I think you're right, the SJ5480 is an OEM code because I can't find any specs. I've heard the the 2N3773/2N6609 is a comparable substitute but I'd have to replace the whole set but I'm not sure if it will work with the rail voltage and the rest of the circuit.
Have a look at the attachment to the very first post in this thread: it has two factory versions of the circuit, one which uses 2N3773s as output and drivers.
I've not done an analysis of the circuit but an implication is that the circuit is not so very sensitive to device selection.
There are many suggestions given by experts for the cure. Output transistors fail due to over voltage, excess current and over temperature (//spot temperature on the transistor) or a fake part. The OP need to post the picture of the heat sink assembly, how the transistors are mounted on the heat sink, to help further and rule out specific issues. Regards.
The amplifier I'm repairing is version 1 with the 2N5630/SJ5480 transistor set. I'd have to modify the rest of the circuit if I choose to use the 2N3773's but I'd rather not. I've determined that one of each 2N5630 and SJ5480 transistor is blown.
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