BBK DV718SI, Service Manual

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2. Working principle
After loading disc in the player, IOA port of MT1389 is defaulted high level to make Q305 saturation
on and form loop together with CD laser power control circuit on CD. At the same time, IOA also goes to
loader PD IC to switch, disc begins to rotate, when servo management system recognizes that the disc
in player is not CD disc, IQA pin outputs low level to make Q305 cut off and make Q303 on, and form
loop together with DVD laser power control circuit on loader to perform disc reading acts. After disc tray
door opens, IOA keeps the state before opening disc tray door. If the player cannot recognize which disc
it is, IQA pin will switch continuously until reading disc or system judges that there is no disc.
Note: Q303 and Q304 are MOS tube
3. Key point voltage (unit: V) is shown as the following table:
Bas e
electrode B
Collector C Em itter E
Grid
electrode G
Drain
electrode D
Source
electrode S
G D S IOA
DVD disc 0 3.86 0 3.81 0.18 0 0 0 0 0
VCD disc 0.64 0.1 0 0 0 0 3.27 0.18 0 3.3
State
Q305 Q303 Q304
Q301
2SB1132-S
Q302
2SB1132-S
R301 4.7R
R302 4.7R
TC302
47uF/16V
TC303
47uF/16V
LDO-AV33
LDO-AV33
LDO2
LDO1
MT1389E
MD1
20/21
XS301
23
20
19
Figure 3.2.4.1 Laser power control circuit diagram
3.2.4 Laser power control circuit
1. Laser power control circuit is shown as in the following figure 3.2.4.1:
2. Working principle:Pin 20/21 of MT1389 is laser power detect signal input pin, pin 21 is DVD laser
power strong/weak detect signal input pin, pin 23 is VCD laser power drive control output pin, pin 22 is
DVD laser power drive control output pin.
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