I tried fixing an old phone I had and succeeded initially. But I put it together sloppily and the menu button wasn't set right, so it wasn't pressing. I took the phone apart again to redo it, but when I put it back together, it no longer would power on. The screen works when I charge it, but I can't turn it on. So I looked at the service manual and it gives a flowchart for trouble shooting. How does one do this kind of testing? With a regular multimeter? And is there an alternative way to power it on that bypasses the power button?
