Need The Power Amplifier For This Yamaha Plus The Predrivers

I'll start with the easy part.  Power amp modules are still readily available, as far as I know.  B & D Enterprises is one trustworthy supplier.  For the CR-840, use STK-0060ii, or perhaps a higher-power part such as STK-0080ii for more safety margin.

If the power amps have failed, be sure to check the emitter resistors - they often suffer an overload and go open.

Now, the hard part:  Original predriver modules have been unavailable for some time, as far as I know.  Some people have adapted still-available pre-drivers from the STK series; that's feasible, but won't exactly duplicate the Yamaha part's amplification function, and it won't duplicate the other function (current-limiting to protect the power amp) at all.  There is quite a bit of discussion of the predriver problem in the Yamaha forum on Audiokarma, so you might want to browse there.

And finally, the advertisement:  I am offering hand-built modules that are a drop-in replacement for the predrivers, duplicating all functions.  I'm brand new to HelpOwl, so I don't know if it gives you a way to contact me, but you could get in touch via the Audiokarma forum, or look on ebay under seller name "lostchipmunk".  The listing on ebay is for a single part, but I can make more.