X61T failing on POST under AC, reboots repeatedly w battery

[cogit0]

I have a Lenovo X61 tablet running Windows XP. Previously I had similar problems with this system. The problems happened about 6 months after the warranty expired, so I've been on my own for the repairs. I replaced the system board, and it looked like it was working. About 3 weeks after I replaced the system board, the battery, the AC adapter, and the RAM, it started having the same problems:
Under AC power (no battery inserted) the system shuts itself down after being stable for 5 minutes, then tried to reboot itself, fails POST, and shuts itself down again in the middle of post. Just plugging in the DC socket powers on the system; I don't hit the power button to turn it on.
Under battery power, the system cannot go past POST at all, and gets caught in a loop where it reboots, gets 5 seconds into POST, shuts itself off, and reboots again. I need to pull the battery to stop this loop.

Again, thinking it was the system board and possibly the DC power lines going into the system board, maybe the socket itself, I replaced these parts. I have tried new RAM as well. I'm exhibiting the same problems as above with entirely new parts, and can't figure out what else it could be.

I've taken out the SATA sub-card that connects DC power to the system board and tried to run it off of battery alone. If it worked, the problem would be in the sub card or the DC power lines. It didn't work.

I've unplugged everything else that is in the system board, short of the CMOS battery, and gotten the same results. I don't know if a dead backup battery would cause these problems, but a multitester is showing +3.0 volts on the battery, and an oscilloscope is showing steady voltage, not jumping.

Any other ideas what the problem could be? Please offer any suggestions! I've been without a laptop for 4 months now, and it's definitely been showing on my productivity, to say nothing of the time I've spend trying to fix the thing.

[pgoelz]

Through all of your struggles you neglected the display. If something was wrong with the lid display or backlight circuits, that could overload the power supply and cause a hard shutdown. Just a thought. You seem to have changed or eliminated everything else.

Can you totally disconnect the lid and use an external monitor? I don't know if it will work without the internal display connected....

Paul

[cogit0]

I tried to unplug the connector from the screen module to the system board and boot from there, but I actually get the same result as above. It looks like it tries to post, I can see the fan spin up (nothing on the external monitor) but dies shortly thereafter.

I don't think that a surge from the screen can really be ruled out completely, but if it's still having similar symptoms without the screen plugged in, it's hard to say that the screen is a likely cause.

Other ideas? I'm about to just scrap this system and get a new machine. Thoughts on the X200t?

[richk]

It might be a bad motherboard (again). That is a sort-of common problem. I have seen that exact same symptom twice on X61 tablets. At the time, I took one of the boards to a guy who does OEM repairs on motherboards and he was unable to repair it.