Tektronix 1740A Series, User Manual

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Circuit Theory
3–32
1740A/1750A/1760 – Series Service Manual
When the signal from the inverted input is switched in, U101C grounds the base
of Q111 and U103 or U102C, which provides the input signal to the emitter of
Q111 through Q127, an emitter follower. The output of Q111 can be backporch
clamped. Because the signal can be deflected positive and negative from center,
a different clamp level is required for positive-going signals than for the
negative-going signals. The sample capacitors, C120 and C121, are charged
whenever the OPT PLS (from the Line Rate Controller, Diagram 7) goes low and
either locally generated TOP or BOTTOM is asserted, causing the output of
U125A or U125B to go low and close either U124A or U124D. The clamped
amplifiers, U126A and U126B, have a dc level at their non-inverting inputs.
When the lightning display is selected this voltage is adjusted to overlay the
backporch levels at the vertical center point of the positive and negative
excursions.
Q112 and Q114 are an emitter coupled pair driving Q116 to output the clamp
level on the collector output of Q111.
The horizontal amplifier is driven by Q180 and emitter follower to provide a
high output impedance. Q151 and Q157 form an active low-pass filter that is
used by all of the component outputs. U162A and U162B are clamped
amplifiers. Their operation is very similar to that of the vertical amplifier
clamps. R159 aligns the top and bottom halves of the display along the vertical
center line, at backporch time.
In order to measure the variations in the horizontal transitions of the lightning
display it is necessary to bright them up. U171 is employed as a dual compara-
tor mixer to provide an output pulse to the Z–Axis Amplifier on Diagram 21. As
the horizontal signal deflects from center both of the active inputs to U171
change. When the changes are compared to a fixed level (the opposite side of
the comparator amplifier) the mixer output changes to create a bright-up level.
Diagram 21 Low Voltage Power Supply
The Low Voltage Power Supply converts the mains line voltage (90–250 Vac) to
supply the power requirements of the instrument. The voltages supplied by the
Low Voltage Power Supply are +40 V,
15 V, and +5 V.
The Low Voltage Power Supply is called a flyback switcher. When switcher
mosfet Q14 is turned on, its drain voltage drops to approximately 0 V. The
current through the 350
H primary winding of T3 begins ramping up. The
voltages present at all secondaries is such that the rectifier diodes are reverse
biased. Energy is being stored in the magnetic field of T3. When Q14 turns off,
Horizontal Amplifier
GBR Intensification
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