Omron 8025G Maintenance Manual page 72

THEORY OF OPERATION
SECTION 4
The video amplifier consists of Ql02 and its associated circuitry. The
incoming video signal (typically 4 V P-P) is applied to the base of Q103. The gain
of this stage can be varied from 12 to 25 with Rl19. the video gain adjustment. QI03
operates as a class B amplifier and remains cut off until a dc-coupled. positive-
going signal arrives at its base and turns the transistor on. Rl18 and R1l9 add ser-
ies feedback to make the voltage gain relatively independent of transistor variations.
This feedback also stabilizes against voltage and current changes caused by ambient
temperature variations.
The negative-going signal at the collector of Ql03 is dc coupled to the
base of Ql04. the video output. Ql04 is an emitter follower that supplies a low
source impedance for driving the cathode of the CRT. Class-B biasing of the video
output allows a maximum available contrast ratio by providing a large video output
signal to modulate the CRT cathode. Typically a video output of about 25 V P-P is
required for optimum contrast.
Overall brightness of the display is determined by the negative potential
at the grid of VI. The normal adjustment range of CRT grid voltage is from +10 to
-100
V
dc. This voltage is set with the brightness control located on the terminal
keyboard. .
The vertical sweep section is made up of QlOl. Ql02. and Q122.
QlOl. a programmable unijunction transistor, with its associated circuitry
forms a relaxation oscillator operating at the vertical sweep rate. Timing is deter-
mined by RC network RI06-108, CI03. and CI04. When power is applied, Cl03 and CI04
charge toward +SS V dc through RI06 and Rl07. The charging rate is set by the time
constant of the network. Capacitors CI03 anrl CI04 continue charging until the anode
voltage of QlOI is within 0.6 V of the gate voltage. When this occurs. QIOI fires,
and Cl03 and CI04 rapidly discharge through QIOI and LIOI to near ground. QlOl then
turns off and· allows CI03 ~d CI04 to recharge to the firing potential.
The gate threshold voltage for QIOl is developed across RIOS, the level
being established by the network RI03. CRIOI, CRI02, and RIOS. This voltage, vari-
able with the vertical hold control (RI03), is typically 4.6 V. Thus CI03 and CI04
must charge to approximately 4 V before QIOI fires.
VIDEO INPUT
VIDEO
..
AMPLIFIER
DRIVER
FROM VIDEO CONTROL CARD
!01031
VERTICAL DRIVE VERTICAL
VERTICAL
~
DRIVE
~
OSCillATOR
AMPLIFIER
FROM TIMING CARD 101011
101021
•
HORIZONT Al DRIVE ADJUSTABLE
HORIZONTAL
DRIVE
..
DElAY ~
MUl TlVIBRATOR
FROM TIMING CARD
10109 1121
+50
VOC lOW VOL TAGE
- -........ , REGULATOR
10121,0105 1081
X RADIATION
PROTECTION
!0111
1
+55
VOC
!0114,1151
VIDEO
~
OUTPUT
CQl041
VERTICAL
~
OUTPUT
101221
HORIZONTAL
~ OUTPUT
101231
~I
~
~
T
HI VOLTAGE
SU1'I'l Y
~
r::I
L::.J
Figure 4-7. CRT display block diagram.
4-24