Originally Posted by Dave Platt

I've just been done on ebay. Both output transistors on a Kenwood A-97
amplifier had blown. Ordered the pair from ebay, installed them,
working, great. Turned it off to put it back together and noticed the
heatsink was really hot on the repaired side. I found that the base
voltage on both transistors was low. Swapped both transistors from the
good side to the repaired side and its now fine. Put the new ones in the
good side the and now the good side has got hot and all the voltages
have dropped the same as they did when they were on the repaired side.

Does anyone know where i can get a genuine pair of these transistors at
a reasonable price?

What was the base-to-emitter voltage on the questionable transistors,
when the amp is idling? How does it compare to the working parts?

These parts (when genuine) seem to be Darlingstons, not just single
transistors. You may have been sold a pair of mislabeled standard
transistors instead - this would probably result in over-biasing and a
high idle current.

Or, possibly they're genuine parts, but have a higher beta than the
ones for which the amp was spec'ed? The beta bin would be a
one-character A, B, or C, possibly stamped on the lower left corner of
the plastic. If that's the case, they might work OK with some
adjustment to the idle/bias settings.

TIP142/TIP147 *might* be acceptable substitutes although their gain
seems lower.