DEC VT220 - 8.2.1 Video Amplifier (Amp) Circuit

8.2.1
Video Amplifier (Amp) Circuit
The video
the video
the CRT.
amp
is a two-stage circuit converting video input from
logic on the logic board into an emitter potential at
The video amp (Figure 8-3) consists of the following circuits.
• Video input (R101-R103)
• Input stage (0101 and R105)
• Output stage (0102, C102, R109, and R108)
• Voltage divider (R106, R107, C150, and C151)
• Frequency compensation (C10l and R104)
Video input
supplied to
potentiometer
when 0101 and
transformer to
to the CRT.
(a 2.5 V peak to peak signal, from 0 V to +2.5 V) is
the base of 0101 through the contrast thumbwheel
(R101). C101 provides for frequency compensation
0102 are conducting. A +60 V input from the flyback
R109 provides the voltage supply for emitter output
With no signal input, 0101 is off, and 0102 has a bias of 7.2 V on
its base from R106 and R107. When a video signal is applied to the
base of 0101, 0101 is turned on, forward biasing 0102. With 0102
on, the potential developed across R109 is dc coupled to the CRT
emitter through RIG8 (R1G8 protects 0102 from transients that can
occur as a result of high voltage arcing at the CRT anode).
Rl0S
2201/2 W
0102
BF457
0101
2N2369A
Rl03
75
R102
150
R109
B20
0102 2W
;~L
Cl--::-=-+--------"---'----'--f-L...:;.::..;-'-----' FROM
FLYBACK
TRANSFORMER
(01001
J2
PiN
FR. OM ~ ViDEO iN
VIDEO 11
lOGiC R101
250B
J2
PINS
Figure 8-3 Video Amplifier Circuit Diagram
8-3