The usual recommendations when dealing with a non-power-up situation would be the following:
- verify that the AC adapter is delivering 16V (older systems) or 20V DC or use a known good adapter, which you have done
- try powering up with only the battery; the DC jack and/or the main fuse may be damaged
- try powering up with only the AC adapter (battery removed)
- try powering up with all of the hardware (except for memory) removed; this means the hard drive, Ultrabay device, MiniPCI card, etc.
- the fuses on the motherboard are identified by "F" numbers; F1, F3, F11, etc. Typical surface mount fuses for the main power look like this: http://www.kawakami-ca.com/images/t22_main_fuses.jpg
The one on the left (clear plastic looking with "FU" marked on top and denoted by the "F2" printed on the motherboard) protects the DC input on T2x motherboards. The one on the right (white ceramic with metal endcaps, with "F11" printed next to it) is the type which protects the battery and Ultrabay.
- look for burnt components on the motherboard. The usual suspects are MOSFETs such as "Q16" in this picture: http://www.kawakami-ca.com/ibm_t2x//let.jpg