cmonex...so you want the details:
whereas you are great at tinkering with software...custom roms and all...I am good at tinkering with the hardware of many gizmo's...fortunately the 720, unlike current cellphones and ipods, is very amenable to simple mods and fixes.
you need to be able to dismantle the 720...there are how to photos on that handheld attitude site...there are a few tricks to getting the back off the lcd...but let's assume you have access to the both ends video ribbon cable and the hinged connector on the mb and lcd.
you need a continuity tester or a volt-ohm-meter...some magnification to see the individual leads coming out of the connectors...attach a needle or straight pin to your meter leads..so you have "microprobe" tips to test each connection point...be certain the cable is seated properly in each connector...no power to the mb, etc Radioshack sells a mini-screwdriver set that has a bit that fits those T-6 torque head screws under the hp/720 stickers on the lower lcd case
(use a needle to get under the logo stickers and lift them off without ripping them
) and on the 720 base/bottom...to get to things...use your fingernail sliding between the case halves to pry it apart at the seams...carefully...prying and sliding.
I use a radioshack vom...set it to measure resistance or continuity
(it beeps when contact is good
)...using your "microprobes" walk down the soldered connectors on each end of the cable...as I recall right side of MB connector corresponds to the bottom/lower end of lcd connector...intact ribbon wire leads will beep and broken leads won't...note which wire is broken.
In my case with the orange and purple horizontal screen pattern...I found one...only one broken wire...and it was the number one or first lead at the bottom of the LCD connector
( just now opened the lcd case to verify...took a few seconds once you know what you're doing
)...so I found a fine insulated wire from a wire cable I had...maybe smaller than a single strand of cat 5...but cat 5 would probably work, so would any fine copper enameled wire probably...I wove this thru the hinge area and soldered the ends to the soldered connector lead on the mb and lcd boards...using a cheap low wattage fine pointed solding iron with a magnifying lens or dollar store reading glasses...pre-solder the cat wire ends so they are "tinned" to keep the heat load down on the lcd and mb.
then just close that baby up...you might want to plug the power cord into the unit and attach the keyboard cable to turn it on before assembly to see if it works. This one wire fix was all it took...and...I am writing this on the very 720 that had the broken video ribbon cable.
by the by...the purple horizontal lines seem to get burned into the screen if left on for anytime in the defective mode...they leave linear shadows that will, however, fade with usage...
if you need more info or photos...let me know and I will try to follow up. Happy hunting.
You can use a 12-volt (max) universal power adapter with multiple plugs, but most of them are way short of 2.6 Ah at 12 V. Caveat emptor. 
