Atec Tektronix-1502 - Manual (Page 33)

TM 9-4935-601-14-3&P
Sweep Inverter with Horizontal Attenuator (Schematic 2)
The Sweep Inverter consists of an operational amplifier U1344; selectable input resistors, R1216, R1217,
R1218, R1219, R1226, R1227, R1323, R1324 (which are selected by the FEET/DIV switch, S1222F) and
selectable feedback resistors, R1148, R1146, R1152, which are selectable by the CABLE DIELECTRIC switch
S1251.
Operational amplifier U1344 is connected in the circuit as an inverter. Its input voltage amplitude is
determined by the position of FEET/DIV switch S1222F, whose input signal is the 4 volt, slow-ramp generator
signal. The FEET/DIV control is selectable from 1 to 200 feet per division in a 1-2-5 sequence. The output of
the FEET/DIV control is fed to the Sampler Comparator by way of voltage offset circuitry R1342 and R1239.
Feedback resistor R0472 is a variable range adjustment for the "OTHER" cable types; OTHER being any
cable with a dielectric material other than solid polyethylene or solid polytetrafluoroethylene.
Distance Offset
The distance offset consists of a compensated current source Q1413, Q1317; a DISTANCE dial, R0271;
and a ZERO REF CHECK switch, S0151.
The constant current of Q1413 is fed into the wiper of the variable DISTANCE resistor, R0271. The
variable calibrated current of R0271 is fed into the summing junction of the Inverters operational amplifier,
U1344. This occurs in all FEET/DIV settings except the 200 feet/div setting. When depressed, ZERO REF
CHECK switch S0151 disables the offset current so that reference checks can be made. DISTANCE CAL
resistor R1315 provides an adjustment for the calibrated current of Q1317.
Horizontal Output Amplifier (Schematic 4)
The Horizontal Output Amplifier consists of Q3221, Q3118, Q3113, and Q3114. This amplifier amplifies
the 0-4 volt, slow ramp signal into a differential voltage of approximately 150 volts as well as shifting the output
level to an average voltage of 85 V. This amplifier gain of approximately 37 times is achieved with a
differential transistor amplifier, Q3221 and Q3118. The gain is controlled by the ratio of resistance R3122 or
R3121, the parallel values of resistors R3217 and R3116, and resistors R3216 and R3212. R3217 is variable to
adjust for crt deflection factor tolerances. The differential amplifier uses 0Q3113 and 0Q3114 as a current
mirror current source. The thermally stable current is set by R3112, R3114, and R3115. R3213 is used for
horizontal positioning.
X-Y Recorder Logic (Schematic 3)
The X-Y Recorder Logic consists of bi-stable multivibrator U2332B and U2332D; gates U2332A and
U2332C and multiplex switches U3223A and U3223B. (U3223A and U3223B are on Schematics 3 and 4.)
During the normal mode of 1502 operation, the output of the multivibrator (PENLIFT: TP2157, yellow) is
low and Q2248 is not conducting. Pin 10 of gate U2332C is low and Q2249 is not conducting. By pressing
RECORD switch S0481B, the RECORD line goes high, the multivibrator changes level, and TP2157 becomes
positive (+5 V). The output of U2332A (pin 3) goes low and the output of U2332C (pin 10) goes high. This
turns Q2249 on. Q2249 keeps C2356 discharged and the output of the Slow Ramp remains at 0 V. By
releasing the RECORD switch, the output of U2332C goes low, which turns Q2249 off. At the same time,
Q2248 is on. This starts the Slow Ramp Generator scanning at a much slower rate due to C2252 being added
to the circuit. When the scan cycle ends, the output of retrace amplifier Q2259 goes high, causing multivibrator
U2332D and B to be reset. This sets the Recorder Logic back to the normal mode of operation.
Components CR2324, R2324, and C2325 are used to ensure that the Recorder Logic is in the normal mode
of operation when the power supply switch is turned on.
Vertical Amplifier
The Vertical Amplifier circuit contains selectable gain amplifier U2136 with positioning control, variable gain
amplifier U2143 to calibrate the instrument for different cable impedances, retrace logic with a fogging
oscillator, and a vertical output amplifier.
Vertical Amplifier. This circuit consists of operational amplifier U2136, and a bank of resistors for gain
selection. The required resistors are selected by switch S2126 (mp/DIV).
The operational amplifier is connected as a voltage follower. The non-inverting input receives its signal
from the preamplifier through R2138 and the positioning signal from amplifier U2131 through R2137. High-
frequency noise is filtered by 0.001
µ
F capacitor C2137. Additional noise filtering can be obtained by activating
the NOISE FILTER switch, S2254, which adds 0.1
µ
F capacitor C2241 to the circuit.
Positioning. Vertical trace positioning is accomplished by offsetting the dc level of the vertical signal.
Operational amplifier U2131 provides the dc offset voltage. U2131 is controlled by R0251A (POSITION) and
R0251B (FINE).
REV C FEB 1980
3-8