Atec Tektronix-1502 - Manual (Page 36)

TM 9-4935-601-14-3&P
The output of the astable multivibrator is connected to the fly-back switching circuitry consisting of Q6349
(an emitter follower) and Q6435 (a high current switch). Q6435 conducts when the output of the multivibrator is
high, so a magnetic field builds up in fly-back transformer T6535 by way of windings 5-6 during the 8p s period.
When Q6435 turns off, the field collapses and the stored energy is transferred to the secondary windings.
Diodes CR6347, CR6441 and CR6442 prevent Q6435 from saturating, causing the switch-off time to be faster.
To reduce the base drive energy, a voltage source that is lower than the battery pack is used. This is supplied
from terminal 3 of T6535.
The +25 V (pre-regulated) supply is used to control the primary regulation. (It is also used for the high
voltage circuitry.) A voltage tap is provided by way of R6358 and Q6357 to create 9.7 V as a reference voltage
for the primary regulator network. This voltage is compared against the combined voltage of 9.1 V zener diode
VR6459 and the base-emitter junction of Q6557. Depending on the voltage level of the 25 volt supply, Q6557
increases or decreases conduction. The amplified collector voltage is fed to astable multivibrator Q6458 and
Q6445 by way of emitter-follower Q6552. This controls the low voltage output duration of the astable
multivibrator.
Anti-Deep-Discharge Circuit. The Anti-Deep Discharge circuit prevents the battery pack from going into
deep discharge, which could permanently damage the batteries. Q6549 and Q6547 are connected as an SCR
network. When fired, both transistors fully conduct, which forces the collector of Q6557 to ground. This
removes all the input voltage to the input of the astable multivibrator, which turns it off and disables the dc
voltage converter.
The SCR network can be triggered when Q6548 conducts. This happens when the battery voltage drops
below 10 volts. The +25 V and +10 V power supplies are used as reference voltages by way of R6549,
CR6546, and R6539. To reset this SCR network, the battery voltage must be removed by turning off the front
panel POWER switch. The batteries must be charged or replaced (unless ac power is to be used) before
further operation of the instrument.
Voltage Regulators. The +165 V, +100 V, +8 V, and -8 V supplies are not individually adjustable. R6358
is used to adjust the +25 V supply to the correct output voltage. The +10 V regulator provides a 10 volt
reference for the +5 V and -5 V regulators.
The 5.1 V zener diode, VR7417, is used in a bridge configuration to provide a reference voltage for the +10
V supply. R6518, R6513, R6413, and R6514 are part of this bridge network. The bridge outputs are fed into
operational amplifier U6515, which drives series regulator Q6427. Zener diode VR6419 is used as a start-up
circuit. Capacitor C6416 provides additional power supply filtering. In order to reduce the energy or power
losses, Q6427 has only a 2 volt collector-to-emitter drop.
Q6331 is the series regulator for the +5 V power supply. The +5 V is referenced against the +10 V with
operational amplifier U6236. The output of U6326 drives the series regulator in an emitter-follower mode.
The -5 V power supply uses Q6227 as a series regulator. U6222 is used as a controlled current source and
the minus voltage connection (VEE) is connected to the base of the series regulator. The standing amplifier
current is shunted by R6222 and only the load fluctuations are used to control Q6227. R6223 is a current
limiter.
Control Logic (Schematic 5)
The Logic board is only used with the TEKTRONIX Y-T Chart Recorder. This board can be removed
without affecting the operation of the 1502. However, if this board is removed, the RECORD switch would
have to be connected to the Vertical board by connecting the cable from the RECORD switch to P28 on the
Vertical board.
The Logic board consists of pulse shaper, U5166A, U5166B, U5166C, and U5166D; binary counter U5147;
a horizontal correction network; Chart Recorder Logic for controlling the paper drive motor; sytlus heat; a Chart
Recorder power switch; and an anti-bounce record circuit. See Fig. 3-7 on Chart Recorder Logic.
Pulse Shaper. The Pulse Shaper is made up of U5166A, U5166B, U5166C and U5166D. When the paper
is moving, a square wave signal (CHART SPEED SENS) is generated by the holes in the paper passing
between a light-emitting diode and a photo-sensitive transistor. The resultant pulses are shaped to a clean
square wave by a Schmitt trigger circuit, U5166B and U5166C. The output signal is capacitor-coupled by
C5167 to one-shot multivibrator U5166A and U5166D. This one-shot produces an output signal at TP5145
(violet test point) which is a positive going pulse approximately 10 ms wide. This (clock) pulse is connected to
binary counter U5147; and to the end of the paper-sensitizer network, R5148.
Counter. The 7-stage binary counter is integrated circuit U5147. The binary outputs are connected to a
resistor ladder network, which forms a digital-to-analog converter. Outputs Q1 and Q7 are connected to AND
gate U5252B so that it recognizes the count number 65. Q2 clocks multivibrator U5127B at count 2. (The
signal from U5127B starts the slow ramp.)
REV B FEB 1980
3-11