I was messing around on my 4230 and accidentally blew an output transistor so did some research on the replacements. Here is the information I gleaned. I forget what I ended up with. I'll pull my board out tomorrow and let you know.
Output Transistors
The correct replacement is a TIP31B TO-220
Elsewhere others have suggested 2N6292 as a replacement.
Hyperion
2SC789 TIP31B 512-tip31b $.43ea. (end of life comments)
Larry
to-220
q11,12 2sc789
512-KSD526Y NPN to-220 bce 80v 4a 30w 3mhz 15-240hfe $0.62
markthefixer
I have used KSD526Y to replace 2SC789's in a 4230. In the supplements to the manual, Marantz recommended using 2SD315D's, but those are NLA. The specs are pretty close.
Strange. I see in the schematic that the output transistors are supposed to be 2sc789 but the actual transistors in the amp are 2sd315 (as the one in the PS). Same TO-66 package.
2sd315 E has a gain of 100-200 and the schematic calls for O-Y on 2sc789. O is a gain of 40-80 and Y is 120-240. And all 2sc789 I find on google are TO220 package. Strange indeed...
The transistor is a Toshiba, the equivalent Sanyo is a 2SD313, Toshiba also had a 2SD526, NEC had the 2SD288 and Rohm had the 2SD1762.
the 2SD526 is an 80v, 4A, 30watt, 3Mhz TO220.
The original is a 70v,4A,30w, with BD241B,243B,579 and 589 all listed as equivalent.