I'm searching for the electronics schematics for IBM terminals 3477 and 3486. I've already googled for it without success. I've to mend this hw and the power side seems ok but doesn't function, so I need electronics schematics to go on. Any advice ?
Check the Internet Archive. They have the schematics for the 3477. (To be fair, these were added just last year, after the OP asked his question.)
In the absense of schematics, you can trace the power and ground lines to the individual chips. If there are datasheets on the individual components, that would help you find out if one or more are faulty (usually it's just one). Another way to do it is with IR imaging - faulty components tend to either be abnormally hot or abnormally cold.
If you manage to trace enough of the board, you'll have the schematics.
You can glean a little more information about the terminal if it's emulated in MAME. I never looked for coax or twinax terminals in there, but the 3151 (ASCII, RS-232), IIRC, is there.