Medion B1997PNST Service Manual page 13

2. In protection mode or out-of-range situation, HUNLock will send 5 V pulse to saturate Q706, then pin 8
of I902 will be low state and open Q509, so G1 will go down to -140V. During the mode change, Mute
acts as same as HUNLock's.
3. The brightness is controlled by CPU, pin 27 and pin 28 comes into pin 20 and pin 21 of I501, DC level can
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be changed by I C bus from 0~5V of D/A output at pin 23 of I501, high voltage causes high current to get
brighter raster, low voltage gets lower brightness.
3-4 Power Supply Section
3-4-1 AC Rectifier
The circuit can accept 90 V to 264 V AC input through D801 bridge diode and C810 filtering to get DC 126
V~364 V for power conversion in T805.
3-4-2 Line Filter
It consists of L801, L802, P801A, C803, C804, C805, C806, C807, C871, C80A, C80C, C80E, T801 and
T802 and meets EMI regulation.
3-4-3 PFC
It consists of T804, Q814, D807, I804 and other auxiliary components. It is similiar to Booster circuit. The
working voltage is provided by D815, C821, R825, C819 and ZD810, but it is not working in Stand-by,
Suspend and Off modes. The output voltage C810 is 180 V~380V DC and generated through input
voltage C816 step up. The maximum voltage is decided by R826 and R827, while the minimum voltage is
decided by C817. In the meanwhile, it will get a similiar to input voltage waveform of input current to
acquire a good PF value.
3-4-4 Power LED Status
1. The LED has 3 leads common cathode with green and amber color for different power saving states. It
is controlled by CPU.
2. Normal : Green light
Amber LED is off because CPU pin 17 is high and pin 18 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 18 is high and pin 17 is low, then green is off and amber is on.
3-4-5 Auto Degaussing
When SMPS works, the 6.6 V power source is applied to Q801 and CPU let Q801 turn on a few seconds,
then drive the armature of RL801 to perform the degaussing function.
3-4-6 PWM Control
1. Start Up
The I802 gets power from C874, C841, R834, ZD807, D802, C859, R806 and pin 7 voltage reaches 9 V
for starting up. The I802 starts oscillation at 20 kHz, sawtooth on pin 4 and pin 6 output to drive Q802/
T805. Once Q802 switches on, D812, C840 set up an 17 V to keep I802 working through D828 auxiliary
Operation Theory
MD 1998LE (B1997PNST)
Page 11