Top: output transistors and heatsink removed prior to
removing the board.
The left channel of this unit was smoked!!!
A catastrophic failure took out much of the components and
burned the board.
* both output transistors
* one emitter resistor
* both driver transistors
* R768 and R771
I contemplated rebuilding this board. The trace side was still in
good shape. I ordered all the parts that needed replaced. But
when it came down to it... I could not do it. I could not put this
board back into the unit. So I relegated it to the parts bin.
I used another board that I pulled from another unit (below)
It is common for lower wattage units to be driven to
their limits. Lets face it, 30 watts does not allow for a
lot of head room. At moderate listening levels 30
watts is sufficient. Push them just a little harder and
the unit is giving all it has. Components start to heat
up. When I get a board that has failed I have to
question the condition of any componet that will run
hot when driven hard.
I used my last set of new NEC 2SC959/2SA606 on the
left channel (long out of production). The right one
kept its originals. All other components replaced on
the left, were replaced on the right as well.
The emitters can take a lot of abuse under the
condition described above, and since there was one
known bad one I decided to replace all the emitters.
The replacements are a set of 5W wire-wound
resistors (3W originals). These are a close match for
the original emitters (which are a type of wire wound
resistor as well... the core is different). It would be
nice if there was a source for the old style emitters.
But, I am afraid there is not. However, inductance is
not an issue for small values such as these (.24
ohms), so... no problem.
Any resistor that is directly in the current supply path
of the output transistors were replaced.