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TM 11-6625-3145-14
Theory of Operation-318/338 Service
INVERTER CONTROL <16>
The inverter control stage, made up primarily of A12U21, provides voltage regulation, a dead-time controller, and a current
limiter. For regulation purposes, A12U21 varies the hold-off time of the Inverter Switching transistor. Under normal
operating conditions, only pin 2 of A12U21 controls the hold-off time. However, either the dead-time controller or the
current limiter can ffect the hold-off time, or stop the inverter operation altogether. The operation of each individual
function of the inverter control stage is described in the following paragraphs.
Regulator. A12U21 acts as a pulse width modulator to regulate the inverter circuitry. It runs the inverter at a constant
frequency and regulates by changing the duty factor of the inverter signal. This is accomplished as follows:
At the beginning of the inverter cycle, A12U21 pins 9 and 10 go high, turning on A12Q34. This provides base drive for A1
1Q149 through A11T140. When A12U21 senses that the -5 V supply has gone below -5 V (via divider network A12R17,
A12R18, and A12R23 to A12U21 pin 2) pins 9 and 10 go low, turning off A12Q32 and thus removing removing the base
drive to A11Q149 and stopping the inverter. A11Q149 stays off until the beginning of the next inverter cycle.
The rest of the power supplies have their own regulation circuitry, which are indirectly controlled by the inverter circuitry.
The -5 V supply is directly regulated by the inverter regulator; the -5 V supply then regulates the "unregulated" supplies for
the rest of the voltages.
Dead Time Controller. The dead-time controller function of A12U21 protects the power-supply components from
damage due to excessive current or voltage. During normal operation, the voltage at pin 4 of A12U21 remains at about
1.5 volts. If the Under/Over Voltage Stop stage turns on, A12U21 pin 4 voltage rises to about 5 volts, and the inverter
stops. The inverter will remain off while A12C53 discharges through al 2Q58, keeping A12VR58 and A12Q59 turned off.
This cycle repeats until the fault is corrected, with the inverter turned on for about 200 ms, and turned off for about 200 ms.
OVER/UNDER VOLTAGE PROTECTION <16>
Whenever the any of the regulated voltages goes out of its specified voltage window the output of A12U51C (for overly
positive voltages) or A12U51D (for overly negative voltages) goes low, causing a12U51 B pin 2 to go low. This causes
A12U51 A pin 1 to go low, which turns off A12Q57, turns on A12Q32, and turns off A12Q33. This allows A12U21 pin 4
(Dead Time) to go high, and thereby shuts off the inverter. A12Q58, A12Q59, and A12C58 hold the inverting input of Al
2U51 A (pin 6) high until the supply voltage is back within its voltage window. At this time, A12Q58 turns on, discharging
the A12R58, A12C58 combination, which allows pin 6 of A12U51 A to go low, thus pulling A12U21 pin 4 (Dead Time) low,
and thereby restarting the inverter.
LOW VOLTAGE RECTIFIERS AND REGULATORS <16>
+12 Volt and -12 Volt Supplies. The rectifiers and the filter components are connected to secondary winding. Regulators
A12U91 and A12U92 provide rectified, regulated positive and negative voltage.
-3.3 Volt Supply. A12U71 is a regulator. If the -3.3 volt supply output is too high, the voltage at pin 5 of A12U71
decreases and the current at pin 9 of A12U71 decreases; that is, the base current of A12Q73 decreases, thus the output
voltage of the -3.3 volt supply is decreased. Transistor A12Q71, A12R80, and A12R81 together form the current protector.
If the -3.3 volt supply is shorted, the voltage on A12R80 and A1 2R81 (over-current sensing resistors) goes high; A12Q71
is turned on, and A12Q73 is turned off.
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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 ...