JVC 250 service manual Yoke Connector Detection

Chapter 7---Troubleshooting

Amplifier PCB the /SWEEP_OK becomes the /VA_OK signal and checks the supply voltages such as +15 V, +6.2 V and +80 V. The /VA_OK LED illuminates (see Video Amplifier PCB LED) if the supply voltages are present and the /SWEEP_OK is low. The /VA_OK also goes to the High Voltage Power Supply. On the High Voltage Power Supply, a logical “high” shuts down the high voltage amplifier that generates the Anode, Focus, and G2 voltages. The /VA_OK signal also goes to the Video Processor PCB where a logical “high” shuts down the G2 control voltages.

Yoke Connector Detection

The CRT Protection circuit also ensures that CRT Yoke connectors are properly installed. It does this with the H_LOCK+/- circuit. This series circuit goes from the Horizontal Vertical Deflection PCB to the Scan Reversal PCB, through each CRT Yoke connector, and back to the Horizontal Vertical Deflection PCB. The H_LOCK+ has +15 V applied to it on the Horizontal Vertical Deflection PCB. If all the CRT Yoke connectors are properly installed H-LOCK+ and H_LOCK- will both measure +5 V. If the any one of the connectors is not properly installed, H_LOCK+ will measure +15 V and H_LOCK- will measure 0 V. If H_LOCK- is at 0 V. This shuts down the horizontal amplifier circuit that generates the horizontal deflection waveform.

CRT Beam Current

Another function the CRT Protection circuit performs is to monitor the beam current of each CRT. The Video Amplifier PCB outputs a current sense line (RED, GRN, and BLU BEAM) from each image amplifier circuits. The Video Processor PCB receives these current sense lines and compares them to a reference. If the beam current of any CRT exceeds 250 µA, the Beam current LED illuminates orange, and the Video Processor PCB reduces Contrast. If reducing Contrast does not bring the CRT beam current below 250 µA, the Video Processor PCB reduces the G2 voltage until the beam current drops below the limit.

7-20 Model 250 Service Manual