Smoking Amplifier

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I have a Sony TA F555 ES amp I'm trying to diagnose to either repair myself or determine whether it would be worth bringing to a repair shop.

There are four 2SC2922 NPN power transistors attached to the large heat sink on the back of the unit. All 4 are smoking, some have burned through the center of the chip.

Any ideas of why the chips are failing? I presume there is another problem that needs repair before these power transistors are replaced.
 
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