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Operation Theory
VM78F
3-3-6 G1, Blanking and Brightness
1. The vertical blanking signal comes from pin 16 of TDA4841. This pulse through Q405 amplified and
converted into negative pulse and sent to G1 for vertical blanking.
2. In protection mode or an out-of- range situation HUNLOCK will send 5 V pulse to cutoff Q407, then G1
will go down to -150V. During the mode change, Mute acts as same as HUNLOCK’S.
3-3-7 Contrast Section
1. Contrast is controlled by I701 through I
2
C bus to I501 (LM1267) directly.
2. Beam current is detected through T402 (FBT) pin 7, C429, R449 and detected voltage feeding into Q432,
Q433, R436, R497, R459, C482 to control I501 pin 22 voltage. When I501 pin 22 voltage drops
below 4.1 V, the ABL function will happen.
3-3-8 H/V size breathing compensation
1. Beam current is sensed as above section (3-3-7 item 2) and this voltage routes through R451, R468, C470
then through R437 to I401 pin 31 for H. size compensation.
2. HV voltage is detected through T402(FBT) pin 11. C481, R4F4, R4F8, R4F9, Q428, R4G0, Q431, R4G1,
then through R302 to I401 pin 21 for V. size compensation.
3-3-9 Dynamic focus circuitry
The dynamic focus is applied to improve the corner focus performance, it includes horizontal and vertical
dynamic focus.
1. Horizontal dynamic comes from C413 and is amplified through T403, L405, H-DY, L402, C411, R417,
C450, C448, R435 and feed to FBT dynamic focus pin (T401 pin 12).
2. Vertical dynamic comes from I401 pin 32 through C315, R322, R326, R321, R323, Q301, R324, R325 and
feed to FBT dynamic focus pin.
3-4 Power Supply Section
3-4-1
AC Rectifier
The circuit can accept 90 V to 264 V AC input through D801~D804 bridge diodes and C808 filtering to get
DC 126 V~364 V for power conversion in T802.
3-4-2
Line Filter
It consists of C803, T801, C804, C805, C808, C819, C807, C817 and meets EMI regulation.
3-4-3
Power LED Status
1. The LED has 3 leads with common cathode to emit green and amber color light for different power
saving states. It is controlled by CPU.
2. Normal : Green light
Amber LED is off because CPU pin 30 is 5V and pin 31 is 0V, only green LED is turned on.
3. Off Mode : Amber light
CPU pin 30 is 0V and pin 31 is 5V, then green is off and amber is illuminated.
