a1708/820-00840 dead CPU?

[savemydevice27]

I am new to mac repair, trying to drive right into some of my dead macs laying around.

I have a a1708/820-00840 board that pulled 20v/.02amps. Had a short to ground on PPBUS_G3H. Removed F7000 to confirm it was PBUS_G3H, which it was. Nothing looked bad so injected 12 volts into F7000 PBUS_G3H pad, which lit up Q7680. Pads P3V3S5_VSW were all short to ground so I removed L7690 which cleared those shorts but I am still short to ground on L7690 on pin 1. I am scared to inject this line with dcps beacuse it has lines to the U0500.

Thanks.

[reformatt]

Re: a1708/820-00840 dead CPU?

PPBUS_G3H although higher voltage, feeds via current sense resistor to the power FET's driving the CPU. So when using voltage injection, it's best to err on the low side like 1V.

So 1V and thermal camera is best way to go for shorts. I've had so many shorted PCH/CPU's that when isolating a fault on the main power rail, I check the low voltage MOSFET's straight away for shorts. Save time troubleshooting a board with a blown CPU. You can sometimes get lucky with the low side FET blowing, rather than the high, which means the short is straight to ground and that saves the CPU.

[ugamazing]

Re: a1708/820-00840 dead CPU?

You can sometimes get lucky with the low side FET blowing, rather than the high, which means the short is straight to ground and that saves the CPU.

Not having the schematics in front of me, could you tell me which FET on the 820-00840 is the low-side one? I've always thought a shorted high or low side FET could kill the CPU. Might have some boards I pull back out if CPU has a slim chance of survival, lol.

[reformatt]

Re: a1708/820-00840 dead CPU?

The FET's for CPU on these boards aren't discrete but integrated into an IC (eg U7210/U7220). You have to take the IC off the board to determine the exact nature of the short. A Vin to SW short (high side driver short) means the CPU will get the full rail voltage and destroy it. SW short to ground (off board) would indicate low side FET shorted.