Dead systems are hard too diagnose. It could be quite a few things which makes tough to fix without following a good diagnostic process.
Wants hard is without having access to spares to slide in and out like a battery, DC-In board & logic board for starters. You then need to test each part discreetly. Referencing schematics and board views of the logic board and using a DVM to measure things.
As you hadn’t spent that much on the system att his point, can you find a second of the same series?