Tektronix 475 - Low-Voltage Power Supply

Circuit Description—475
Output Amplifier
The push-pull signal from the Gain Setting Amplifier is
connected to the Output Amplifier. Each half of the
Output Amplifier can be considered as a single-ended
feedback amplifier, which amplifies the signal at its input to
produce a voltage output to drive one of the horizontal
deflection plates of the CRT. Both halves of the Output
Amplifier are basically the same with only minor differ
ences; therefore, only the upper amplifier (drives the +
horizontal deflection plate) and the differences will be
explained.
Q1172 and Q1174 are connected in a Darlington
configuration. Q1234 is a feedback amplifier in the
feedback path for the output amplifier. Q 1234 sets DC
levels and reduces the impedance of the feedback path.
Q1202 and Q1204 form a complementary amplifier that
provides the output signal to drive the + deflection plate of
the CRT. C R1173 clips excessive signal amplitudes and also
provides temperature compensation for several of the
emitter-base junctions in the output amplifier. R1175,
C1233, and C l 179 are high-frequency compensation adjust
ments. R1256 centers the output signal swing within the
dynamic swing capabilities of the output amplifier.
There are two signal paths through each half of the
Output Amplifiers. Slow signal transitions are applied to
the CRT through Q1172, Q 1174, and Q1204 with feedback
via R1233 and R1235. C1235, C1233, and C1178-C1179
compensate the amplifier and provide a fast AC signal path
from one side of the amplifier to the other. The lower
amplifier (drives the — horizontal deflection plate) is
basically the same as the upper except for two things.
Q1188 is a relatively constant current source for Q1182
and Q1184. VR1216 shifts the DC level of the signal from
the collectors of Q1182 and Q1184 to the emitter level of
Q1214.
LOW-VOLTAGE POWER SUPPLY
General
The Low-Voltage Power Supply circuit provides the
operating power for this instrument from six regulated
power supplies. Electronic regulation is used to provide
stable, low-ripple output voltages. Fig. 3-8 shows a detailed
block diagram of the Power Supply circuit. A schematic of
this circuit is shown at the back of this manual.
Power Input
Power is applied to the primary of transformer T 1400
through Line Fuse F1401, POWER switch S I401, Thermal
Cutout S1402, Line Voltage Selector switch S1403, and the
Regulating Range Selector. Line Voltage Selector switch
S1403 connects the split primaries of T1400 in parallel for
115-volt nominal operation, or in series for 230-volt
nominal operation. Line Fuse F1401 should be changed for
each nominal line voltage (current rating of fuse for
230-volt operation is one-half the current rating of fuse for
115-volts).
The vacant windings between pins 10, 10A, 11, 12, and
12A of T1400 are intended for use with the optional
Inverter Circuit Board (Option 7). This allows the instru
ment to be operated from an external DC power source or
an 1106 Power Supply.
Secondary Circuit
The —8 volt, —15 volt, +5 volt, +15 volt, +50 volt, and
+ 110 volt supplies are series-regulated supplies. U1418,
U1454A and B, and U1464A and B are high-gain amplifier
cells with differential inputs. These amplifiers monitor
variations in the output voltages and generate error signals
to maintain relative constant output levels. Additionally
unregulated voltages are used from the +15 volt and +50
volt supplies.
Q1492, Q1498, and their associated circuitry monitor
the unregulated voltage in the +50 volt supply and provide
a visual indication (via the LOW LINE indicator on the
front panel) when the applied line voltage falls below the
lower regulating limit selected by the Regulating Range
Selector assembly. Q1482 provides operating power to the
CRT graticule lights that is adjustable via the SCALE
ILLUM control R1480.
CRT CIRCUIT
General
The CRT Circuit provides the voltage levels and control
circuits necessary for operation of the cathode-ray tube
(CRT). Fig. 3-9 shows a detailed block diagram of the CRT
Circuit. A schematic of this circuit is shown on Diagram 12
at the rear of this manual.
High-Voltage Oscillator
Q1318 and associated circuitry compose the high-voltage
oscillator that produces the drive for high-voltage trans
former T1320. When the instrument is turned on, current
through Q1316 provides forward bias for Q1318. Q1318
conducts and the collector current increases, which de
velops a voltage across the collector winding of T1320. This
produces a corresponding voltage increase in the feedback
winding of T1320, which is connected to the base of
Q1318, and Q1318 conducts even harder. Eventually the
rate of collector current increase in Q1318 becomes less
than that required to maintain the voltage across the
collector winding and the output voltage drops. This turns
3-16