Adcom GFA-585 Service Manual page 3

The
output stage consists
of
two sets
of 5 parallel transistors
operated as
emitter followers, driven by another
pair
of
emitter followers. This configuration minimizes
distortion
caused by
varying load
impedances. The
output transistors
have 0.33-ohm
ballast resistors to
ensure
current sharing
and
bias
stability.
The
AC-input
power
supply board includes
a power-on delay
relay to
reduce
the turn-on current surge
in
the
AC
power
line,
jumpers
to
set the supply voltage range,
and an
amplifier
bias
delay.
CAUTION
DO NOT
use any
type of variable
AC
supply,
such
asavariac,
to slowly turn
on
the
GFA-585
without
first
shorting
R506
(4.7
ohm/20
watt).
R506
is
an
integral part of the turn-on, inrush-suppression
circuit
and
failure to
comply
will
burn
out
R506.
TEST
PROCEDURES
AM
tests
are performed
with a
1
20V,
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
83
watts
into
an
6-ohm
load
for at least
1
0 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. OO NOT CONNECT RCA
INPUT-JACK
GROUNDS
TO BLACK
OUTPUT-TERMINAL
BINDING POSTS,
DAMAGE TO
THE GROUNDING SYSTEM OFTHEAMPLIFIERMAYRESULT.
SOkHz low-pass
filter is
employed
during
THO
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 load,
and
is
derived
with a dual-time-based oscilloscope reading the slope
of a
full-power
(1
20V
peak-to-peak)
5kHz square wave.
To
avoid
damaging
output
networks R1 34/C1
1
3
and
Ri 84/C1 63
DO
NOT OPERATE THE
AMPLIFIER
ATPULL-POWER. SINE-WAVE ABOVE
22i(Hz
OR FULL-POWER
(1
20V
PEAK-TO
PEAK)
SQUARE WAVE ABOVE
SkHz.
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 PROCEDURESand
with
NO
SIGNAL
IN,
set bias controls (R1 25,
R175)
to
midpoint.
2.
Connect a
millivolt
meter across
TP301 andTP40l.
3.
Turn
amplifier
on and
allow a 3
to 5
minute
settling
period.
4.
Adjust
BIAS
control
R125
to
obtain
either
a + or
—
24mV
(±
ImV)
indication on
the
millivolt
meter.
5.
Connect
a
millivolt
meter across
TP351 and TP451.
6.
Adjust
BIAS
control
R175
to
obtain either
a +
or
—
24mV
(± ImV)
indication
on
the
millivolt meter.
7.
To check
for
proper
bias setting,
remove
millivolt
meter
ar>d
apply
input signal to
obtain
83
watts
into
8
ohms
for
1
minutes
with the
amplifier's
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
24mV
(±
1
mV).
2