Tektronix 549 Instruction Manual page 43

Circuit Description— Type 549
Horizontal Amplifier
The Horizontal Amplifier converts the single-ended saw­
tooth output of the Time Base Generators into a push-pull
signal suitable for driving the horizontal deflection plates
of the CRT.
The gain of the amplifier may be varied by a factor of
five with the M A G N IFIER switch. Controls are also provided
for horizontal positioning.
The sawtooth waveform from the Time Base Generator is
coupled to the input cathode follower through a RC network.
This network attenuates the input signal and provides means
to compensate the input circuitry for optimum frequency
response.
The H O R IZO N TA L P O SITIO N
the DC level on the grid of V343A. Th is alters the DC level
on the signal path through the amplifier and changes the
DC voltage applied to the CRT horizontal deflection plates
which shifts the horizontal position of the trace.
Possible damage to the target area during initial turn
on time is prevented by pulling the beam to the left and
off the CRT screen. C335 couples a negative pulse through
diode D334 and R334 to the grid of V343A when the — 150
volt supply turns on which charges C340. This charge decays
at an RC time rate and provides ample time for all voltages
to stabilize before the beam is allowed to return to the CRT
screen area.
Coupling between the input cathode follower and the
driver cathode follower is made through the M A G N IFIER
switch. W ith the M A G N IFIER switch in the OFF position, the
signal from the input cathode follower is attenuated by a
factor of five through the network of C348, in parallel with
the series combination of R348 and R349. Variable potentio­
meter R348 adjusts the length of the sweep by varying the
amplitude of the sawtooth applied to the grid of V343B.
Variable capacitor C348 is adjusted to provide optimum
linearity of the sweep. To provide magnification of the
sweep, the network is removed when the M A G N IFIER switch
is turned to the X 5 position.
The gain of the Horizontal Amplifier is controlled by a
negative-feedback circuit. A portion of the signal at the
left-hand deflection plate is fed back to the input of driver
cathode follower V343B. Norm/Mag Regis R358 establishes
the DC voltage applied to the feedback loop.
By changing the DC voltage at this point, the starting
position of the unmagnified sweep can be adjusted so it w ill
correspond with the starting position of the magnified sweep
in the center of the graticule.
The cathodes of the output amplifier tubes V364A and
V384A are connected through a degenerative network which
includes the Mag Gain adjustment.
is adjusted when the M A G N IFIER switch is in the X 5 posi­
tion. C375 in parallel with the Mag Gain is a linearity
adjustment for very fast sweep speeds.
The output signal from the paraphase amplifier (output
amplifier) drives the output cathode followers.
followers are used to provide the current drive for the
capacitance of the horizontal deflection plates and stray cir­
cuit capacitance. To help supply the high current required
at high sweep rates, V398 is connected in a cascode con­
3-14
and FIN E controls vary
The Mag Gain R375,
Cathode
figuration with V364B. Plate current of a pentode remains
fa irly constant even when the plate voltage decreases. A
flat topped pulse, derived by differentiating the positive
going sawtooth from the cathode of V384B through C390
and R390, is applied to the grid of V398 to boost current
output during the sweep or trace period.
of this flat-topped pulse is proportional to the sweep rate;
therefore, more current boost is provided as the sweep rate
is increased.
Bootstrap capacitors C364 and C384 are used to help sup­
ply the charging current for the plate circuits of V364A and
V384A respectively at fast sweep rates.
When the LOCATE button SW 380 is depressed, power is
applied to both L362 and L553 provided both OFF switches
of the UPPER and LO W ER
switches are closed (pushed in).
L362 and L553, both reed switches SW 362 and SW 553 close.
This decreases both the horizontal sweep and the vertical
signal deflection amplitudes. SW 380 also sets the intensity
in the CRT circuit to a preset level.
W ith either of the two STO RE buttons pushed in, the cor­
responding O FF switch is opened and L362 or L553 w ill not
energize with the closure
LOCATE switch does, however, shift the DC level slightly
positive on the grid of V384A. This results in a beam shift
to the left of the CRT graticule area or to the locate zone.
W ith single sweep operation, the beam can therefore be
positioned vertically without affecting stored information.
Storage System
The basic operation of the storage CRT was described
earlier in this section. The electrodes of the CRT and cir­
cuits peculiar to the storage feature of the Type 549 are
shown in the storage circuit diagrams in the schematic section.
The storage circuits contain the following: Voltage divider
and operational amplifiers which set the voltage levels of
the CRT storage electrodes; erase pulse generating circuits
with an optional automatic erase feature to erase written
information; enhance pulse generator circuit to provide a
controlled amount of enhancement to the CRT storage target;
and integrate circuitry to sum writing beam current of a
repetitive signal.
The operating voltage levels of the target backplates are
stabilized by operational amplifiers. These provide a con­
stant voltage source to these electrodes.
V1064A and B operates as a cathode follower operational
amplifier for the upper target backplate and sets the oper­
ating level for the upper backplate. V1164A and B sets the
operating level for the lower backplate.
Backplate Level adjustment R1055 establishes the oper­
ating level of the target backplate and sets the current out­
put of the operational amplifier when the CRT is shifted to
the ready-to-write state (STORED switch pushed in). Adjust­
ment R1050 sets the DC balance of the two target back-
plates.
The voltage that is applied to each backplate depends
on the position of the corresponding STO RE switch (SW 1005
and S W 1105). In the conventional
voltage is set by the Non Storage Level adjustment R1169.
The amplitude
SCREEN STO RA G E selector
W ith power applied to
of the LOCATE switch. The
mode the backplate
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