DEC VT220 Technical Manual page 178

The anode output is rectified internal to Tl, and filtered by
graphite coatings at the CRT end of the anode wire, which can
connect or disconnect only at the CRT end.
The operational voltage (+60 V) used
brightness control circuits is output
regulated and filtered by 0100 and L204.
by the video amp and
from pin 2 of Tl and
The cutoff grid potential is taken from pin 4 of Tl and regulated
by L203 and
D202.
The regulated potential (+750 V) is routed to
the CRT cutoff grid via a voltage divider network (R2l9 and R224,
filtered by C219 and C220), and to R220 in the dynamic focus
circuit.
The focus grid potential is picked off of the focus potentiometer
(R22l) with a voltage range at R221 from +500 V to -170 V. The
positive range is developed from the same Tl output used for the
cutoff grid potential, while the negative range is developed from
input from the charge pump circuit in the horizontal output
circuit. The actual focus grid value sent to the CRT is developed
across R222 in conjunction with the value supplied by the dynamic
focus circuit (refer to section 8.2.5).
8.2.4 Vertical Deflection Circuit
The vertical deflection circuit develops the potentials supplied
to the vertical windings of the yoke assembly. These potentials
provide for deflection of the CRT electron beam during vertical
trace and retrace times.
vertical deflection
is
a linear current rise to the vertical yoke
windings during a complete scan of the screen (each scan line of
the scan defined by horizontal trace and retrace activity). A
complete scan of the screen occurs at a rate of 60 vertical trace
periods r second. Vertical retrace time is defined by a vertical
sync signal input from the video logic (VER IN). VER IN results in
a steep drop in the current supplied to the vertical yoke
windings, returning the placement of the CRT beam to the top of
the screen.
The vertical deflection circuit (Figure 8-14) consists of the
following circuits.
•
•
8.2.4.1
circuit
periods.
to move
through
windings
Vertical processor
Vertical output
vertical Processor Circuit The vertical processor
provides the base control for defining trace and retrace
It develops a ramping output used by the vertical output
the CRT beam from the top to the bottom of the screen
linear increase of current flow through the vertical
of the yoke assembly.
8-12