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Chapter 5---Electronics
Model 250 Service Manual 5-47
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 G
1
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 (G
1
_BIAS) control voltage (0-5 V) that comes from the brightness control
on the Video Processor PCB. The G
1
_supply (-200 V) comes from the High
Voltage Power Supply. The G
1
regulator regulates the -150 V G
1
supply up to
about -80 V. The blanking pulse is added to the -80 V and the G
1
bias adds an
offset for brightness control. This voltage is distributed to each of the CRT’s G
1
control grids.
G
2
(Black Level) Voltage
The Video Amplifier PCB receives the G
2
supply voltage (1200) from the High
Voltage Power Supply. It receives the G
2
control lines from the Video Processor
PCB. The Video Amplifier PCB regulates the G
2
supply voltage down to about
1100 V and distributes it to each G
2
regulator (one for each CRT). The G
2
control
line from the Video Processor PCB controls the output of each G
2
regulator
according to the G
2
(Black Level) command setting in the Convergence Menu.
The output goes to the G
2
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 G
1
and G
2
regulators effectively shutting down the all the CRTs.