I blew the output transistors on my Kenwood KR-720 (too painful to discuss) and need to replace the following parts:
2SC2577
2SA1110
2SC2590
2SA1102
0.22 OhmK resistor (MPC78)
0.22 Ohm, 3W resistor
Welcome to the ever-exciting world of vintage gear repair!
I am no expert and I'll share the following few things I've learned as long as you keep in mine these are just one perspective:
There are thousands of discrete transistors that have been manufactured and used in audio gear over the last decades. LOTS of them. The biggest difference between them is how does each perform in a given application.
For instance, one transistor model, e.g. 2SC458, might be able to do duty as a switch, as a voltage regulator, as a voltage amplifier, and more... but it won't necessarily be a _good choice_ in each of those different uses it could fill.
So, when someone (like NTE) says "KSC1845 crosses to (can replace) 2SC458", that answer is not always useful. That "cross reference" might only cover when the original device was used in a circuit for amplification but not for switching. You need the specs for a given transistor _as used in the circuit_ along with the specs of a new replacement device to really get a good replacement match.
If you only look up a susnstitute in a cross ref guide, or only use an online substitution site like alltransistors.com", or only use a thread here on AK that gives out non-application-specific cross reference, then you could easily put the wrong substitute in place while feeling like you're doing things right.
So, when I started out, I found asking here on AK "what is a good substitute for 2SC1344" when I added "for use in the preamp board of my Pioneer SX-450 at component number Q402" was the most-successful and least-expensive way of finding good substitutes. And I learned a lot about transistors from reading all the posts from many members during that thread.
A great example was finding a substitute for a preamp low-level amplifier like 2SC458. On my own I had guessed KSC945 would be perfect, only to have many people teach me "yes, the KSC945 will work, but it isn't as low noise as the KSC1845. So, for audio preamp work, use a lower noise device so long as the other specs work, too, and your resulting circuit will operate the same but it will be quieter than before."
In this particular case, it sounds like you have reasonably starts with searching for an original transistor on Mouser.com or similar, only to find out Mouser's database of cross references doesn't show an exact-replacement modern-production device. Perfectly understandable.
Most 60's to 80's transistors aren't made any more, so there are few modern direct replacements. That's ok, since the modern replacement you _do_ find will be better in most if not all ways.
So, one way to get good advice on which transistor replacements to look for is to make a thread that says something like "looking for transistor substitutions" (like you did) and to include the brand and model number of the unit you're working on or the origins transistor model numbers you need to replace. That will help the interested AK'ers with some experience with that unit to notice your thread quicker and drop in to give their experienced advice. (And not just have people like me with limited experience or E.E. chiming in).
If you have the same luck Ive had here, you'll get responses that not only tell you which modern devices are good substitutes, but also _why_ they are good substitutes or _where_ they found their info. Then you better know how to fish the next time.
Good luck!
Doug