Service manual

Page 13

MD 1772LB (B1770NSL/T)

MEDION

4-3-7 Contrast Section

1. Contrast is controlled by I701 through I

C bus to I501 (TDA4886) directly.

2. Beam current is detected through T402 (FBT) pin 7, C432, Q418, VR402, R453, Q419 and detected

voltage feeding into R510, R509, R507,C523 to control I501 pin 24 voltage. When I501 pin 24 voltage

drops below 5V, the ABL function will happen.

4-3-8 Dynamic focus circuitry

The dynamic focus is applied to improve the corner focus performance, it includes horizontal and vertical

dynamic focus.

1. Horizontal and vertical dynamic comes from I401 pin 32 and is amplified through C427, R450, Q416,

Q415, Q414, R498 and feed to FBT dynamic focus pins.

2. This amplifier need 700V voltage supply, it comes from FBT pin 2 and rectified through L406, D414 and

C426.

4-4 Power Supply Section

4-4-1 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.

4-4-2 Line Filter

It consists of C801, C802, C803, C816, C820, C823, C807 and T801 and meets EMI regulation.

4-4-3 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 18 is high and pin 17 is low, only green LED is turned on.

3. Standby / Suspend : Yellow light

CPU pin 17 and pin 18 are low, then green and amber LED are turned on. That is yellow.

4. Off Mode : Amber light

CPU pin 17 is high and pin 18 is low, then green is off and amber is illuminated.

4-4-4 Auto Degaussing

When S801 turns on, pin 10 of I701 will send a signal to Q802 and turns on RL801 for degaussing. After 4

seconds, it will turn off RL801 automatically.

4-4-5 PWM Control

1. Start Up

The I801 (MC3842) gets power from R831, R830, C812 and pin 7 voltage reaches 16 V for starting up.

The I801 starts oscillation at 22 kHz, sawtooth on pin 4 and pin 6 output to drive Q803/T802. Once Q803

switching on, D806, C804 set up an 15 V to keep I801 working through D821 auxiliary voltage.