Omron 8025G Maintenance Manual page 71

THEORY OF OPERATION
SECTION 4
The positive output of CR3, filtered by C3, is fed to the regulator assembly
via J6-5 and motherboard lines 53-56. The negative output is filtered by C2 and
applied to J62-4 and motherboard lines 17-20. Two IC voltage regulators, Z2 and
Z3, provide regulated -12 V dc and +12 V dc, respectively. Capacitors C7 and C8 pro-
vide additional filtering. Output bypass capacitors, ClO and Cll, improve response
time by attenuating transients.
The voltage across the secondary of Tl is rectified by full-wave rectifier
CRI and CR2, filtered by CI, and applied to J63-1,2,3, and motherboard lines 59-62.
The voltage on lines 59-62 is applied to IC voltage regulator Zl, which
maintains a constant voltage on motherboard lines 71-80, and 65-66 at currents below
700 milliamps. The level of this output is set by R4 while C6 provides additional
filtering of the input to Zl and C9 improves transient response. The voltage drop
across Rl controls the current through Ql.
4.4.2 CRT Display
The CRT Display in the terminal is basically a IS - inch television monitor.
It provides visual readout of input data from the keyboard or from external devices.
Block Diagram Analysis. As shown by the simplified block diagram in Figure
4-7; the CRT Display is made up of four major sections: video, vertical sweep, hor-
izontal sweep, and low-voltage regulator.
An adjustable closed-loop voltage regulator provides a constant output to
all stages in the CRT Display. The X-ray protection circuit prevents any X-radiation
due to line voltage surges or regulator failure. Under those conditions, the circuit
disables the horizontal drive multi-vibrator.
Video inputs are amplified and inverted by the video amplifier driver. The
video amplifier output is coupled directly to the video output stage, which provides
a low-impedance source for driving the cathode of the CRT.
The vertical oscillator is a relaxatiqn oscillator synchronized to the ver-
tical interval which is s'et by the vertical drive input. A sawtooth output from the
oscillator is directly coupled to a driver amplifier, the output of which is applied
to the vertical output. 'fhe vertical output in turn is transformer coupled to the
yoke.
Horizontal drive pulses, after being delayed by an
~djustable
delay cir-
cuit, are applied to the horizontal drive one-shot. This delay compensates for in-
herent deflection-circuit delays. The output of the one-shot is coupled to the
horizontal output stage, which supplies the correct horizontal scanning currents
and the high-voltage pulses for the high-voltage supply.
Circuit Description. Refer to schematic diagram 95-146-02 in Section 8
and to the voltage waveforms shown in Figure 4-8.
The video section consists of QI03 and Ql04 with their associated cir-
cui try.