I was junking a moderately priced combo CD, AM/FM/Casette, from 1998,
that I got out of the trash and which wouldn't play CDs anymore, and
it had an array of five output trnasisttors, instead of the usual
four. All are the same size, on one big heat sink. No schematic
of course.

The part numbers are obscured but I can see the ends of several.
1 B1370
2 B1020
3 B1415 (or 01415, or D1415, or ?1415
4 B1020 same as 2
5 same as 3

What is the fifth transistor likely for?

The 2SB1020 (PNP) and 2SD1415 (NPN) are complementary pairs of power
transistors... absmax around 100 volts 7 amps. Presumably the "usual
four" that do the current amplification.

The 2SB1370 is a PNP in the same TO-220FP package. It seems to be
spec'ed out as a driver.

I'd lean towards agreeing with the speculation that this is being used
to regulate the output-stage bias voltages or currents somehow - to
provide thermal tracking. This is more commonly done with one or more
diodes in the bias string (one set per channel) but there may have
been some tricky way of doing it with a single thermally-coupled
transistor on the heatsink which was simpler and/or cheaper to
implement.

Maybe (and this is sheer speculation in the fact of an acute lack of
schematic) this one transistor controls two separate idle/bias strings
via a current-mirror arrangement of some sort?

Or, maybe it's just a "Whoops, too hot!" emergency shutoff switch
circuit, to keep the player from going into thermal runaway if the
owner tries to bend metal and break walls by turning the poor beast up
to 11?

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!