extremely hot but it was not doing it in this case so that basically told me to start focusing on the CPU side of things.
I checked around a few capacitors (C1683, C1681, C1680 & C1682) to see if i had one of the main CPU voltages. On the faulty board i got 0v but when i checked against a working board, the voltages on those capacitors was around 0.8v. This told me I was missing one of the main CPU voltages. I checked on the schematic and found i had lost PPVCORE_S0_CPU. Tracing this signal back to it's source, Q7520 i found this component was not functioning properly. Upon further checking I found that the input signals to Q7520 where missing. Therefore tracing further the source of those signals I found that they originate from U7400 (main CPU control IC).
This now meant I had to figure out why U7400 was not working. I checked to make sure i had 5v (PP5V_S0) on pins 15,24 and 40, which I did. Please note you will only see the 5v on those pins when the board is powered on due to the power on state signal S0. I went back to the working board to make sure what voltage was present on pin#2 CPUIMVP_TON which was 12.5v. I subsquently found out from the working board that this voltage is present when the board is powered off.
I checked the faulty board and guess what, this voltage was not present so i checked the input of the this voltage which passes through a resistor R7402 and low and behold, i had 12.5v on one side (PPBUS_S5_HS_COMPUTING_ISNS) of the resistor but not the other. This told me the resistor had to be faulty. I therefore replaced it with one from a scrapped board. Just had the magsafe plugged in but did not power on (same as i did with the working board), measured pin#2 of U7400 and now i got 12.5v. I therefore connected everything back onto the logic board, RAM, FAN, LCD. Powered on and it displayed with the flashing icon...another success.
Rog )
South657