JVC Model 200 service manual Video Amplifier

Chapter 2---System Description

2.3.5 Video Amplifier

The Video Amplifier Board (VAB) is located at the back side of the projector under the necks of the three CRTs. It contains three separate video amplifiers, one each for red, green, and blue. The outputs from these video amplifiers connect directly to the CRTs and provide all electrical connections to the CRTS except for the anode voltages.

The following functions are provided by VAB:

•Amplification of video signals and driving the cathode of all three CRTs

•Sensing the cathode beam current for all three CRTs

•G1 regulator for all three CRTs

•Blanking drive section

•Phosphor protection for all three CRTs

•G2 regulator and adjustment of black level (screen) for all CRTs

•DC restoration for the video signals

•CRT interface for focus, heater voltage and ARC ground

The Video Amplifier Board I/O diagram (Figure 2-11) and the list of Inputs and Outputs (Table 2-8) provide information for the technician to perform module level troubleshooting.

G1 Regulator for three CRTs

The input supply for the G1 grid of the CRTs is G1_SUPPLY and is about -200V. The VAB regulates this supply to about -80V to - 30V. This supply can be adjusted by the G1_BIAS line. This adjustment provides the brightness control function. The G1_BIAS signal is an input of the VAB and is generated by the Video Processor Board. In the event of a failure, such as lack of deflection (a high on the /SWEEP_OK line), this output goes to the most negative value of the input supply (about -200V).

G2 Regulator and black level adjustment for CRTs

The VAB also regulates the G2 grid of the CRTs. The supply for the G2 grids (about 1200V) is generated by the HVPS. This

G2_SUPPLY is an input to the VAB. There are also three control inputs (RED_G2, GRN_G2, and BLU_G2). The G2 voltage of each CRT is adjusted individually by using these control lines. This adjustment provides black level control. The G2 output voltages are pulled down during shutdown mode.

2-30 Model 200 Service Manual