(from TM 11-866)
21. POWER SUPPLY UNIT. -- Power Supply Units RA-74-C, RA-84-B, and RA-94-A are alike except for variations in the power transformers (T1, T2, and T3 respectively). These differences are described in detail in PAR. 2 and PAR. 7e. The power unit furnishes "A", "B", and "C" voltages for the receiver. The "A" or heater voltage, is 6.3 volts a-c obtained from a separate secondary winding (1-2) on the power transformer. "B" voltage is obtained from the center-tapped high-voltage secondary (7-8-10) connected to the plates of the "B" rectifier tube VT-145 (V1). After this voltage is rectified, it is filtered by the combined action of the first filter choke (L1) and the first two 8 microfarad sections of filter capacitor C1. This provides 380 volts d-c for the plates of the power output tubes in the receiver. Further filtering by the second filter choke (L2) and another 8 microfarad section of C1 provides 250 volts d-c for the plates of the remaining tubes in the receiver. Approximately 100 volts d-c for the screen grids of the receiver tubes is obtained from the tap on the bleeder resistor (R1), which is by-passed by the remaining section of C1. Negative "C" voltage is obtained from a tap (9) on the high voltage secondary connected to the filament of the "C" rectifier tube VT-80 (V2). The rectified output from the plates of this tube (V2) is filtered by the three 8000 ohm sections of resistor R2 and the four 3 microfarad sections of filter capacitor C2. When connected to the receiver, the voltage at the end of this filter is approximately minus 50.
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