JVC 250 service manual Video Amplifier PCB - Operation

Chapter 5---Electronics

RGB_HEAT (neg.) - negative side of CRT heater voltage (ground level).

ARC_GND - CRT ground back to High Voltage Power Supply (CRT Protection against arcing).

/VA_OK - CRT Protection signal to the High Voltage Power Supply (low = good, high = bad). This signal is becomes /HV_ENA at the High Voltage Power Supply.

Video Amplifier PCB - Operation

Image Amplifier

The main function of the Video Amplifier PCB is to amplify each of the image signals going to the cathode of the CRTs. The Video Amplifier PCB amplifies the image signal with a gain of approximately 80.The DC Restore from the Video Processor PCB is added to the amplifier circuit and the image signal is sent to the CRT. The Video Amplifier PCB uses the +80 V from the Low Voltage Power Supply to bias the image amplifier circuits.

G1 Regulation and Blanking

The Video Amplifier PCB provides G1 regulation for the blanking pulses. The Video Amplifier PCB receives the blanking pulse timed to both the horizontal and vertical syncs from the Raster Timing Generator PCB. It also receives the DC offset (G1_BIAS) control voltage (0-5 V) that comes from the brightness control on the Video Processor PCB. The G1_supply (-200 V) comes from the High Voltage Power Supply. The G1 regulator regulates the -150 V G1 supply up to about -80 V. The blanking pulse is added to the -80 V and the G1 bias adds an offset for brightness control. This voltage is distributed to each of the CRT’s G1 control grids.

G2 (Black Level) Voltage

The Video Amplifier PCB receives the G2 supply voltage (1200) from the High Voltage Power Supply. It receives the G2 control lines from the Video Processor PCB. The Video Amplifier PCB regulates the G2 supply voltage down to about 1100 V and distributes it to each G2 regulator (one for each CRT). The G2 control line from the Video Processor PCB controls the output of each G2 regulator according to the G2 (Black Level) command setting in the Convergence Menu. The output goes to the G2 grid on the CRT.

CRT Filament (Heater) Voltage

The Video Amplifier PCB receives the Filament (heater) voltage (+6.2 V) from the Low Voltage Power Supply and distributes it to the filament of each CRT. The filament or heater heats the cathode and causes it to emit electrons that are accelerated toward the anode of the CRT to form the image on the face of the CRT.

CRT Protection Circuit

The Video Amplifier PCB plays a major role in the CRT Protection circuit. The/SWEEP_OK from the Horizontal Vertical Deflection PCB becomes /VA_OK on the Video Amplifier PCB. If the /VA_OK goes high, the Video Amplifier PCB shuts down the G1 and G2 regulators effectively shutting down the all the CRTs.

Model 250 Service Manual 5-47