Compaq CRT Monitor s7500 Service Guide - Page 15

Video Amplifier and OSD On Screen Display function

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HP S7500/MV7500/CV7500 2. Regulation The DC O/P voltage is proportional to the auxiliary voltage, so I801 pin 4 senses the feedback voltage from the divider D806, R809, C810, VR801,Q803, Q801 and R821to compare with thebuilt-in 1.5 volts reference voltage for error amplifier operation. Finally pin 6 can modulate the different duty cycle by VR801 setting to achieveregulation purpose. 3-4-6 Synchronization 1. NormalMode The sync pulse from FBT (31 kHz~69 kHz) via R812, R811 and C815 to pin 5 of I801 to keep I801 synchronizedwith horizontalsync inputfrequency. 2. Power SavingModes: Standby/Suspend Because there is no pulse from FBT, so the free-run frequency SMPS works at 20 kHz by I801 itself. 3. Override The horizontal free run frequency is about 63.2 kHz under override condition, SMPS is synchronized to this frequency. 3-4-7 O.V.P. If the auxiliary voltage is higher than 25 volts makes pin 3 of I801 is limited to have the OVP activated. 3-4-8 O.P.P. The excess current of T802 through I801 pin 1, 2, make Vs>1.1V internal of I801, then limite the power. 3-4-9 Step Up Power Supply For FBT 1. The B+ of FBT is proportional to horizontal frequency, that is the higher frequency, the higher voltage. The basic voltage is 58volts from T802 pin 10 via L811, D813, C824 andthe gate control of Q451 comes from I401 pin 6 via Q450, Q434, Q435 and R4E5. The duty cycle is controlled by I701 pin 23 (PWM pin Adj.) via R719, C741, R4F6,R4F5, R4F1. 2. The regulation and boost up (from 68 V to 160 V or more, on demand). The H.V. is set at 25 kV (zero beam) by I701 pin 23 which senses the secondary O/P from FBT. The booster comprises Q451, L410, L411, D450, C425 and T802 to offer the required B+ for different frequency modes. 3-5 Video Amplifier Section and OSD (On Screen Display) function 1. RGB signal inputs are terminated by R501, R531 and R561 then pass through the coupling capacitors C503, C533and C563 to IC501 LM1237 preamplifier. 2. The amplifier RGB signals (0~3 Vpp) are adjusted by I2C bus, I501 pin 23 is for clamp pulse which comes from pin 16 of TDA4841 to set up equal clamp level. 3. The pre-ampvideo stageis amplifiedby I501 (LM1237/NT6812), it's includingOSD functionand control by I701 pin 12, 13 (I2C). 4. The video outputstage 15 is amplified by I502 (LM2469/LM2466). 5. The RGB cathodes cut off are adjusted by I503 (LM2480)pin 8, 7, 6, which comes from I501 pin 16, 15, 14 to adjust cut iff voltage level by I2C bus. 6. Under override condition, "NO SIGNAL" will show up on the screen. Page 13

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Page 13
HP S7500/MV7500/CV7500
2. Regulation
The DC O/P voltage is proportional to the auxiliary voltage, so I801 pin 4 senses the feedback voltage
from the divider D806, R809, C810, VR801, Q803, Q801 and R821 to compare with the built-in 1.5 volts
reference voltage for error amplifier operation. Finally pin 6 can modulate the different duty cycle by
VR801 setting to achieve regulation purpose.
3-4-6 Synchronization
1.
Normal Mode
The sync pulse from FBT (31 kHz~69 kHz) via R812, R811 and C815 to pin 5 of I801 to keep I801
synchronized with horizontal sync input frequency.
2.
Power Saving Modes: Standby/Suspend
Because there is no pulse from FBT, so the free-run frequency SMPS works at 20 kHz by I801 itself.
3. Override
The horizontal free run frequency is about 63.2 kHz under override condition, SMPS is synchronized to
this frequency.
3-4-7 O.V.P.
If the auxiliary voltage is higher than 25 volts makes pin 3 of I801 is limited to have the OVP activated.
3-4-8 O.P.P.
The excess current of T802 through I801 pin 1, 2, make Vs>1.1V internal of I801, then limite the power.
3-4-9
Step Up Power Supply For FBT
1.
The B+ of FBT is proportional to horizontal frequency, that is the higher frequency, the higher voltage.
The basic voltage is 58 volts from T802 pin 10 via L811, D813, C824 and the gate control of Q451 comes
from I401 pin 6 via Q450, Q434, Q435 and R4E5. The duty cycle is controlled by I701 pin 23 (PWM pin
Adj.) via R719, C741, R4F6, R4F5, R4F1.
2.
The regulation and boost up (from 68 V to 160 V or more, on demand). The H.V. is set at 25 kV (zero
beam) by I701 pin 23 which senses the secondary O/P from FBT. The booster comprises Q451, L410,
L411, D450, C425 and T802 to offer the required B+ for different frequency modes.
3-5 Video Amplifier Section and OSD (On Screen Display) function
1.
RGB signal inputs are terminated by R501, R531 and R561 then pass through the coupling capacitors
C503, C533 and C563 to IC501 LM1237 preamplifier.
2.
The amplifier RGB signals (0~3 Vpp) are adjusted by I
2
C bus, I501 pin 23 is for clamp pulse which comes
from pin 16 of TDA4841 to set up equal clamp level.
3.
The pre-amp video stage is amplified by I501 (LM1237/NT6812), it’s including OSD function and control
by I701 pin 12, 13 (I
2
C).
4.
The video output stage 15 is amplified by I502 (LM2469/LM2466).
5.
The RGB cathodes cut off are adjusted by I503 (LM2480) pin 8, 7, 6, which comes from I501 pin 16, 15,
14 to adjust cut iff voltage level by I
2
C bus.
6.
Under override condition, “NO SIGNAL” will show up on the screen.