Adcom GFP-750 Service Manual page 12

the RIN-
and
LIN-
lines to signal
ground. For
all
ottier sources,
RIN- and
LIN- are connected
to
signal
ground
via
K161 and K178.
TAPE
input
mode
connects
the tape
inputs to
RIN-t-
and
LIN+ through
K144
to
the
preamp
circuit
AND
disconnects tape out jacks
J132AarKlJ132B.
MONO
relay
K166
AC
couples
the
RIN+
and
LIN-t-
lines
together through
0163
and
0164
for
mono
operation.
The
discrete preamplifier
circuit
is
the
same
for
both
left
and
right
channels.
Unbalanced
operation
of
the
left
channel
is
described
here.
Reference numbers
are
different for the right
channel.
Q1 88 and
Q228
are constant current
sources
regulated by
transistors
Q194
and 0234
respectively.
0209
and
0249
are
constant current sinks regulated by
0206
and
0246
respectively.
01
96 and
0236
operate as
a
differential pair
at
audio
frequencies.
The
LIM+
signal
is
AO
coupled
to
0196
gate through
0191 and
01
92.
The
LIN- signal (which
is
shorted
to signal
ground
for
unbalanced
sources)
is
AC
coupled
to
0236
gate through
0232
and 0231
.
The
signal
voltage
variation at
0196
gate
modulates
the drain current
of
01 96
directly
and
the
drain current of
0236
indirectly
via
R210, 021
1
,
021
2
and
balance
pot
R330B. The
produced
variations
cause
similar
current variations
through
drain
resistors
R202 and R242
producing
the
output voltage signals
at
the drains of
0196
and 0236. The
two signals are equal amplitude
and
opposite
polarity.
These
output signals are
AO
coupled through
capacitors 0199, 0189,
0239
and 0229
to
the
output jacks J108,
J106 and J115A-Dfor
either
balanced
or
unbiUanced
output (regardless
of input
mode). Relays K114 and K109
determine the output
mode
(STEREO, REVERSE).
In
passive
mode
LIN+ and
LIN- are diverted
around
the
preamp
circuit
by K118. K131, K135 and
K168.
Only volume
pot
R331 remains
in
the signal path.
PROCESSOR
relays
K136 and K126
divert
the
LIN+
signal
through
the
PROCESSOR
jacks
J123D
out
and
J123B
in
to the
LOUT+
output.
MUTE
relays
K122
and
K1
1
2 short the
LOUT-^ and
LOUT-
signals
to
ground
at
the
volume
pot
wiper.
Control
Circuitry
Microcontroller
U53 performs most
of
the
state
switching
of the
GFP-750.
Its
operating frequency
is
set
by X54,
055 and 056
to
12MHz.
Port
1
of
U53
(pins
13 -
20) is
used as
both
an
input
and
output.
The
input pins receive the
remote
control signal, monitor the
POWER
switch,
S41
9,
the
PROCESSOR
switch,
S422, and
the
SOURCE
rotary
encoder, S412.
The
output pins
control the
balance
pot
motor and U68
latch enable.
U53
Port
3
(pins 1-5
and
21-23) outputs
a
parallel 8-bil
control word. This
8-bit
word
is
latched through
U66
from
input
pins 2
-
9
to
output pins
12
-
19.
This
latched
word
controls the 8
relay drive transistors
for
the
CD,
TUNER.
AUX
1.
AUX
2.
TAPE.
POWER, MUTE
and
PROCESSOR
sources.
When
the user
changes sources a
new
8-bit
word
is
produced
at Port
3 and
latched
through
U68
to
change
the
state of the
relays.
The
latching of the
word
is
marked by a momentary 5V
pulse on
U68
pin
1 1
U53
Port 0 (pins 6-8) controls
the
volume
motor.
The U53
reset
pin (pin 9)
time constant
is
set
by C50 and R1
.
Pin 9
will
be
pulled high
momentarily
at
power
up
when
back
panel switch
S3
is
switch^
on.
Remote
control signals are received
either
through
J95 and Q62
or IR receiver IR370.
These
signals are
sent
to
U53
via buffer/amplifier
063.
The
REVERSE
and
MONO
relay
circuits,
Q84, K109, K114,
Q82and
K166,
are
driven
directly
by
MODE
switch S401.
PASSIVE mode
relays
K118, K131,
K135 and K168
are driven
directly by
S416.
U359
and
transistors
0350,
0351
,
0354, 0355, 0364, 0365,
0366
and
0369
drive the
balance and
volume
motors.
11