OK, my brother in law has a 2100 that he bought new. It was subjected to decades of the typical sort of abuse while used on tour and for local gigs, by his R & R and country rock band members. It functioned flawlessly until it was dropped hard a few years ago. One channel went out.
I offered to take a look it it.
I read here in the AK Yamaha section a thread which suggested common problems with dirty connectors. I gave it a basic cleaning without taking anything inside apart, then cleaned (with CRC QD contact cleaner) and re-tensioned the pins in all the connectors inside. That did it and now it works fine.
Every now and then we get lucky.