Tektronix 556 Instruction Manual page 78

a Q1517 collector to emitter drop which equals the difference
between the rectifier output voltage and 150 volts. When the
regulator circuitry is adjusted to produce — 150 volts for a
given rectifier output, any change in output voltage with
respect to ground causes a bias change on Q1494 of oppo­
site polarity to the output voltage, and of about one half
the amplitude. For example, if the — 150 volts output began
to increase toward — 151 volts, the voltage at the emitter
of Q1494 would become more negative by approximately
the unit of voltage change (due to the constant voltage
across V I482), while the voltage at the base of Q1414
would become more negative by approximately one-half
the unit of voltage change (due to the voltage divider action
of the resistance network), resulting in an increase in forward
bias of approximately one-half the unit of voltage change.
Q1504 and Q1513 with their associated circuitry form
two emitter follower circuits. Error signals developed in the
collector circuit of Q1494 are coupled to the base of Q1517
via these two emitter followers. The two emitter followers
provide isolation between the error sensing circuit and the
series regulator. The associated circuitry is configured to
provide error signal limiting for regulator circuit protection
and to provide phase shifting to prevent self-inflicted reg­
ulator circuit oscillations.
Q1517 and shunt resistor R1519 form a series regulator
circuit. The circuit functions as a variable resistance in series
with the — 150 volt load circuits. Any change in the rectifier
output voltage, whether due to line voltage or load current
changes, is compensated by an equal change in voltage
across the series regulator circuit.
Since the compensating changes in voltage across the
series regulator are reactions to changes in the output volt­
age with respect to ground, there is a temporary change in
output voltage during the regulator circuit reaction time.
Filter capacitors C l498 and C l499, connected between the
— 150 volts output and ground, suppresses the effects of
these voltage changes to a level negligible to the opera­
tion of the instrument.
Fuse 1482 w ill open when the current through it exceeds
approximately .75 amperes. Since an excessive current must
be present for a finite interval before FI482 w ill open, R1517
is provided to limit surge currents through Q1517 to a level
within the transistor's dissipation capabilities.
+ 100 Volt Supply
The input to the +1 0 0 V supply is the output of the sec­
ondary winding connected to terminals 13 and 14 on the
power transformer T1401 and the silicon diode bridge D1452.
In addition to its other loads, the +1 0 0 V supply provides
current to the +2 2 5 V supply and then through the 225 V
supply to the 350 V supply. Also, the +1 0 0 V supply must
provide a total current o f 300 mA to two series strings of
filaments (see the Heater W iring diagram). The plug-in units
become part of the series filament string.
units are removed while the oscilloscope is operating, the
+ 100 V supply is capable of remaining within its dissipation
limits despite the 300 mA change in load.
The configuration of the + 100 V olt Supply regulator
circuit and its reaction to a change in output voltage is
similar to the — 150 Volts supply regulator circuit.
®
Q1454 with its associated circuitry forms the error sensing
circuit. The base of Q1454 is connected to a resistance net­
work that includes the +1 0 0 volt output adjustment R1459.
Since R1459 is connected between the + 100 Volt output and
the regulated — 150 Volt output, any change in the +100
volts with respect to the — 150 Volts is applied as a change
signal to the base of Q1454. D1450 provides thermal com­
pensation for Q1454 and reference to ground.
Q1463 and Q1464 with their associated circuitry form a
circuit similar in configuration and function to the circuit
formed by Q1504 and Q1513 in the — 150 Volt supply circuit.
Q1467 and Q1477 are connected in parallel and with the
shunt resistors R1476, R1477 and R1478 form the series regu­
lator circuit.
Fuse F1478 and current limiting resistors R1464 and R1469
protect the circuit from excessive currents.
+ 225 Volt Supply
The AC output of the secondary windings between termi­
nals 15 and 16 is rectified by diode bridge D1432 and added
to the + 100 volt supply output to produce the +2 2 5 volts
supply.
The +2 2 5 volts supply is similar in configuration and
operation to the +1 0 0 volts supply. Q1434 with its associ­
ated circuitry forms the error sensing circuit. D1430 provides
+ 100 reference and temperature compensation for Q1434 and
R1430 is the +2 2 5 V olt output adjustment. Q1443 and Q1444
with their associated circuitry form the emitter follower cir­
cuits. Q1447 with the two shunt resistors R1445 and R1446
form the series regulator circuit. The regulating circuit is pro­
tected by fuse FI446 and current limiting resistors R1442 and
R1443.
+ 3 5 0 Volt Supply
The AC output of the secondary windings between termi­
nals 19 and 20 is rectiifed by diode bridge D1412 and added
to the + 2 2 5 volts supply through the series regulator (Q1427)
to produce the +3 5 0 Volt supply.
Q1414 with its associated circuitry forms the error sensing
circuit. The base of Q1414 is connected to the junction of
two equal value resistors. Since these two resistors are con­
nected between the +3 5 0 Volts output and the — 150 Volt
output, about one-half the amplitude of any change in the
+350 Volts with respect to the regulated — 150 Volts is
applied to the base of Q1414.
Q1424 with its associated circuitry forms an emitter fol­
lower circuit that provides isolation between the error sensing
circuit and the series regulator circuit. The output of Q1424
is the signal drive for Q1427 in the series regulator circuit.
Q1427 and shunt resistor R1427 form the series regulator
circuit. Current limiting for Q1427 is provided by R1429.
If the plug-in
In the event of a circuit malfunction, the + 350 V supply
circuitry is protected from excessive currents by FI 424 and
the output supply voltage is prevented from decreasing sub­
stantially below the + 2 5 0 volts supply level by D1440.
The power supply circuits in instruments SN 100-1999 are
configurated similarly to the circuits described above. They
produce the same output supply voltages and the supply
Circuit Description— Type 556
3 -3 9