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The main function of the Flyback Transformer (FBT) is to generate a 25,000 volt
(EHT) potential for the anode of the picture tube. This voltage times the beam
current is the power that lights up the phosphor on the face of the picture tube.
At 1.5mA beam current, for the 2793 monitor, the FBT is producing almost 38
watts of high voltage power. The FBT also sources the focus voltage, screen grid
voltage, filament power, and has two more secondaries which are used for control
functions. The FBT has a built in high voltage load resistor which stabilizes the
EHT, for the low beam current condition. This resistor also discharges the EHT,
when the monitor is turned off, which improves the safety of handling the
monitor.
The Horizontal Output transistor is mounted to the rear frame which acts as a
heat sink. The collector conducts the 900 volt primary flyback pulses which should
not be measured unless the equipment is specifically designed to withstand this
type of stress. A linear ramp current is produced in the horizontal yoke by the
conduction of the horizontal output transistor (trace time). A fast current reversal
(retrace time) is achieved by the high voltage pulse that follows the turn off of the
horizontal output transistor. This pulse is due to the inductive action of the yoke
and flyback transformer.
The Horizontal Driver supplies the high base current necessary to drive the
horizontal output transistor which has a beta as low as three. A transformer is
used to step up the current from the driver circuit and also protects the horizontal
output transistor from a continuous turned on state. A special clamp circuit is
connected to the transformer which reduces the turnoff time of the horizontal
output transistor for reduced power dissipation.
The
Horizontal Control incorporates a variable sync delay and a phase locked
loop to generate the horizontal timing. The H POS. adjustment, on the remote
control board, sets the sync delay time which controls the picture position. The
phase locked loop uses the flyback pulse to generate a sawtooth wave which is
gated with the delayed sync pulse to control the horizontal oscillator.
The sync pulse from the LA7851 triggers a one shot in the LA7838 which clamps
the vertical ramp generation capacitor to 5V during the first half of vertical
retrace. The ramp generation capacitor then charges via a constant current set by
an external resistor. This resistor is connected to the V SIZE pot, located on the
remote control board, for the vertical size adjustment. The vertical drive is a
differential amplifier which compares the ramp voltage to the yoke return
feedback current. The yoke feedback current and voltage circuits are used to set
the vertical linearity. The vertical Output is a power driver, with thermal
protection, which drives the vertical deflection yoke. It also has a special pump up
circuit which doubles the output voltage during vertical retrace. This voltage
doubler also increases the efficiency of the circuit since the high retrace voltage is
not present across the power driver during the trace time.