Yamaha EF6600A Service and Repair Manual
Model code EF6600A. The Yamaha service manual that documents it is LIT-19616-CM-99, which also covers the EF4600A.
The factory put the two in one book because they are built along the same lines despite the difference in output. Where a specification is shared, one entry serves both; where it is not, the manual says which model it belongs to.
Service content includes the sequence for taking the generator down and reassembling it, inspection measurements and the limits that decide whether a part is reused, torque specifications, wiring diagrams for the generating and control circuits, maintenance intervals, and troubleshooting organised around what the machine is doing.
Read EF6600A off the data plate, match it to the list at the front, then work from the procedures as written.