Yes, all the amps are on one board.
Here is the service manual. The pertinent pages are 98, 99 and 100. There seem to be a lot of parts already unavailable according to that manual. You can not use a multimeter in place of a scope.
The only thing I could suggest is that you isolate the preamp board output from the right front preamp and switch it to an input of a channel you are not using. I don't know if you have the chops for that or not.
I'm not surprised it gave trouble and that receivers seem so often to have such a short life. Building like this is what I call thrown together.
I use pre pros and external amps. If not it is new receiver time.