664
Chapter 13
Display/Power Supply Section
A6 Power Supply Assembly
Buck Regulator Control
See function block H of the A6 power supply schematic diagram in the
component-level information binder.
The buck regulator control pulse-width modulates the buck regulator
and provides a synchronized signal to the DC-DC converter control
circuitry. The buck regulator control has two feedback paths. The first is
the output of the buck regulator, which provides coarse regulation. The
second is the feedback circuit which samples and compares the +5 Vdc
output of the output rectifier.
U202B and associated circuitry senses the output of the input rectifier
and will turn off U203 if the voltage at TP108 is less than
approxi170 Vdc. Also, it will not allow U203 to start up until
this voltage e215 Vdc. A low on the output of U202B will also
clear the overcurrent latch in the DC-DC converter control circuitry.
Thermal shutdown occurs when RT201, mounted on the main heatsink,
reaches a temperature of 100 C. When this occurs, the voltage at U203
pin 13 exceeds 0.6 V and inhibits pulses to the buck regulator.
R203, R204, U211, and associated circuitry provide feedforward for
U203. This makes the loop gain independent of input line voltage and
cancels 120 Hz ripple by more than 10 dB.
U202C and its associated circuitry permit the power supply to start up
at low line voltages at low temperatures. At low line voltages U202C
will draw charge away from C206 through R205. This allows the buck
regulator to turn on and draw current through the thermistors in the
input rectifier. This warms up the thermistors, thereby decreasing their
resistance and increasing the voltage at TP108. When the voltage is
sufficiently high at TP108, the output of U202C will open and C206 will
be allowed to charge normally.
U202A converts the sawtooth at TP204 to a squarewave to drive the
DC-DC Converter Control circuitry. The frequency of the sawtooth is
determined by the resistance at pin 7 of U203 and the capacitance at
pin 8 of U203.
Summary of Contents for 8564EC
Page 17: ...25 1 General Information ...
Page 37: ...47 2 Adjustment Diagnostic Software ...
Page 77: ...89 3 Manual Adjustment Procedures ...
Page 129: ...161 3a Manual Adjustment Procedures 3335A Source not Available ...
Page 142: ...175 4 Assembly Replacement ...
Page 194: ...Chapter 4 257 Assembly Replacement Procedure 13 A21 OCXO Figure 4 34 A21 OCXO Mounting Screws ...
Page 196: ...259 5 Replaceable Parts ...
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Page 224: ...303 6 Major Assembly and Cable Locations ...
Page 234: ...315 7 General Troubleshooting ...
Page 238: ...Chapter 7 319 General Troubleshooting Introduction Figure 7 2 Ribbon Cable Connections 1 of 3 ...
Page 239: ...320 Chapter7 General Troubleshooting Introduction Figure 7 3 Ribbon Cable Connections 2 of 3 ...
Page 242: ...Chapter 7 323 General Troubleshooting Introduction Figure 7 5 Service Cal Data Menu ...
Page 271: ...352 Chapter7 General Troubleshooting Block Diagram Description Figure 7 6 Functional Sections ...
Page 283: ...364 Chapter7 General Troubleshooting Block Diagram Description ...
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Page 287: ...377 8 ADC Interface Section ...
Page 291: ...Chapter 8 381 ADC Interface Section Introduction Figure 8 2 A3 Test Connector Pin Locations ...
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Page 322: ...427 9 IF Section ...
Page 356: ...Chapter 9 461 IF Section A5 IF Assembly Figure 9 13 Detailed IF Adjust Signature 5 ...
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Page 379: ...487 10 Controller Section ...
Page 394: ...521 11 Synthesizer Section ...
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Page 454: ...593 12 RF Section ...
Page 489: ...628 Chapter12 RF Section A15 RF Assembly Figure 12 10 10 MHz TTL Reference at U304 Pin 13 ...
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Page 492: ...633 13 Display Power Supply Section ...
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Page 505: ...671 14 Component Level Information Packets ...
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