Yamaha M70 amp

[sam13]

lost audio for left ch speakers, i pulled the drive transistors and the power trans for testing ( their are 4 drive and 4 power transistors ) i use a transistor testor that i picked up at ratshack and the only problem i found was with one of the drive transistors, my question is would that one drive transistor be enough to knock out the whole left chanel or should i be checking other parts.