DEC VT220 Technical Manual page 180

The vertical sync input initiates retrace each time it goes low
(60 times per second). However, the E301 is capable of
free-running trace and retrace, in the event no retrace sync
signal is sensed from the video logic. R302 and C304 define E30l
internal free-running oscillation at 60 Hz
(~l
Hz).
At the start of a vertical trace, E301 outputs a voltage potential
on pin 4. This potential is stabalized and output to the vertical
output circuit. Feedback from that circuit is input to E30l at pin
10. This feedback is applied across R306 and compared with the
adjustable (R307) vertical linearity value at pin 1 of E301. The
feedback at pin 10, and the value at pin 1 are used by E30l to
monitor gain and to compensate for any drift in output voltage
value.
The output potential at pin 4 of E301 is ramped upward throughout
the trace period. At vertical retrace, the output is returned to
zero, removing the ramped potential from the vertical output
circuit.
Current regulation on the output of E301 is controlled by an
adjustable regulator (R303), which can increase or decrease the
height of the area of the screen scanned during a vertical trace.
8.2.4.2 vertical Output Circuit -- The vertical output circuit
develops the potentials used to drive current through the vertical
windings of the yoke assembly.
The vertical output circuit (Figure 8-16) consists of the
following circuits.
• Voltage divider network (R31l-R3l4 and C309)
• Yoke current drive components (C3ll, C3l2, R3l7, and Jl)
4.7K
' 9 10
TO/FROM { ..
VERTICAL
PROCESSOR
----r--+----,----,
R312~
562K
1 ~"O
C309
47
10V
C311
1000
R313 16V
47.5K
1%
R314 R315
1 1
TO
DYNAMIC
FOCUS
lC313)
PART
C312 OF
1000 Jl
16V
R317
100
1/1W 4
PART
OF
YOKE
ASSY
Figure 8-16 Vertical Output Circuit Diagram
8-14