Sony D98 chassis Operation Manual page 6

IC604
PQ6RD83B
Heater
Regulator
1
+8V O
I
VC
G
3 4
Heater
ON/OFF
When remote on/off is set high, Q656 is turned on and Q658 is turned off; this disables active off and suspend modes
regulation loop.
Active On Mode Feedback
As stated earlier, when Q656 is on, the active off and suspend mode regulation is disabled. At this time, IC652 takes over and
the B+ line is regulated. IC652 is a shunt regulator and regulates the B+ voltage by sinking current from IC603 to ground.
The reference voltage at pin 2 determines the sink current into pin 3. The reference voltage is set to 2.495 volts by resistor
divider R659 and R660. Since the B+ is connected to the resistor divider, any changes in voltage is detected at the reference
terminal; the shunt regulator then tries to force the reference voltage to 2.495 volts by sinking more or less current into pin 3.
As mentioned before, IC603 biases its transistor accordingly and IC601 responds by changing the switching duty cycle in
order to keep the output voltage at 183 volts. The other components connected to pins 2 and 3 are for compensating frequency
response. This completes the feed back loop for active on mode.
4. Protection Circuits
There three protection circuits: OCP, OVP and secondary short circuits. OCP is pulse by pulse and is performed on the primary
side. OVP detects excessive output voltages. To detect secondary side short circuits, there is a circuit called B+ detect and it is
monitored by the microprocessor. The following will explain the operation of these functions.
OCP
OCP is activated if there is too much current passing through the power MOSFET, Q602. This condition will occur if either
the B+ line or 80 volt line is shorted.
R609 is the current sense resistor. Current through this resistor will cause a negative voltage drop in reference to ground.
R608, R606 and C615 couples this voltage drop to IC601 pin 4. If the voltage level exceeds –0.18 volts, the IC sets pulse by
pulse current limit and the output voltages are in affect very low. This condition is often audible and characterized by a
'chirping' sound.
OVP
The function of OVP is related to IC601 and the threshold is approximately 19.4 volts. This is sensed at pin 7 of IC601. Under
Feedback Circuits
Heater
2
out
Q601
D610
+5V BACK UP
+8V
R636
Q606
IC605
TDA8138A
3
1 2
C662
C623
R617
D618
R666
R635 R637
D619
Q607
R638
C636
- -
6
4
7
6
+5V
D607
C634 C635
REMOTE ON/OFF
+16V