Sony XM-1502SX Training Manual page 12

C
Q104
B
1
C123
AUDIO VIA
LINE SW.
IC102/E
+50V
INSET
47k
R144
R143
DC
47k
FD BK
-50V
FIGURE 1-8 - Bi-PHASE OUTOUT STAGE TROUBLESHOOTING
Referring to the main schematic of Figure 1-8, we can see that if the voltages at the emitters of Q103 and Q104
are both 0.6V but of opposite polarity, they will conduct equally (balanced), producing the same voltages at their
C2 collectors (also of opposite polarity). The driver stage (Q105, 6) that follows is symmetrical in the same
manner. Their collector load resistors are 100k (R139 and R140). The junction of these resistors connects to the
speaker and to feedback resistor R143. The speaker completes the current path to an audio ground. (Ground
side of the speaker not shown.) Feedback resistor R143 provides an offset correction voltage to the center of the
voltage divider string (Q103 & Q104 / B1) if the top and bottom amplifiers components were not perfectly
symmetrical.
Troubleshooting
Troubleshooting to a stage separated by a coupling capacitor is the job of a scope. Once the defective stage has
been found, a DC voltmeter and mental logic are the most valuable tools to detect damaged parts within that
stage. The output stage consists of parts that can be grouped: differential amplifier(s), drivers and output transistors.
The differential amplifiers and drivers are balanced and are all set at the verge of conduction (class B). Even
without the output transistors present, the differential amplifiers and drivers can produce a perfect signal on a
scope. Of course since the output transistors are not connected, a (speaker) load must not be connected. This
is the basis for the troubleshooting method and testing.
Note: Repair procedures you and I have used for more common single differential amplifier do not work in this
Bi-Phase dual differential output stage system. The Bi-Phase system tends to balance well and mask the
problem, so separating the parts of the output stage to isolate the problem further requires a new method /
procedure.
+50V
R133
C
2
1
47k
B
1
P
B
Q104
2
E
C
-REF
R144
(+39V)
2.2k
+
+REF
(+39V)
C
1
N
B
1
REF.
REF.
R134
47k
-50V
D103
Q106
15V
E
P
C
2
R132
R128
4.7k
B
220
1 2
-50V
Q107
BIAS
CONTROL
+50V
R131
4.7k
Q103
C
2
N
D101
Q105
15V
E
B
2
DC FD BK
10
1. Car Power Amplifier
-50V
47
OHM
-2.5V
N
R140
100k
R139
100k
0.22
OHM
Q108
R135
Q110,Q112
220
N CH.
P
+2.5V
47
OHM
R143 +50V
8.8k
D
Q109,
Q111,Q113
P CH.
S
0.22
OHM
LEFT
SPEAKER
OUTPUT
S
D
13.A125 1489 11/12/02