Tektronix 453 User Manual

The Error Amplifier is connected as a comparator. Refer-
ence voltage for the comparator is provided by zener diode
Dl 114 which sets the base of Ql 114 at about -9 volts. The
base level of Ql 124 is determined by voltage divider Rl 121-
Rl 122-Rl 123 between the output of this supply and ground.
Rl 122 is adjustable to set the output voltage of this supply
to
-12 volts. Rl 119 is the emitter resistor for both com-
parator transistors and the current through
it divides be-
tween Ql 114 and Ql 124. The output current of the Error
Amplifier stage controls the conduction of the Series Regula-
tor stage (through
Ql 133). This output current changes to
provide a constant, low-ripple -12-volt output level. This
occurs as follows: The -12-volt regulator maintains equal
voltage at the bases of the Error Amplifier transistors Ql 114
and Ql 124. If the -12 Volts adjustment Rl 122, is turned
clockwise, the current through
Ql 124 increases (Ql 124 base
tends to go more positive than the base of Ql 114) and the
current through Ql 114 decreases. Decreased current through
Ql 114 produces less voltage drop across Rl 117 and the base
of Ql 133 goes positive. The emitter of Ql 133 pulls the base
of
Ql 137 positive to increase the current through the load,
thereby increasing the output voltage of the supply. This
places more voltage across divider Rl 121-Rl 122-Rl 123 and
the divider action returns the base of
Ql 124 to about -9
volts. A similar, but opposite, action takes place when Rl 122
is turned counterclockwise so the base of Ql 124 is more
negative than the base of
Ql 114. The -12 Volts adjustment
Rl 122, is set to provide a -12-volt level at the output of
this supply.
The output voltage is regulated to provide a constant volt-
age to the load by feeding a sample of the output back to
the Series Regulator,
Ql 137. For example, assume that the
output voltage increases (more negative) because of a change
in load or an increase in line voltage. This negative-going
level at the output is applied across the voltage divider
Rll21-Rll22-Rll23 and the base of
Qll24 goes negative
also.
This reduces the current flow through Ql 124 which
allows
Ql 114 to conduct more and its collector goes nega-
tive. When the collector of Ql 114 goes negative, the bias
on Ql 133 is reduced, resulting in reduced current through
the Series Regulator,
Ql 137. Reduced current through Ql 137
also means that there is less current through the load and
the output voltage decreases (less negative). In a similar
manner the Series Regulator and Error Amplifier stages com-
pensate for output changes due to ripple.
The Short-Protection Amplifier stage, Q 1129, protects the
-12-Volt Supply if the output is shorted. For normal opera-
tion, the emitter-base voltage of Ql 129 is not enough to
bias it on. However, when the output is shorted, high current
is demanded from the -12-Volt Supply, and this current
flows through Rl 129. The voltage drop across Rl 129 becomes
sufficient to forward bias Ql 129 and its collector current
produces an increased voltage drop across
Rl 117. The
increased voltage drop across Rl 117 reduces the current flow
of both Ql 133 and Ql 137 to limit the output current.
+ 12-Volt Supply
Rectified voltage for operation of the + 12-Volt Supply is
provided by 01142 A-0. This voltage is filtered by Cl 142
and connected to the + 12-Volt Supply Series Regulator and
to the High-Voltage Oscillator stage in the CRT Circuit. Ref-
erence voltage for this supply is provided by voltage divider
Rl 151-Rl 152-Rl 153 between the regulated -12 volts and
the output of this supply. The -12 volts is held stable by the
©!
Circuit Description-Type 453/R453
-12-Volt Supply as discussed previously. If the + 12-volt
output changes, a sample of this change appears at the
base of
Ql 154 as an error signal. Regulation of the output
voltage
is controlled by the + 12-Volt Series Regulator stage,
01167, in a similar manner to that described for the -12-
Volt Supply. The + 12 Volts adjustment, Rl 152, sets the
output level to +
12 volts. Dl 152 provides thermal com-
pensation for the
Error Amplifier. Cl 164 improves response
of the regulator circuit to AC changes at the output.
Shorting protection is provided by Ql 159 and Rl 159. If
the output of this supply is shorted, Ql 159 is biased on to
limit the conduction of the Series Regulator in the same man-
ner as described for the
-12-Volt Short-Protection Amplifier.
Dl 164 protects Ql 154 when the output of this supply is
shorted.
+ 75-Volt Supply
01172 A-D provides the rectified voltage for the +75-
Volt Supply. Cl 172 filters the rectified voltage which is con-
nected to the +75-Volt Series Regulator. Reference voltage
for
this supply is provided by voltage-divider Rl 181-Rl 182-
Rl 183 between the regulated -12 volts and the output of
this supply. Since the -12 volts is held stable by the -12-
Volt Regulator circuit, any change at the base of Error Ampli-
fier Ql 184 is due to change at the output of the +75-
Volt Supply. Regulation of the output voltage is controlled
by
Error Amplifier Ql 184-Ql 193 and Series Regulator,
Ql 197, in a manner similar to that described for the + 12-
Volt Supply. The +75 Volts adjustment Rl 182, sets the
quiescent conduction level of the Error Amplifier stage to pro-
vide an output level of
+75 volts. The output of the + 150-
Volt Supply (unregulated) is connected to the Error Ampli-
fier to provide the required collector supply for stable opera-
tion. Zener diode
Dl 209 establishes a vol age at its cathode
of about +
108 volts. Then, Rl 186, zener diode Dl 185 and
Rl 185 drop this voltage to the correct level for the operation
of Ql 184. 01182 provides thermal compensation for the
Error Amplifier.
Ql 189 provides current limiting for this supply through
Dl 188. Quiescently, Ql 189 is off under normal operating
conditions,
Dl 189 is conducting and Dl 188 is reverse biased.
However, when the output
is shorted, the increased current
flow through
Rl 187 biases Ql 189 on and its collector goes
negative.
This forward biases 01188 and reverse biases
01189. Now
Ql 189 limits the collector current of Qll 97
through Q1193. Fl437 (F1172 below SN 39320) also pro-
vides overload protection. Dl 198 protects the +75-volt
supply from damage if it is shorted to the -12-volt output.
+ 150-Volt Unregulated Supply
Rectifiers 01202 and Dl 212 provide the unregulated out-
put for the + 150-Volt Supply. From serial number 30820
through SN 39319, the output from this secondary winding
also provides the operating potential for the cooling fan
motor through
Rl 200. Serial number 39320 and above, this
secondary winding contains additional taps to provide the
operating potential necessary for the cooling fan motor.
The output of the +75-Volt Supply is connected to the nega-
tive side of the + 150-Volt Supply to elevate the output level
to + 150 volts. Diodes Dl 202 and 01212 are connected as
a full-wave ·center-tapped rectifier and the output
is filtered
by Cl 202-Cl
204-Rl 204 to hold the output level at about
+ 150 volts. Fuse Fl 204 protects this supply if the output
is shorted.
3-27

Tektronix 453 Specifications

General

Brand	Tektronix
Model	453
Category	Test Equipment
Language	English