Hv Hold-Down Circuit; Beam Protector; Scan Failure Protection; G Board (Power Supply) - Sony TRINITRON GDM-1601 Service Manual

3·4·8. HV Hold·Down Circuit
Feedback voltage, normally 9.00 V, from the HV
block is input to the positive input of comparator
lCI (1/2) on the DD board.
It is compared with a
reference voltage (VHV) selected hy R305, R345,
and R314 of 9.60 ±O.IO V. Normally, the output of
lCI (1/2) is low. When the voltage at the positive
input exceeds VHV, the output voltage at pin
CD
goes high and switches on Q309 thru hold·circuit IC I
(1/2) on the DE board. The collector of Q309 is con·
nected to the collector of Q302 to disable the HV
drive pulse when Q309 switches on, holding down the
high voltage. The high voltage value when hold down
initiates can be calculated as follows:
HP (PRT)
=
9.60 (±0.10) x 3000 (+1/-3%)
=
2B.80
~li~ kV
3·4·9. Beam Protector
If
the CRT anode current increases beyond a prede'
termined value, the beam protector circuits inhibits
high voltage operation. This monitor incorpotates
two individual beam protectors to provide double
over current protection.
Anode current is supplied by the secondary of the
FBT and flows thru series resistors R307 - R31 O.
Voltage on pin
®
of connector D·6 decreases as the
high voltage current increases. Since the bleed cur-
rent flowing to high-voltage feedback resistors and
focus resistors is constant, any change in anode cur-
rent is proportional to the voltage drop across sense
resistors R307 - R31 O. When anode current exceeds
a threshold value, the voltage at the positive input of
IC2 (2/2) on the DD board drops below that of the
negative input (VHV), and the output ch8nges from
a "high" to a "low" state. IC2 (2/2) is configured as
a comparator with hysteresis and the output is held
"low". As a result, high-voltage pre driver Q302 is
inhibited and prevents excessive current flow to the
CRT.
ICI (2/2) on the DD board comprises the second
beam protector and functions in a similar manner to
lC2 (2/2). The current threshold of beam protector
#2 is set at a higher value than the threshold of beam
protector #1.
3·4-10. Scan Failure Protection
Scan detection circuits are utilized to inhibit HV
generation and protect the CRT phosphor upon
loss of horizontal or vertical deflection.
The collector voltage for Q302 (HV predriver) is
obtained . by rectifying the retrace pulse from the
HOT. Upon loss of horizontal deflection, no voltage
is supplied to the collector of Q302 and HV is in-
hibited.
3·5
Vertical deflection flyback pulses are rectified by
D 106 and charge capacitor C327 to turn Q30B "on".
However, when vertical defleciton fails, Q30B is
switched "off" and Q307 is switched "on". The
collector of Q302 is held low by Q307, inhibiting HV
operation.
3-5. G BOARD (Power Supply)
3-5-1. AC Line Filter
The AC line filter consists of an input filter, and
transformers Tl and T2, and capacitors CI, C28,
C30, and C31 which are mounted on the G board.
3·5·2. AC Voltage Selector
AC input voltage is selected by switch S902 on the
connector panel. When set for AC 100 to 120 V, the
voltage rectifier circuit is configured as a voltage
doubler. When set for AC220 to 240 V, the voltage
rectifier circuit is configured as a full-wave rectifier.
3·5·3. Degauss
The degauss coil is mounted on the metal CRT shield
to automatically demagnetize the CRT. The me-
chanical relay RY52 and one of the two posistors
THPI and THP2 provide an attenuating AC current
to the degauss coil L90 l.
Transistor Q56 controls
RY52 and a time constant circuit, R65 and CB8,
allows a degaussing duration time of approximately
five seconds at each turn on cycle. Until RY52 is
seiected on, AC current thm the posistor causes its
resistance to increase and the current is gradually
attenuated.
3·5·4. OVP Circuit
The over voltage protection (OVP) circuit is designed
into the AC primary line circuit. The circuit operates,
for example, when applying an AC input voltage of
220 V to the monitor with the AC voltage selector
switch set to 100 - 120 V. When the switch is set to
this position, a series circuit consisting of neon lamp
NL I and resistor R4 is connected across the AC
,
primary lines (from Live to Neutral). The neon lamp
fires at a voltage of 230 ±34 V and results in a blown
fuse F901. The maximum AC voltage which triggers
this protection circuit is set at 164 ±24 V, with the
voltage selection switch set to 100 - 120 V AC.
3·5·5. DC Power Supply (Regulator)
DC supply voltages (+145 V <GDM-1602: 120 V>,
+60 V, +15 and -15 V) are generated by a Y -Z type
switching mode regUlator. Two regulators are uti-
lized, one for the +B voltage and one for +B2, +15,
and -15 voltages. This description is for the +B
regulator.
lC51 compares a feedback voltage from the +B line to