TM 11-6625-3145-14
Theory of Operation-318/338 Service
Figure 4-14. 318/338 Simplified diagram of the power supply circuit.
Thermistors A11RT101 and A11RT102 limit the surge current when the power supply is first turned on. After the
instrument is in operation, the resistance of the thermistors decreases so that they have little effect on the circuit. When
the instrument is turned off, the Inverter Control stage turns off the inverter, preventing it from discharging A11C121 and
A11C122; A11C121 and A11C122 discharge slowly, through A11R121 and A11R122, to allow for thermistor thermal
recovery. This ensures sufficient thermistor resistance to limit the turn-on surge current to a safe level. Since A11C121
and A11C122 discharge slowly, dangerous voltages exist within the power supply for several minutes after the POWER
switch is turned off.
Varistors A11VR101 and A11VR102 are surge voltage protectors. If a peak voltage greater than 240 volts is present on
the line, A11VR101 and A11VR102 will conduct and quickly open line fuse F1 to interrupt the input power before the
instrument can be damaged.
INVERTER START NETWORK <15>
Capacitor A11C132 is charged by a constant current source, made up of A11Q131, A11VR131, and A11R132. When the
charge of A11C132 reaches the voltage level set by A11VR135, A11CR138, and the base-emitter voltage of A1Q135,
transistors A11Q135, A11Q134, A11Q140, A11Q139, and switching transistor A11Q149 are turned on, A11C132 is
discharged, and energy-storage capacitor A11C155 for A12U21 <16> and A12U1 <16> is charged. When the charge on
A1C155 reaches the voltage level required to start A12U21, A12U21 charges A1C143 through A11T140. This causes
transistor A11Q141 to turn on and A11Q131 to turn off. This disables the inverter start network after the instrument is on.
4-38
Summary of Contents for 318
Page 119: ...318 VERIFICATION AND ADJUSTMENT PROCEDURES ...
Page 182: ...338 VERIFICATION AND ADJUSTMENT PROCEDURES ...
Page 253: ...318 ___________________ TROUBLESHOOTING TREES ...
Page 344: ...338 TROUBLESHOOTING TREES ...
Page 517: ...TM 11 6625 3145 14 318 338 4434 923 318 Block Diagram ...
Page 518: ...TM 11 6625 3145 14 318 338 4434 924 338 Block Diagram ...
Page 519: ...TM 11 6625 3145 14 318 338 4434 925 318 Acquisition Module Wiring Diagram ...
Page 520: ...TM 11 6625 3145 14 318 338 4434 926 318 338 Mainframe Wiring Diagram ...
Page 521: ...TM 11 6625 3145 14 318 338 4434 926 338 Acquisition Module Wiring Diagram ...
Page 522: ...TM 11 6625 3145 14 318 338 4434 928 Figure 9 1 318 A01 Input A Board Component Locations ...
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Page 528: ...TM 11 6625 3145 14 Figure 9 3 318 338 A03 ACQ Control Board Component Locations ...
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Page 542: ...TM 11 6625 3145 14 Figure 9 8 318 338 A10 CRT Board Component Locations ...
Page 544: ...TM 11 6625 3145 14 Figure 9 9 318 338 A11 Inverter Board component Locations ...
Page 546: ...TM 11 6625 3145 14 Figure 9 10 318 338 A12 Regulator Board Component Locations ...
Page 551: ...TM 11 6625 3145 14 Figure 9 12 338 A01 Input A Board Component Locations ...
Page 553: ...TM 11 6625 3145 14 318 338 SERVICE ...
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Page 555: ...PIN 058584 ...