Protection Circuit - Oscillator Stop 2 (Offset) - Sony XM-1502SX Training Manual

When the amplifier gets hot, the TH thermistors decrease resistance, increasing the voltage at IC901/pin 16. If
the voltage reaches the trip point of 1.2Vdc, IC901's internal power supply oscillator stops, removing supply
voltage to the amplifier. IC901 additionally outputs 5Vdc from pin 3 to keep the amplifier latched OFF and the
Thermal LED ON via R1.
THP thermistors - If the THP thermistors are used, the thermistor circuit is changed. THP thermistors are
normally less than 5k ohms cold and increase with temperature. In the similar circuit of Figure 1-10b, an increasing
thermistor resistance also brings the voltage at IC901/pin 16 up.
Batt
+12V
Figure 1-10b - Thermal Stop Circuit - THP Thermistors used (simplified)
Testing
To find out which thermsitor keeps triggering the thermal protection LED / circuit, run the amplifier and measure
the voltage at each diode / anode (D1 in Figure 1-10 or TH801-813 in Figure 1-9) that is connected to the
thermistor. Normally each point is 0.1Vdc (cool room temperature). The trip voltage is 1.9Vdc before the diode.
A voltage near the trip point means the related thermistor is bad or the transistors near that thermistor are
overheating. Adjust the bias in that channel or switch the suspected thermistor with another on the board. Idle
current is typically 1 amp at 13Vdc (no input or load).
Protection Circuit – Oscillator Stop 2 (Offset)
Concept
The offset protection circuit monitors both amplifiers for an average voltage of 0Vdc and shuts off IC901's
oscillator if this voltage averages above 0.6Vdc. The latch circuit external to IC901 keeps the oscillator OFF as
long as REM input voltage (power ON) is present.
MAIN BD.
LEFT
CN804 + -
R153
470k
Q118-Q113
L
OUTPUT
R253 470k
R
STAGES
+ -
CN805
RIGHT
5V Reg
14
in IC901
12
OFFSET ERROR
Q803
R806
OFFSET +
N
Q804
OFFSET -
P
C803 +
R807
47
R808
R810 N
1M
R809
Q805
INV
-14.2V
FIGURE 1-11 - PROTECTION CIRCUIT 2 - OFFSET
THP Thermistor Circuit
D1
.1V
R2
THP-xxx
R1
14.2V
R822 R823
C808 +
22
P
Q811
LATCH-1
14.2V
R825
Q810
N
R826
14
1. Car Power Amplifier
Protection
0.17V
Circuit in
16
IC901
3
0V
Thermal LED
CN806/
CN807
0VDC
R814
1
D803
OSC. TO DC-DC
CONV. IC901/PIN 1
D807
Q114-6, Q214-6, Q812-3
OUTPUT CURRENT
DETECTION
9A125 1484
LED BD.
D810
OFFSET
11/12/02