ViewSonic P90F Service Manual download pdf (Page 23)

When the voltage at pin 17 of I501 drops under 6V, contrast of display start to decrease. R167

set the light output where the contrast (or full white screen light output) start to drop. If the

beam current is too high, Q118 will conduct and shutdown PWM controller (I101).

Pin 7 of FBT provide negative bias for Grid 1 of CDT:

Pin5 of I310 provide vertical blanking to base of Q108 via D113. Q108 collector pulse drops

63V to give a proper blanking level at Grid 1 of CDT. When monitor turns off, pin3 (HUNLOCK)

is pull high to turn off Q108 via D108. Grid 1 of CDT is then pulled down to most negative

voltage from pin7 of FBT to protect CDT. This action is so-called spot killer.

8-5 Dynamic focus circuit

The vertical dynamic focus is from pin 32 of I301 and feed to base of Q105 via R105 and C103.

The horizontal dynamic focus is taken between L402 and Horizontal S correction capacitors. It

is step up (amplified) to the certain level voltage (600V approx. at 94kHz Timing Mode) and

combine with Vertical dynamic focus waveform through L403, and then feed to PIN 11 of FBT.

The dual trace parabola wave-form plus the DC focus voltage gives sharp and clear display for

the whole screen.

The two variable resistors on top of FBT provide focus setting.

The bottom one set Grid 2 voltage of CDT.

8-6 Video and CDT drive circuit

8-6-1 VIDEO PREAMPLIFIER (I501)

The signals come from PC through P901 to CDT drive board (PWB-0237). Pin7 (Red), pin5

(Green), pin3(Blue) feed to pin11, 6,2 of I901(video pre-amplifier). Pin 4, 9, 13 of I901 are

OSD R, G, B video signals input coming from OSD IC I503. Pin 12 of I901 is the input for OSD

blanking signal which blank the normal video as OSD video signal is active. Since the video

inputs are AC coupled to I901, so I901 need a clamping signal (at pin 19) from H. sync (IA01

pin 34) to give DC restoring at I901 outputs. The horizontal blanking signal is provided from

PWB-0235. Via P902A, it connects to pin 27 of I901. The video outputs of I901 will have the

level darker than the video black level during the horizontal retrace time. Pin 20 and 21are IIC

input for MCU (IA01) to control video. By IIC bus MCU will initiate I901 and control the contrast,

brightness, R/G/B gain, and R/G/B bias.

Pin15 is an ABL control input. By detecting the beam current, it will reduce the contrast when

the beam current exceeds the setting point.

8-6-2 VIDEO DRIVE AND OUTPUT CIRCUITS

After being pre-amplified, the R. G. B. video signals are output from pin35, pin32, and pin29 of

I901 respectively. The amplitude of the signal at theses output are about 3 ~ 4 V p-p. Those

video signals are connected the I904 at pin 7, 6, 9 respectively.

The I904 is video output amplifier to amplify the R, G, and B signal. They offer about 40 V p-p

signal in amplitude to drive the cathodes of CRT.

L912, L942, L972, are the peaking coils, for the compensation of high frequency response.

The DC off-set bias voltage is set up by R902, R903 at pin30 of I901.

The cathode cut-off setting voltages required for white balance are obtained from pin23(r),

pin25 (g), pin26 (b) respectively, These output magnitude can control the conduction of

transistors Q903, Q933, Q963 individually. The DC bias of the cathodes are then varied to

optimize the color temperature of back ground.

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P90f-1

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