[julie_m]

One output transistor failing should ring an alarm bell. It did not die by itself -- something killed it, and that something will still be there to kill the replacement transistor if you don't fix it.

You'll have to unsolder and test every transistor in the faulty channel. Start with the other output transistor. If it measures short C-E, you have a fighting chance. Both output transistors short C-E (or blown to bits; that can follow a C-E short fault) is often caused by a short circuit on the speaker wiring and if you are lucky, they will be the only components damaged. If and only if the other output transistor is short C-E, fit any matched pair of reasonably-powerful transistors (BD138/139, TIP41/42 or similar) and power the unit up (via a lamp limiter at first won't hurt, if you have one). If the speaker output is at 0V DC, try connecting a test speaker (not one of your best ones) and a CD player or other audio source. You should get some sort of sound through it, but do not turn it up too loud as the temporary output transistors probably will not be good for as much power as the originals. If the original transistors were especially high gain types, you may get less output and distortion set in sooner. If the smoke stays in for an hour or so, you can breathe a sigh of relief and order yourself the exact replacement parts.

If the other output transistor tests OK, or any other fault than short-circuit collector to emitter, then it will be pointless trying temporary transistors yet. You will just have to unsolder and test every transistor to find which one(s) are faulty. While you are at it, take the opportunity to check for any overheated resistors, bulging capacitors, dry solder joints and so forth.