Tektronix 442 Service Manual page 71

Theory of Operation—442 Service
TV Trigger Circuit
Active devices Q2164, Q2174, Q2176, U2156B, C, D,
and associated circuitry comprise the TV trigger circuit.
When the MODE switch is in TV, R2186 is disconnected
from +8 volts which allows trigger signals from the
collectors of Q2134 and Q2136 to pass through the TV
trigger circuit. (In AUTO and NORM, the +8 volts applied
to R2186 biases Q2164 to saturation.)
The sync separator circuit consists of Q2174 and
associated circuitry. It processes sync-positive pulses
when the SLOPE switch is inthe +OUT position and sync-
negative pulses in the —!N position. Transistor Q2174
produces large positive-going pulses from negative-going
sync signals at the collector of Q2164.
In the TV field mode (SEC/DIV switch set for .1 ms or
slower), Q2176 is saturated (since base is grounded), and
the integrator (composed of C2174, C2176, R2174, and
R2176) is switched into the circuit (effectively connected
to +8 volts through saturated Q2176). The integrator
filters out the horizontal syne pulses, leaving only the
integrated vertical sync pulses, which trigger the TV
Schmitt trigger U2156B and U2156C.
In the TV line mode (SEC/DIV switch set for 50 us or
faster), Q2176 is turned off (base open), disconnecting
C2174 and C2176 from +8 volts. Capacitors C2174 and
C2176 no longer integrate the pulses, thus allowing both
the horizontal and vertical sync pulses to pass through to
the TV Schmitt trigger.
Schmitt Triggers
The Schmitt trigger for the NORM and AUTO triggering
modes consists of U2156A, U2156B, and associated
The Schmitt trigger for the TV triggering mode is
U2156B, U2156C,
U2156D, and associated circuitry.
Resistors R2154 and R2178 determine the sensitivity.
When the MODE switch S2150 isin AUTO or NORM, +8
volts is applied to R2156, which causes pin 1 of U2156A to
go HI enabling Schmitt trigger U2156A-U2156B. At the
same time, pins 12 and 13 of U2156D are also HI, disabling
Schmitt trigger U2156C-U2156B.
A Trigger signal from
Q2152 triggers Schmitt trigger U2156A-U2156B to
produce a logic trigger signal at pin 6 of U2156B.
SWEEP AND HORIZONTAL AMPLIFIER
(Diagram 5)
Sweep Generator
The sweep is produced by a Miller Integrator circuit
consisting of Q2242, Q2244, and Q2246. Asweep ramp is
initiated at the collector of Q2246 when pin 3 of U2234A
goes LO, and is terminated when pin 3 goes HI (see Timing
diagram, Figure 3-1).
In the NORM triggering mode, pin 2 of U2212A is HI,
allowing a positive-going trigger signal at pin 1 of U2212A
to cause pin 3 of U2234A to go LO (via U2212D and
U2234C). This reverse biases CR2233 and CR2234, and
allows the timing capacitor (selected by SEC/DIV switch
$2550) to charge, producing asweep ramp atthe collector
of Q2246. When the sweep ramp reaches about 12 volts,
Q2274 turns on. This causes pin 7 of U2224Ato goLO, pin
8 of U2234C to go LO, and pin 3 of U2234A togo HI. When
pin 3 of U2234A goes HI, CR2233 and CR2234 are forward
biased, terminating the sweep. Pin 7 of U2224A remains
LO for a length of time (hold-off time) determined by
C2275, C2274, R2271, R2274, and HOLD OFF control
R2272. Three hold-off times are selected by SEC/DIV
switch $2550 and varied by R2272. After the selected hold-
off time, U2224A pin 7 goes HI. This allows the next trigger
signal to switch pin 3 of U2234A LO and again start the
sweep.
In the AUTO triggering mode, when no trigger signal
occurs at pin 11 of U2224B for about 50 ms, pin 10 of
U2224B goes LO, causing the sweep to start after the hold-
off time ends. This allows the sweep
to free run and
provide a reference display. When a trigger
signal is
present, pin 11 of U2224B goes HI, then LO (when trigger
signal ends), and the time constant of C2226 and R2226
prevents pin 10 from going LO as long as the repetition
rate of the trigger signal is higher than about 20 Hz.
REV A MAY 1980