I fairly recently got my hands on a 7800 intended for the scrap heap. The power supply plug was in somewhat bad shape and the power jack was broken off. When I opened it up I discovered that the soldering points for the power jack was almost completely ruined, as you can see below. However with some careful consideration I managed to find a work around, fitted a new power plug and jack, and got it to power on.
However I got nothing but a black screen and some humming/buzzing. Also, I found it strange that the plug wasn't a standard RF jack, but rather the same type found for connecting a monitor to an Atari ST computer. Curious that I am, I decided to open it up further and well, as you can see in the second picture, someone has been a busy little bugger. It has certainly been heavily modified during its lifetime. Still, this brings me no closer to a solution. Anyone know what might be wrong with the system or is it a lost cause?