DEC VT220 Technical Manual page 174

Q202 is turned on and off by current flow through the secondary of
T201 in the horizontal driver circuit. During the second half of
retrace, Q202 is turned on to saturation. At other times, Q202 is
backbiased by T20l, quickly turning Q202 off to minimize
dissipation.
The Q202 collector voltage is sourced at +12 V and boosted through
the collector voltage network to +15.5 V.
The charge pump circuit inverts the large positive voltage swings
at the collector of Q202 into -170 V bias used by brightness
control and dynamic focus circuits.
The horizontal linearity circuit provides adjustable (L20l)
control of current flow through the horizontal windings of the
yoke assembly during trace and retrace scanning. L20l provides
variable inductive reaction for adjusting current flow and varying
the trace width. L202 provides nonlinear reactance that varies
with es in yoke current. R217 and C212 provide correction for
nonlinearity caused by the decaying exponential rate of current
increase in the yoke.
The output components switch yoke current back and forth between
C213 and C216.
Figure 8-8 provides an illustration of the waveform developed at
the horizontal wi ings of the yoke assembly during trace and
retrace riods. Points T0 through T3 are identified in this
figure as references within the waveform. Figures 8-9 through 8-12
provide illustrations and descriptions of the current flow
conditions that result in the waveform shown in Figure 8-8.
N-~"'"
I THROUGH
YOKE
- 0 AMPS
t ~m===~~===-1r!=:l_OIlOLTS
r
l.
L
IIOLTAGE
i
I
II
ACROSS
I I
YOKE
T,j;3 TO
T2
. .
..-..
Figure 8-8 Horizontal Deflection Waveform
8-8