Sadly these things are a bit touchy so if they decide they want to be recalcitrant the only option is to either swap parts until it works or just give up. Without in-depth troubleshooting data and precision soldering tools available there isn't much that the average hobbyist can do to fix a component-level failure on these. Sometimes you can get lucky and find a bad fuse or broken resistor or something but more often than not there's a fault somewhere that you can't see, let alone access, and most of the chips on the board are custom and/or NLA, so even if you did have a $10k SMT rework station it wouldn't do you any good because you can't get new parts.
Not to be too much of a downer but unless you've got a couple machines for parts it's not likely you'll get one of these, or many of its contemporaries, in working condition for very long. You really need to be able to do (or know someone nearby who can do) a part swap until it works and then go from there: if the I/O card is bad, then you can repair/replace that rather than continue fussing with the logic board or processor card. If the processor card ends up bad, you can order a replacement or an upgrade. If the logic board is bad, you can try to break out an oscope and start probing some test points I guess, or just try to replace it. But it's difficult to narrow it down without a cache of spares for testing.
Anyway is there a good video on YouTube for these things? Occasionally I'll see something posted here, sometimes by members such as JDW but normally it's from random people covering something. 65scribe kind of has a vintage Mac review channel that does minor repairs. I've considered doing a channel filled with vintage Macs among other things but the production values would be very low (I record it, edit for content, then upload it: no intros, no make-up, no star wipes, no fancy dress, no in-line ads, no studio, no music, no trick editing; it's just me and the machine in my natural habitat) and it would be unscripted and presented as if I was casually talking to friends rather than a live studio audience with the aim of making money. I have enough 2400 parts and accessories that I could cover a good amount of info about them, in addition to many other machines (I could do the complete Pioneer MPC series, PowerBook G3 series, PB 5300 series, PM 52/62/53/63xx models, Color Classic series, etc) so it would be more than just the one video.
