Black screen, solid sleep light and max fan speed boot failure

I have a Macbook A1342 that’s currently not booting up.

When I press the power button, one of two things will happen depending on power source:

MagSafe charger connected - upon pressing the power button, the machine will turn on. However, the sleep LED is solid white and screen is black. After some time, the fan slowly speeds up and then it cuts off. After this, it’ll automatically reboot and after a bit, the fan will spin very fast. It’ll remain this way until I cut the power. The screen is black the entire time and the sleep LED remains solid white if the machine is on.

So first power on results in a rather quick power off just as I hear the fan rev up, and it’ll reboot and this time not cut power, fan then hits full speed and stays this way.

Battery - it will simply reboot loop. It will wait until the fans start speeding up and then cut power, and reboot. It’ll keep rebooting itself until I disconnect the battery or hold the power button.

This happens with RAM seated or not seated. It also happens with just the board and fan connected outside of the Macbook’s case.

Any idea what might be wrong here?

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Do you hear the bong or hear beeps when you boot it with no RAM? It’s likely a logic board issue either way.

This is random and might not help at all but does the backlight turn on? I have an A1342 with a faulty logic board and it gives me a black screen but with backlight (check the Apple logo on the other side of the screen)

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@nagayoshi765 I found out a while ago that the issue is the logic board. The ISL9504 chip on the board gets EXTREMELY hot while the Mac is in the black screen solid sleep light state. I looked on a schematic I could find online and it's a critical chip (U7400). It does a few things for the CPU from the looks of it.

I have no clue how this chip managed to kill itself, but I guess I should be more careful when dealing with old Macbook logic boards like this. It's the second one that has suddenly acted up after I've taken it apart. The first one died when I replaced the thermal paste on the CPU and GPU. Another chip (i625 marking, along with some other characters) got killed on that board.

I don't know what is killing them. I've disassembled my laptop many times and each time has been fine. Maybe it's static or something I don't know, or that the logic boards are so old that they're just prone to fail more easily. It's baffling.

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