Tektronix 1740A Series, User Manual

Page: 74 / 368

Circuit Theory
3–10
1740A/1750A/1760 – Series Service Manual
Diagram
3
Vertical Input
The Input MUX is an 8-to-1 multiplexer (U79) controlled by the Line Rate
Controller on Diagram 7. It selects one or a combination of the input signals to
drive the Pix Out, the Filters, the Internal Video Sync Separator (Diagram 7), and
the flat input of the Filter Multiplexer (Diagram 4).
The Pix Out Amplifier is a negative feedback operational amplifier, U81, whose
output is 2V peak-to-peak across a 75
load (R316). Q23 increases the dc level
of the Pix Out signal when the line strobe pulse from the Line Rate Controller
(Diagram 7) occurs. Strobe pulse width can be up to 15 lines in duration.
The multiplexer is made up of two 4-to-1, Line Rate Controller asserted
multiplexers, U74 and U75. Selection of an internal reference channel is
controlled by two control lines and the chip selects. If neither chip select
(REFCH SEL2 or 3) is asserted, there will be no output. This occurs when
External Reference is selected.
When the INT EXT control line is asserted (goes low) U70D and U70A close to
drive Q19 from the EXT REF input. Q19 is an emitter follower providing a high
impedance output that drives a sync separator on Diagram 7. Q17 clamps the
input side of U70A to ground when Internal Reference is selected.
The output of the Input MUX drives the luminance filter through R302. L5 and
L6 are adjusted for a white bar square corner while C131 is adjusted for
minimum chrominance. The filtered output drives U72B to output the lumi-
nance signal for the Picture Monitor mode and component analog applications
(J13) for the 1760–Series. The output signal from U72B also drives the Diff
Step Filter.
The input stage of the Diff Step Filter is an active low-pass filter (U72A) that
drives an integrator consisting of C111 and R249. The integrator circuit converts
staircase risers into sharp spikes that are amplified by a factor of approximately 5
by U65.
There are two chroma filters, driven in parallel, by the Input MUX. Construction
of the filters is identical, with the component values selected for the appropriate
subcarrier frequency. The PAL filter is centered on 4.43 MHz and the NTSC on
3.58 MHz. U80 the chrominance amplifier is driven by either the video signal or
the test circle oscillator, depending on whether U76B or U76C is closed.
Input MUX
Pix Out
Horiz Ref MUX
Luminance/Diff Step
Filters
Chroma Filter
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