Yes, the very first thing to check is the condition of the main power connectors. On the PDB (power distribution board), the huge one, bottom right, locate the connector with the blue/white and the red wires, J101 (on the right side of the board) . In what condition is this connector? And the pins on the board itself? Can you post pictures of this connector. A picture of the fuse holders as well is a good idea, some fuse holders might show signs of aging too.
Besides that, unplugging J101, few times can help clean the pins but it's a temporary solution.
Also make sure J210 on the CPU board (board to the left) is in good condition, you can unplug this one a few times as well to try and clean it.
Do the same with the ribbon cable, unplugging each connectors a few times in hopes of cleaning the pins.
Also send a picture of your CPU board from the battery holder down to the connectors, to check for battery acid damage..
It's possible that you only see garbled text on the display because it doesn't finishing booting properly, first thing will be to get the machine to boot, then focus on the display if needed.