Sounds like this is going to require some disassembly and inspection. Does external VGA work when the AC adapter is connected? If so, it could be an issue with powering the backlight from the external power jack, so something in between could be wrong, whereas the path from the battery to the backlight is fine.

Mister Xiado wrote:

Sounds like this is going to require some disassembly and inspection. Does external VGA work when the AC adapter is connected? If so, it could be an issue with powering the backlight from the external power jack, so something in between could be wrong, whereas the path from the battery to the backlight is fine.

I just realized I was using a power jack that was 5 volts and 3 amps too high, could that do this kind of thing?

And luckily I didn't fry it, it still powers on, and the screen still shows without the power charger.

The amps don't matter but volts do. Using higher voltage is most definitely not recommended as it can destroy the battery (with fireworks) and the charging circuit.

Tiido wrote:

The amps don't matter but volts do. Using higher voltage is most definitely not recommended as it can destroy the battery (with fireworks) and the charging circuit.

Yeah, I think that was the problem, I contacted the seller and he said it worked fine when plugged in his charger, which was a much lower voltage than mine. I'll see how it runs when I get the right one.

Your luck in not frying it probably has to do with some IBM engineer’s happy foresight in designing the voltage regulator. If it wasn’t working properly you would have been pushing all of that extra voltage through the machine, and because your power supply can provide all of that current, it probably would have been fatal.

The reason I say this is because sometimes you can get away with using a higher voltage supply if it can’t provide the amperage needed at that voltage to fry the circuits.

Probably the only effect you might care about is that you have been charging your battery at a higher rate.

Edit: when ordering your new power supply remember that you need the amps to be equal to or greater than that of the original supply. The amperage rating is really just a measure of how many amps the supply is capable of providing. How much it actually does provide depends on how much the machine is demanding at the moment. So if you don’t have enough amps, the unit might work ok until you try to do something that consumes a lot of power like running the CD drive or a WiFi card.

As far as I know all those old thinkpads including 770 models use 16V 4.5A power supplies, so I don't think 5V would fry it if the polarity of the power supply was correct...
Not sure why anyone would use just random power supply without checking anything beforehand though...