Upgrading output transistors

[Bigdrums]

I am working on a friends "Setton" Model BS5500 stereo power amp. One channel was not working, had blown fuse and found two of four output transistors were shorted. One 2ST5949 and one 2ST2121. These are now obsolete and so I purchased 4ea MJ21195G and 4ea MJ21196G. This would be to replace all the outputs on both channels. My question is for test purposes, is there any risk in just replacing the two shorted outputs so as not to risk the other new ones?

[Enzo]

You want the transistors on an output to be the same. Well in a push pull, you want all the pushes to be the same and all the pulls to be the same. Clearly the two polarities can be different. Main reason is they need to share current. This is especially hard for them if they are made by different processes. If one transistor were to turn on at a slightly lower voltage, it would hog the current and the second one would not do any work. Or put the other way, if one turns on a little later, then it is the other one doing all the work. In either case it would be like using only half as many transistors. Leads to early failures.

Shorted outputs often stress their drivers, so be sure to check the drivers carefully, or just do as we do here, just replace any transistor driving a shorted output device.