Laptop troubleshooting forum: Need expert troubleshooting help!

This is not another novice laptop computer issue. This is one of those issues where you might suggest shipping it off to the manufacturer for repair, but that is not an option. I intend on repairing this myself, but have run out of ideas, and need some suggestions.

My parent's Gateway laptop is about 6 years old. For years, the primary issue has been a faulty power jack, forcing you to hold the plug to the right to get any charge. A few weeks ago, I set out to repair this by purchasing a replacement jack, and soldering it into place.

I disassembled the laptop, as I have done a half dozen times, all the way down to removing the motherboard. After inspecting the old power jack, I realized that it had been brazed, not soldered, which means I would have to postpone the repair work. So I started to reassemble the machine.

Upon completion, the system would not power on at all, with a full battery. After testing the plug, and the battery, I decided to take everything apart again to see if I forgot to connect something. Everything was in place, so I unplugged all components, from the hard drive and LCD screen to the CPU fan, down to the Wi-Fi card. With everything unplugged, I carefully connected the power, and booted the motherboard. All the LEDs came on, except the HD LED, which is understandable with the Hard Drive being unattached. So I unplugged, hooked up a component and tried again.

It worked up to the point that I plugged in the CPU fan. I even had the LCD screen plugged in and powered on. Without the CPU fan attached, I'd get a post error saying "CPU fan failure" and an immediate shut down. With the CPU fan attached, I'd get a quick flash of the main power LED. Sometimes the main power LED would stay solid, but no other LEDs lit up, no LCD screen, and the fan would make a short attempt to come on, but to no avail.

I tried pulling the RAM, resitting the CPU, resetting the CMOS, starting over from having no components connected, and finally decided to replace the CPU fan. The new fan seems to be working, but new problems have come to the surface.

Now the computer powers on, the power LED stays lit, all the other LEDs flash once at start-up except the HD LED, and the LCD screen does not come on. That made sense to me when the Hard Drive wasn't attached, but now it doesn't make sense to me. I tried removing the Hard Drive from it's sleeve and checking the circuit board. I even plugged it back in and had it power up once, even boot into Windows, only to shut off once Windows had loaded, and not work again.

So now I'm stuck at this point. It seems that everything wants to work, but it's not reading the Hard Drive. Also, the CPU fan will start up slowly but then it revs up for a few secs before shutting off completely. This happens every single time I power on the system, except for the one time it actually booted normally into Windows.

I still need to replace the power jack, but will need to rent the right equipment first. But obviously something else is wrong. I get the same results whether the plug is attached, or if I'm running of the fully charged battery.

First thing I would love to find is a website specializing in all the LED codes and beeps for all the different manufacturers to troubleshoot post errors. I just can't seem to find that on Google for some reason. And hopefully someone with some real expertise might have some advice for me, other than ship it off to Gateway.