TM 11-6625-3025-14/ET426-AA-MMA-010/E154 VII210/T.O. 33A1-8-902-1-1
Each input signal is applied to an isolating emitter follower which biases the pulse signal so that pulse tips are
negative (below ground level). The emitter-follower outputs then control modified bistable multivibrators in which the
negative pulse tips lock the multivibrator into a predictable mode. A modified Schmitt trigger operation is achieved with a
uniform level output of sharp risetime. The outputs are then routed to appropriate pattern generation circuitry. For
instance, the sync signal is applied to emitter-follower Q1 where impedance is transformed and biased. The pulse is then
routed through diode CR1 to the base of Q2 where the negative tip locks operation of the bistable in one direction with Q2
collector "high". Transistor Q3 is saturated to the "low" state by current through R8. When the negative pulse ends the
emitter of Q1 goes high and Q2 is biased "on" through R5. Since voltage at the base of Q2 is limited to the base-to-emitter
drop, Q1 becomes reverse biased. Transistor Q3 then becomes non-conductive with the collector going "high". Additional
current through R6 reinforces this stable state. The circuit remains in this mode until another negative input pulse tip
causes CR1 to conduct again. This circuit operation develops both positive and negative pulses of sharp risetime and
uniform amplitude for each of the four input signals.
The video card (figure FO-8) processes all pattern information into the output video signal. Pattern signals are
coupled into the card through one of three routes. Most are emitter coupled at the common junction of Q10 and R24
through connector pin W. Emitter coupling allows the fastest risetime, since it is a very low impedance. This is especially
important for the resolution function.
The emitter-coupled inputs (pin W) are connected to the output of this stage at the junction of R24 and Q10
emitter, where all video elements are brought to a common point, but inverted. This low impedance point drives Q9, which
establishes the proper signal polarity at fast risetimes. The signal is then routed to emitter-follower Q8 and black level
clipper R19 and Q7. The clipper is used to adjust digital signal black level as close as possible to the black level of the
gray scale signal.
The digital signal is applied to emitter-follower Q2 and the incoming gray scale signal is applied to Q1. The
emitter-follower pair operates as a level control, clipping off the digital video signal as directed by the gray scale input from
the front panel level control. In this way the video level is controlled without sacrifice in digital signal rise and fall time. The
video signal is routed to an emitter-follower network which mixes the blanking and sync signals to form the composite
video.
The blanking and sync signals enter the card through pins A and F, and processed in similar fashion. For
instance, the blanking signal is routed through bias network R48 and R49 to Q19, whose collector load is connected to -5v,
creating a pulse from zero to -5v. If blanking is not used (as in the video only signal mode), Q20 is turned on by switching
a -5v level to pin E, creating an "off" level near
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