Test Procedures - Adcom GFA-555II Service Manual

TEST
PROCEDURES
All tests are
performed
with
a 120V,
low-distortion (less
then 2%),
AC-power
source,
8-ohm
resistive load, (except
slew
rate),
and a
signal source
of not
more
than
600 ohms.
Tests are performed
after
warming up
the
amplifier
at
66
watts
into
an
8-ohm
load
for at least
10
minutes.
All
grounds
during testing are referred
to
the
ground
of the
black output
terminals.
EXCEPT FOR RCA
INPUT-JACK
GROUNDS AND ANY
SIGNAL-GENERATOR GROUND. DO
NOT CONNECT RCA INPUT-JACKGROUNDSTO BLACK
OUTPUT-TERMINAL
BINDING POSTS,
DAMAGETO
THE GROUNDING SYSTEM OF THE
AMPLIFIER
MAY
RESULT.
80kHz
low-pass
filter is
employed
during
THD
distortion
measurements.
Signal-to-noise
measurements
are
"A"
weighted.
Damping
factor
is
measured
by
comparing
the 20-watt-output voltage with
and
without
an 8-ohm
load.
Slew
rate
is
measured
with an inductive
lo
ad,
and
is
derived
with a
dual-time-based
oscilloscope reading the slope
of
a full-power (120V peak-to- peak) 5kHz
square wave.
To
avoid
damaging
output networks R901 /C901
AND
R951
/
C951
DO NOT OPERATE THE
AMPLIFIER
AT FULL-POWER,
SINE-
WAVE ABOVE
22kHz
OR FULL-POWER
(120V
PEAK-TO-PEAK)
SQUARE WAVE ABOVE
5kHz.
IMPORTANT
BEFORE PROCEEDING
WITH ADJUSTMENTS,
MAKE SURE
AMPLIFIER
IS
AT
ROOM
TEMPERATURE.
BIAS
ALIGNMENT
1
.
With set-up as per the
first
paragraph
of
TEST
PROCEDURES
and
with
NO
SIGNAL
IN,
set bias controls (R1 19
and
R169)
to
midpoint.
2.
Connect
a
millivolt
meter across
TP201 and TP301.
3.
Turn
amplifier
on and
allow a 3
to
5 minute
settling
period.
4.
Adjust
BIAS
control
R1
1
9
to
obtain
either a + or
—
1
0mV(±
1
mV)
indication
on
the
millivolt
meter.
5.
Connect
a
millivolt
meter across
TP251 and TP351.
6.
Adjust
BIAS
control
R169
to
obtain either a or
—
lOmV
(±
ImV)
indication
on
the
millivolt
meter.
7.
To check
for
proper
bias
setting,
remove
millivolt
meter and apply
input signal to obtain
66 watts
into
8
ohms
for
10
minutes
with
cover
on.
8.
Remove
input signal
and
connect
the
millivolt
meter as
in
Step 2 and
step
5. Let amplifier
idle until
bias
stabilizes
and
readjust
to 1
0mV
(± i
mV).
2