Take a look at how the power regs are installed. Mine had only glue attaching the heat sinks to the PCB and it had all decayed and come loose. Because, you know, HEAT. The HS is drilled for a screw and there's a hole in the PCB for it but none was installed. that's just lazy.
A lot of Yamaha gear is known for cracked PCB traces and cracked solder joints. These can be a bear to find and fix.
I just did this with a C-80. Had to clean up all the dried glue, remove the trans and HS', pair of resistors (replaced) and polypropylene caps (replaced) and rework everything. Found some perfect fit screws in my junk piles and added the insulated shoulder washers for a clean install and no more glue. Funny they all have holes and sinks set up for screws but they're never in there. Some bean counter must have decided it was more cost effective. It was also another one that I had to remove and grind/burnish/polish a corroded ground busbar on. I hit them with a coat of clear lacquer so they won't tarnish before re-installing.