Operating Description - Pioneer QX-949A Series Service Manual

Operating Description
The input signal (LT &
RT)
enters
the Se
basic
decoder (M51651P), where 4-channel
signals
LF,
RF, LB
and RB are obtained by the Se
decode
matrix, then
these signals enter
the gain
control,
back blend and logic cirbuits. The front-back logic
produces
a
positive voltage
with
a
CF
sound
source, and a negative one with
a
CB source. This
voltage
passes
through the time constant circuit
and
is
applied
to
the
gates
of
the MOS FETs for
LF
and
RF gain control.
As these FETs are P channel
enhancement types,
their
equivalent internal resistance
decreases
only
when a
negative
voltage
is
applied.
Front (LF
&
RF) output
signal levels are attenuated
with
a
CB
sound source.
For rear control,
wave matching logic produces
a
negative
voltage with
respect
to
a
front
single signal
(LF or
RF)
and a positive voltage with
respect
to
a
rear
single signal
(LB or RB). Front control
is
also performed by
producing the
reverse
polarity
of
these voltages.
The rear control
voltage
passes through the time
constant
circuit and
is
applied
to
the
gates of
MOS FETs
for
LB
and RB gain control. The
front
control
voltage
passes through the time constant
circuit
and is applied to
the
gates
of the junction
FET
for
back blend and the MOS FETs
for
LF
and
LB
gain
control.
As the
junction FET
is an N
channel depletion
type, LB and RB
are
normally
blended,
but the
device becomes open when
a
negative
voltage is applied.
The detector outputs of
the full logic IC (CX-049)
with
respect
to
sound source are
as
shown
in
the
following
table.
*Gain
control
operates (attenuates)
with (-)
detecting
mode.
*xFront
bach logic
output
B
is
not
employed.
x**Back
blend is
not p?rformed
only
when
waue
matching
logic output F
mode is
(-).
LF
RF LB
RB CF
CB Gain
control*
FIB
logic
W/M
logic
F 0 0 0
0
+ LF,
RF
B 0 0
0 0
+ IF iF
F
+
+
0
0
LF,
RF***
B + +
0 0
LB,
RB
CAUTI ON
The
gain
control
lC
(CX-718D)
is an MOS (metal oxide semiconductor) type and subject to
dielectric breakdown from
static electricity. Note
the
following precaution
when handling.
*Do
not
remove the aluminum cap
from
the
tC until
it
has
been instalted
in
teh
circuit.
First
solder
the
lc
to
the
circuit
board,
then remove the aluminum
cap.
?2