When one channel out of two is still working, most techs can work off of that. If they have to, due to not having a tech manual. The working channel is the reference for testing/voltages/current/etc and usually a fix for the dead channel is not that far away.
This becomes problematic if the entire amp is dead, or cannot be turned on. It’s then a game for those who like to hurt themselves or prove how good a tech they are, but financially, having the tech manual makes the most sense for a repair shop or tech, that wants to have a viable financial future.
but, in reality, no tech manual for a given power amp that, underneath any and all of the given conditions encountered, still has working channels...underneath that.. the amplifier is ultimately fixable. It just requires more swearing and intelligently applied grunt work. Which, depending on the scenario, can be financially non viable in a repair business scenario, but very (near 100%) doable for the determined and capable tech (outside of time/money constraints).
as stated, I knew of techs who would never touch a piece without a tech manual available..and other techs who spent their entire tech business career doing over half or more of their repair work, in some cases most (~90%) of their work...was done with no tech manuals. There are only so many ways to design and implement audio circuits.
