Sony XM-1502SX Training Manual page 18

I DET-L
Q115
R166 R285
R158
I DET-L
Q116
SAMPLE OUTPUT DRIVER PAIR
+50V
Q108
+AUDIO
N CH.
Q105/C
R145
R148
-AUDIO
Q109
Q105/C
P CH.
-50V
Operation
Refer to Figure 1-12. The same current that flows in the speaker also flows through the output transistors and
their emitter resistors (R145 and R148). By sampling the voltage across the two emitter resistors using Q114,
the maximum speaker current can be detected. The values of R158, R159 and the gain of the Q114 transistor
set the maximum current.
When the current threshold is reached, 0.6Vdc momentarily appears across Q114's b-e junction. Q114 conducts
and lowers the 14.2Vdc at the input of the latch at Q812/collector. The Q812 and Q813 latch change the input
LOW to a constant (latched) output HIGH. This HIGH from Q813/collector is applied to light the Over Current
LED and stop the power supply oscillator (IC901/pin 1) via D807.
Testing
An Over current LED indication is often caused by a short circuit in the speaker wiring or too many speakers are
used. Within the amplifier, an output transistor that is lower in gain than the others could cause the Overcurrent
LED to latch ON. If you can run the amplifier for at least 5 minutes at the rated output without an Overcurrent trip,
the problem is external to the amplifier in the car wiring.
14.2V
I DET-R I DET-R
Q214 Q215
R266
R258
14.2V
I DET-R
I ERROR
Q216
14.2V
R158
2.2k
AUDIO
OUTPUT
(SPK)
R159
1k
FIGURE 1-12 - PROTECTION CIRCUIT 3 - OVER CURRENT
Q803-5, Q810-1
OFFSET DETECTION
CIRCUIT
R827
Q812
10k
P
Q813
R829
0V
LATCH
-2
R828
N
C809
+
4.7
R830
R185
10k
N
Q114
I DET-L
16
1. Car Power Amplifier
D803
OSC. STOP
TO DC-DC
CONV. IC901/PIN 2
D807
R813
2
LED BD.
10A125 1485
OVER
CURRENT
11/12/02