High Voltage Hold-Down Circuit; Beam Protector Circuit; Crt Protector; H, V Static Convergence - Sony Multiscan GDM-2036S Service Manual

3-4-6. High Voltage Hold-Down Circuit
A feedback voltage which is nonnally 9V is initially sent from
the focus pack and it is supplied to the (+) INPUT of IC203
(2/2) where it is compared with (- ) input voltage. This
voltage is selected so that 9.9
±
0.2V is obtained at R508,
RS09 and RSlO. Thus, pin (j) for IC203 (2/2) on the D3 board
is set at "Low".
If high voltage increases unusually and the (+) input voltage
for IC203 (2/2) exceeds the ( - ) input voltage, pin (j) changes
to "HIGH" and "HIGH" level voltage is output from pin @ of
the connector D-l causing Zener diode D207 on the N board
to continue to operate and input the voltage to pin
@
of
IClOl. At this point, the X-ray protector circuit in IClOl on
the N board operates and stops the drive pulse for the output
at pin @ of the IC so that the high voltage circuit is
suspended.
When the X-ray protector circuit is activated the IC302 (2/2)
latches "HIGH" level and causes 0501 to tum on and hold
down the high voltage.
The high voltage point at which voltage hold down starts is
calculated by the following fonnula.
HV(PRT)=9.9(
±
0.2) x
3000~ (%)=29.7:~:kV.
3-4-7. Beam protector circuit
The beam protector circuit is a kind of ABL circuit which
stops high voltage supply in instances when anode current for
the CRT increases beyond the necessary level. This circuit
features two beam protectors.
Anode current is detected by removing the secondary current
(CRT current
+
high voltage resistor current
+
focus current
+
G2 resistor current) as voltage by detection resistors RSll to
RS14.
ABL current flows through these resistors, the voltage at
TP403 drops. At this time, bleeder current flowing to the high
voltage resistors is constant and if CRT current increases, the
(- ) input voltage at IC203 (1/2) drops below the (+) input
voltage and changes output pin
<D
of IC203 to "HIGH" (beam
protector (1).
If the beam protector circuit (1) fails to operate for some
reason, IC202 (beam protector (2» operates in the same
manner as beam protector (1). These outputs are connected to
pin @ (HV STOP) of the connector D-l and hold down the
high voltage. Beam protector (2) is designed to operate at a
higher current than beam protector (1).
3-4-8. CRT protector
The CRT protector detects the H. ACT signal by rectifying the
secondary output at T202 in order to protect the fluorescent
surface of the CRT from burning when the horizontal
deflection circuit malfunction and stops operating. Also, for
the vertical deflection circuit, the CRT protector rectifies the
V middle point voltage of pin
®
of D-13 connector and
detects the V. ACT signal.
GDM-2036S
If H. or V. deflection circuit stops, 0502 or 0503 tum off to
stop the high voltage circuit in the same manner as the hold
down circuit.
3-4-9. H, V-Static Convergence
H. STAT and V. STAT signal (DC) is controlled and output
from the N board.
H. STAT is amplified approx. 300 times by IC303 (1/2) and
0703.
The collector of 0703 is connected to the CRT and controls
the H. STAT convergence plate voltage.
V. STAT is amplified by IC303 (2/2), 0701 and 0702, and
drives NTC.
3-5. H4 BOARD
From Dl to D7 are the select switches. They are LEDs to
indicate the selected items. From D8 to D12 are the matrix
circuit to read the states of push switches SI to S5 on the front
panel.
3-6. M1 BOARD
In this section, RGB signals input to J611 to J613 and HD and
VD signals input to J604 to J605 are output to J606 to 610; at
the same time, HV and VD are sent to the Bl board.
At J606 to J61O, the tenninal resistor becomes open when
BNC connector is inserted. As for HD and VD, tenninal
resistor values can be selected at S-I.
3-7. GBOARD
3-7-1. AC Input-Degauss clrcult-Rect & smoothing
section
<D
After passing through the AC inlet and the filter section,
AC is supplied to the rectifier diode. For 5 to 6 seconds
after power on, the rush current is restrained by
thennistor THl. (Once RYI turns on, THI is short-
circuited.)
VDRI (varistor) is equipped to protect against AC noise.
®
Degauss circuit
-29-
As RYI is open when the power on, the degauss current
flows through posistor THPI. The varistor VDR2 clamps
in the AC 200V system of excessive voltage. After 5 to 6
seconds from power on, RYI is short-circuited and the
degauss loop is cut off.