Tektronix CURVE-TRACER 576 Instruction Manual page 75

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obtained through the use of fiber-optic readout. Fiber-optic
readout involves the use of plastic fibers of very small
diameter, called light tubes, for transferring light from one
place to another. The light tubes are designed so that the
light incident at one end of the tube is transmitted through
the tube to the other end. If the output end of the tube is
viewed directly, the output light looks like a small dot. This
transmission of light occurs even if the light tubes are bent
at slight angles. In order to form a character, many I ight
tubes are arranged so that their output ends, the dots of
light, are in the configuration of the character to be
formed. The input ends are then arranged so that they
receive their incident light from the same light source. In
some cases it may take two or more light sources to form
one character. Whenever the proper light source (or
sources) is illuminated, the desired character appears. It is
the purpose of the readout circuitry, therefore, to light the
readout lamps so the deflection factors they indicate cor-
respond with the CRT display deflection factors deter-
mined by the positions of the VERTICAL and HORIZON-
TAL switches, the MODE switch, the DISPLAY OFFSET
Selector switch, the AMPLITUDE switch and the .1 X STEP
MULTbutton.
The inputs for the readout logic come from logic lines
whose logic levels are controlled by the switches shown on
the Readout Switching and Interconnections schematic, or
by externally provided logic levels. The form of the inputs
is a high-low code. Normally all inputs are high and the
code is determined by switching some of the logic lines to
ground. Ground reference is generally provided directly as
part of the switch. However, in the case of the vertical and
horizontal switches, ground is provided through saturated
transistors 0900 and 0943 respectively. If lows are applied
to pins 7 and 20 of J363, these transistors are turned off. In
this case ground reference for the affected logic lines must
then be provided externally.
The readout logic (see Readout Logic schematic) pri-
marily consists of integrated circuit decoders. These de-
coders receive inputs from the incoming logic lines in terms
of the above-mentioned switch code. This input code is
then translated into a high-low lamp code which appears on
the output logic lines. Each of the output logic lines is con-
nected to a readout lamp (see Readout Lamps schematics)
and each lamp illuminates one character or part of a
character. A low on a readout lamp causes the lamp to light.
The intensity of the readout is determined by the 0 to 4.5
volt supply.
The readout logic circuitry also generates a lamp code
which produces a readout of beta or transconductance
(gml per division. This {3 or gm readout lamp code is ob-
tained by dividing the vertical lamp code by the steps lamp
code.
The decoders which control the horizontal deflection
factor readout are U951 and U953. Inputs to these de-
coders are controlled by the HORIZONTAL switch, the
DISPLAY OFFSET Selector switch or by externally
©I
Circuit Description-Type 576
applied inputs to J363. Outputs from these decoders go to
the horizontal readout lamps. As an example of how a lamp
code is generated, assume that the HORIZONTAL switch is
set to .5 V COLLECTOR and the DISPLAY OFFSET
Selector switch is set to NORM (OFF). Due to the closing
of contacts by the HORIZONTAL cam switch (see the
Readout Switching and Interconnections schematic). lows
are applied to the inputs to U951 and U953 at connectors
13, T, and S of P950 (see Fig. 3-13). The other inputs to
the horizontal decoders are held high. The output lamp
code resulting from this input code is lows at lamp input
connectors F, I, J, L, A, C, D and E. The resulting PER
HORIZ DIV readout is 500 mV, which corresponds with
the .5 V COLLECTOR position of the HORIZONTAL
switch.
Decoders U956 and U960 control the vertical deflection
factor readout. Inputs to these decoders are controlled by
the VERTICAL switch, the DISPLAY OFFSET Selector
switch, the MODE switch and externally applied inputs to
J363. Outputs from these decoders go to the vertical read-
out lamps. The horizontal and vertical decoders are also
affected by the logic inputs, at pin U, pin Y and pin 12 of
J950, whose logic levels may only be determined
externally.
Decoders U965 and U970 control the step amp I itude
readout. Inputs to U965 and U970 are controlled by the
AMPLITUDE switch, the STEP MULT .1X button and
externally applied inputs to J361. Outputs from U965 and
U970 go to the steps readout lamps.
The beta or gm generator consists of U974, U975 and
U976. The input code received by these decoders is a com-
bination of logic levels coming in part from the vertical
lamp code, and in part from the steps lamp code. The out-
puts from these decoders go to the beta readout lamps.
0960 and 0974 decode the logic levels appearing at pins 13
and 15 of U960 and pins 13 and 15 of U970. 0977 and
0979 provide a means of lighting the 1
A
lamp (connector
Bl) whenever the 2,5 lamp (connector AR) is off.
POWER SUPPLY
Low Voltage Power Supply
The Type 576 can be operated either from a 115-volt or
a 230-volt line voltage source. The low voltage power supc
ply (see Fig. 3-14) consists of a single transformer, T701,
which has nine secondaries. This supply provides six regu-
lated voltages: -75 volts, -12.5 volts, +5 volts, + 12.5 volts,
+15 volts and +100 volts. It also produces a regulated vari-
able voltage of 0 to 4.5 volts, one unregulated voltage of
+50 volts and an AC voltage to drive the POWER ON light
and the GRATICULE ILLUM lights. In addition the
windings providing a source of clock pulses for the step
generator and the CRT heater are among the nine secon-
daries of T701. All the regulated power supplies are
completely short proof.
Input Circuit. When the POWER switch is switched tc
ON, line current flows from the input, P701 (see Power
Supply schematic), through power switch SW701, fuse
F701, Thermal Cutout TK701 and into the primary wind-
3-21