Star Tech Journal

D
STAR*TECH Journal
September 7992
REPAIRING AUDIO AMPLIFIERS
(PART 2)
Audio amplifiers are an area of
repair where a good procedure
can give one skill and confidence.
There are only a few basic de-
signs, so there are not many sur-
prises involved, and they do need
repair from time to time.
Last issue ·we started by looking
at the function of each stage and
v;r_e will now apply that knowledge
to practical troubleshootlll:g in
this issue.
AMP BLOWS FUSES
Suppose we look at a typical ser-
vice problem: The amp blows
fuses.
After the power supply is cleared,
this points to the output stage.
Very few parts of the amp can
draw enough current to blow a
fuse, even in failure mode.
WHICH CHANNEL
Our first question, in the case of a
stereo amp, is determining which
channel has the short. The first
level of troubleshooting is to sit
down under a good light and give
the unit a real good visual inspec-
tion. Look for burned, cracked
parts, especially transistors and
resistors. Remove any you find
and start making a list of parts
you need.
If you are still not sure, a good
way to gently find trouble is to
put the unit on a variac and
slowly bring the voltage up.
When the current reads about an
amp, just feel around the output
transistors and resistors. Some-
thing will be getting warm pretty
soon.
When you know which channel is
bad, the first step is to remove the
outputs from the circuit. Because
of the low impedances, these
must be removed before they or
anything connected to them can
be tested.
TRANSISTOR TEST
ings from a known good part you
should be able to make a judge-
ment call.)
Find some bad transistors? Good,
we're on the trail. Next check
those big, low ohm emitter resis-
tors. From there go back to the
drivers. If they are OK, then
merely replace the outputs.
IN-CIRCUIT TEST
Once the outputs are out of the
circuit, everything else can usu-
ally be checked in-circuit. You
will not get the perfect diode/open
readings you get out of circuit,
but you will usually be able to tell
good from bad. When in doubt,
pull one out and re-check it.
Work from the outputs on back,
and as soon as you get to good
parts, stop. Failures tend to eat
their way from the outputs back,
so as soon as you hit good parts,
you are ready to re-assemble.
A simple test of the junctions is
sufficient in most cases. By this I
mean diode readings for EB and
BC, open in both directions for
CE.
VERIFY
Darlingtons are harder to test, as
they often have some extra stuff
inside, but by comparing read-
At about this point on a stereo
amp I will want to stop, power up
the unit, and verify that the other
channel is good. Thus, when I get