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From your description, it could be several things and here's what's needed for verification:
- AC adapter is kaput; best way to determine (without endangering any other equipment) is to measure the voltage at the DC plug. Refer to the Hardware Maintenance Manual for proper procedure: http://download.lenovo.com/ibmdl/pub/pc ... 551_02.pdf
If you don't have a voltmeter, an alternative is to try plugging the AC adapter into another 20V Thinkpad and see if it works there. Words of warning
: If the AC adapter is putting out more than the 20V it is supposed to, you risk damage to the other laptop.
- AC outlet is dead; you said you "thought maybe it was the wall outlet, but no". Just to be clear, did you try plugging in a light or appliance into the same outlet and verify it's getting AC power or plug the AC adapter into another outlet you know works?
- Fuse is blown inside Thinkpad; assuming that the AC adapter and wall outlet are not the problem, then it's possible that the fuse that protects the DC input inside the laptop has blown. Only way to verify is to take the system apart and measure the fuse with an ohmmeter.
- Thinkpad's DC jack is broken; a very small possibility is that the DC jack inside the tablet is busted. That would prevent the 20V from being delivered to the motherboard.
- motherboard issue; something else besides the fuse and DC jack is causing the problem. This would mean changing out the motherboard as it's not too practical to troubleshoot and replace any individual parts on the board.