In the last week the backlight on my Thinkpad T60 has been going off for no apparent reason, usually after about 2 minutes after logging on. After a bit of experimentation, I found that it happens only when the charger is plugged in (with or without the battery). I tried another battery and another charger (all configurations) with the same behavior, so I don't think it is a battery or a charger issue: it could be software, or it could be a circuit board component that handles power switching gone bad.

I found this remark:

The only think i can think of is that the backlight is turning brighter when you plug in the ACadapter, most do. And you have a problem with the inverter and the additional power draw overloads it. Go into powermanagement and set the screen to the same brighness on AC as when on battery, apply, then plug in the charger and see.
(I don't know what an inverter is! But overload I do understand.)

Three questions:

1) how do I set the brightness so that when the charger is plugged in it remains low?
2) Is there any way to diagnose the power changes on the board or circuit board power problems?
3) Any tips whatsoever?!

Right now my workaround is to charge the battery, work for an hour or two, suspend, charge, then unplug charger and repeat. It does make me take a break, but not always when it is convenient....