Normally the outputs are changed in pairs because the failure of one tends to stress the other enough to make it the next failure. I don't have the service manual so I can't tell what Q51 or Q52 are, but when the outputs go bad (shorted usually), you must also check the driver circuit and the bias transistors. It is very unusual for only the outputs to be bad. Check all of the resistors in the area as well. Low valued ones may be open and since the are often flameproof resistors, there will be no physical indication that they are bad.

Keep us posted.
Dan