Tektronix 455, Instruction Manual, Page: 118 / 246

Instrument Options—4 5 5 /A 2 /B 2
the emitter o f U2232D goes positive enough to overcome
the charge on C2242, a positive-going signal is supplied to
the base o f U2218E through emitter follow er Q2246. This
causes U2218E to conduct through CR2245 and help charge
C2242 to the new positive peak. As a result o f this process,
the signal at the collector of U2218E is an amplified and
inverted version of the positive sync pulses at the emitter of
U2232D. These negative-going sync pulses are level-shifted
by R2252 and R2254, then regenerated and buffered by the
Schmitt Trigger composed of U2232C and U2218B. The
output of the Schmitt Trigger is supplied to the T V LINE
positions of the A COUPLING and B SOURCE switches
through CR2257 and C2258 in the negative (—) position of
the A SLOPE/SYNC switch or through CR2251 and C2251
in the positive (+) position o f the A SLOPE/SYNC switch.
V E R T IC A L SYNC RECOGNIZER
The negative-sync output at the collector of U2218B is applied
to the input of the Vertical Sync Recognizer composed of
U2218D and U2218C. The vertical sync pulse is wider than
the horizontal sync pulse. Therefore, when the vertical sync
pulse arrives, U2218D is turned o ff longer than fo r a horizontal
sync pulse allowing C2262 to discharge through R2262. When
the vertical sync pulse ends, U2218B turns on and conducts
heavily to recharge C2262. The heavy conduction of U2218D
causes its collector to go sufficiently less positive to turn on
U2218C. The result is a negative-going, short-duration pulse
occurring at the end o f the vertical sync pulse. This short
duration pulse is supplied to the T V FIELD position o f the
A COUPLING switch through R2267 and coupling capacitor
C2267.
Option 5 Page 10
REV. A, OCT 1975