Hi Dino,
There is an official distributor for service manuals authorized by yamaha germany: Schaltungsdienst Lange in Berlin.
https://sdl-online.de/dokumentationen/
They sell the service manuals. They sell copies, not originals, but the quality is very good.
And regarding your problem: I am quite sure, that the circuit for the powerup-reset acts too fast. This ciruit is usually a simple ‘unload a capacitor and send a reset when the capacitor is loaded fully again’. If the capacitor degrades, the load time will be faster, so the reset comes too early. A very simple test is to add a momentary switch, which shortens this cap.