Power Supply Protection; Latch; +135 Volt Over Voltage; +135 Volt Over Current Protection - Sony KP-43T70 Training Manual

Power Supply Protection

Overview
The RA-3 chassis employs over voltage and over current protection for
the +135 volt line. The standby +5 volt line is protected against over
voltage and over current. There is also an 11V LVP/18V OVP protection
circuit

Latch

Shut down occurs whenever a condition in one of the protect circuits causes
the Q655 to turn ON. When Q655 turns ON, Q654 also turns ON. This
drops the drive voltage to Relay Drive Q652/B, turning it OFF. When
Q652 turns OFF, the ground return path for the power relay opens and
the unit shuts OFF.

+135 Volt Over Voltage

The +135 volt line is input to the protection circuit through D672 and then
to a voltage divider consisting of R661 and R660. The voltage developed
across R660 is applied to IC651/5 Non-inverting input. IC651/6 Inverting
input has 2.5 volts applied to it from the voltage divider consisting of R663
and R662. This voltage is divided down by the Standby +5 volt line. If the
voltage at IC651/5 rises above the 2.5-volt reference, then the output at
IC651/7 will go HIGH. This High output is then applied to the latch circuit
and to the Self-Diagnostic section on the A board. When this occurs, the
Timer LED will flash three times.

+135 Volt Over Current Protection

The over current protection circuit works by monitoring the voltage divider
network that consists of R659, R657 and R654. R654 is connected be-
tween the negative side of the +135 volt bridge rectifier and ground. Any
rise in current in the +135 volt supply will cause the voltage across R654
to become more negative. This changes the voltage that is input to IC651/
2 Inverting input. IC651/3 Non-inverting input is connected to ground. If
the voltage at IC651/2 becomes negative, a HIGH will be output at IC651/
1. This HIGH output is applied to the latch circuit and also to IC002 Sys-
tem Control on the A board for the Self Diagnostic feature. If this occurs,
the Timer LED will flash twice.
3

Standby Unregulated OVP

Current from T602/9 is rectified by D667 to produce a positive supply at
D667/K. This supply voltage is applied to IC655/I and D675/K. D675 is a
10-volt zener diode. When the voltage present at the input of IC655
reaches around 11.2 volts, D675 will begin to conduct. When this occurs,
Q655 will turn ON which activates the latch.

Standby +5 Volt OCP

Current from T602/9 is rectified by D651 to produce a negative voltage at
D651/A. This voltage is connected to the Standby +5 volt line through a
voltage divider that includes R686, D664, R688, Q658/B-E and R689. As
more current is drawn by the circuits on the Standby +5 volt line, the
voltage at D651 becomes more negative. When this voltage is low enough,
Q658 will turn ON and activate the latch. D676 is used to enable the soft
start circuit during power up or when there is a sudden rise in Standby 5
volts.
11V Low Voltage Protection and 18V Over Voltage Protection
Q657, D669 and associated components make up the 11V LVP and 18V
OVP. Normally, D669, a 13V zener, is biased below its zener point and is
OFF. A severe drop or loss of the 11V line will cause the zener to break
over and conduct, causing Q657 to turn ON and activating the protection
latch circuit. In addition, any rise in the 18V line that causes D669 to
conduct will also turn ON Q657 and, in turn, the latch circuit.
D669
C678
MTZJ13
+11V
R685
RELAY
DRIVE
FROM
Q652/C
11V LVP/18V OVP
R684
D688
Q657 MTZJ-24A
R682
R683
G BOARD
+18V
TO
Q655/B
LATCH