CTX CFA1797B
Service Manual

204.0 DESCRIPTION OF CIRCUIT

CFA1797B
56

4.0 DESCRIPTION OF CIRCUIT
1.
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Power supply circuit The AC voltage input connected to EMI circuit and rectifier circuit. R102, R112 and Switch (SW101) will provide DC voltage to IC101, when switch on. While IC101 works normal, The IC101 will Auto-detect output voltage of power supply from Pin2 and correct the duty cycle of Pin6 output pulse to compensate the variation of output voltage. The output of IC101 Pin6 connected to power MOSEFET to drive the power transformer T101. When power MOSFET is on, the energy stored in the primary winding of T101. Once MOSEFET is off, the energy transfer to the secondary and charges the output capacitor to get the stable DC voltage.
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(EEPROM). The outputs of IC501 were connected to other function circuit (IE. H-size, H-phase, V-size, and V-center�). Also, the user can adjust picture from keyboard and the data will be saved into IC502 automatically. For the O.S.D. mode, when O.S.D. manual is active CPU will inform the O.S.D IC (IC603) to send O.S.D BLK signal to blank the video signal from VGA card, and the O.S.D. IC will send O.S.D. R.G.B. Video signal to show the O.S.D. manual on screen. H.V. regulation circuit The IC401 is a P.W.M IC; Pin6 is P.W.M output, which connected to Q407, Q409. The Q402 is H.V. output switching transistor. Q401 and D401 are damper device provide the path of damper current. Q421 is a feedback voltage from FBT Pin13 connected to PIN2 (IC401) and correct the duty cycle of PIN6 output pulse. Video output circuit Video circuit consists of video preamplifier IC601 and output cascade amplifier IC604. IC601 is a video processing IC equipped with three DC amplifiers to pre-amplify R.G.B. signals from 0.7V to 4.2V. The R.G.B. GAIN & BIAS control signal are from IC501 via I2C-bus to DAC IC601. The IC601 control to video gain. The output of pin 24, 25, 26 are connected to BIAS circuit of each R.G.B. signal�s amplification and thus achieve a well balance able white picture. The O.S.D. R.G.B. (Pin4, 9,13 of IC601) is the input of O.S.D. Video signal. The IC604 is R.G.B. drive to capable of driving CRT.

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Oscillation circuit The functions H-size, H-phase, V-size, Vcenter, Side-pin, Parallel�.are designed inside into IC301. The Pin25 is X-RAY protect input. When H.V output circuit is abnormal (H.V too high) the X-RAY protect circuit will shut off the horizontal output, H.V. also will be shut down. The Pin28 is B+ control driver output. The Pin26 is horizontal square wave output drives horizontal output. The Pin24 is parabolic output. The Pin23 is vertical output. The Pin1 & pin2 are H-SYNC & V-SYNC input. The Pin3 is vertical blanking output. The Pin 20, 22 for vertical oscillator. The Pin 6, 8 for horizontal oscillator.

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Vertical output circuit The Pin23 of IC301 is vertical sync output connected to IC201 (Amplifier), Pin5 of IC201 is vertical amplifier output drives the vertical deflection directly.

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Horizontal output circuit The DC-DC is to generate a DC voltage (B+) for horizontal output circuit. The CPU (IC501) controls the H. size adjustment function of IC301 via the I2C Bus. The output of IC301 (Pin6) connected to gate of Q311 to make the Q311 switching. When Q311 is ON, the energy stored in T304 and the energy released via D313 when Q311 turned OFF. The B+ applied to Horizontal Yoke and supply the power for horizontal deflection. The more the B+ will get the bigger the horizontal size.

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Micon circuit The IC501 (CPU) will detect polarity and frequency of input H.V. Sync. The CPU will determine the mode of input timing (preset or users mode) and load mode data from IC502

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