A1418 820-00431-A short PP5V_S4

[sdogs]

Good morning everyone,
I have an imac 21 A1418 with logic board 820-00431-A that has power problems.
Specifically, with power on, only one LED (1) lights up and when I go to press the power button it goes off for an instant but nothing happens, then it turns green again.
I also tried another power supply but the result did not change.
I measured the various binaries in diode mode but everything seems to be normal, apart from the PP5V_S4 which is clearly shorted.
I measured the various voltages and they are all present.
I have tried removing C4702 and C4602 and other components around them including R7300 , u7300 , c7300 but the short is still there.
I injected 1v on pin 1 C4702 and had 3 amps but with thermal imager I noticed only slight heat on Q7650 (could it be bad?) belongs to 5V S4 Regulator circuit.

Any suggestions ?

[mon2]

Re: A1418 820-00431-A short PP5V_S4

Remove all power -> meter in resistance mode -> what is the exact resistance to ground on PP5V_S4 ?

The 3A may not be enough to force the offending part to be visible -> can you increase the current? (for now, keep the voltage @ 1V).

Likely it is even safe to increase the voltage since the 5 volt rail is not linked to the CPU power rail. The leakage of the high side mosfet of the CPU rail is always the risky one but that mosfet will operate from the PPBUS_G3H power rail which is not this one.

For this reason, the rule of thumb is to keep the injection voltage as low as the lowest power rail on the PCB.

If your power supply does not offer higher current, it should be safe to consider to dial the voltage to 2V and test again. We are using the 'short killer 30A' model from Aliexpress. Works well and can spike out, they claim upto 30A. Often, for us, @ 8A or under, we can cause the offending capacitor to crack open. Usually is a tantalum.

[sdogs]

Re: A1418 820-00431-A short PP5V_S4

thanks for the reply, I gave it a try and replacing Q7650 is working fine now ;-)

[sdogs]

sorry