Nokia 447ZiPlus Service Manual page 21

17” colour monitor
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6-13. Brightness Control Circuit
The -157V is obtained from the FBT # PIN7 negative fly-back pulse via rectifier D489, through R484,
R482, D357 to supply Grid 1 of CDT(G1). This G1 voltage is normally set up at -70V.
The brightness control of the monitor is achieved by changing the G1 voltage via Q495, Q497. The control
voltage is derived from the pin26 of I501. The variation of the DC voltage on the base of Q495 via R492
will change the G1 voltage via R498, thus picture brightness change.
6-14. Spot Killer Circuit
The spot killer circuit is included in order to prevent from damaging CRT while power off monitor.
During the power on due to the voltage drop on R351, D348 from -157V, the #B, #E of the Q348 is
reverse biased. So Q348 remains non-conducting, whereas when powered off, the negative polarity of the
C349 will drop drastically to around –225V, this change will lead to the Q348 conducting, therefore draw
the G1 voltage down to much more minus voltage to cut off the beam current.
6-15. Blanking Circuit
The vertical blanking pulse is produced from pin5 of I330 via R337, C337, R339, R340, and D338. It is
also combined with horizontal blanking signal via C342. Both of the signals are fed to the amplifier Q341,
Q353. The output-blanking signal is then fed to G1 via C355.
6-16. Horizontal Position (Raster) Circuit
The adjustable horizontal raster centering is achieved by using the circuit of D485, D486, R485 by select
jumper on P402 in either a DC+CS AC current element or a DC-CS'AC current element. The positive or
negative magnitude of this AC current compare with the central zero level can decide the picture scanning
inclines to left or right side, via flow in or flow out of the L485 into the yoke assembly. The background
raster should be centralized with respect to the edge of the bezel by proper jumper selection on p402

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