Hughes JVC 200, Service Manual

Page: 44 / 140

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