(b) Plus and Minus 15 VDC Supply.

TM 55-4920-401-13&P resistance check circuit (fig. 1-24) and the heater probe power relay (fig. FO-1). (b) Plus and Minus 15 VDC Supply. O ne of the 37 vac secondaries of T1 (fig. 1-25) is full- wave rectified by diodes CR5-CR8 on the switch- power board to forma positive and negative supply. The positive supply is regulated to +15 vdc by Q1, Q2, and A1 and filtered by C3. The + input of Al is referenced to 8.95 vdc generated by CR15. The negative supply is regulated to -15 vdc by Q3, Q4, and A2 and filtered by C4. The + input of A2 is ref- erenced to ground. (c) Display Lamp Supply. On the switch- power board diodes CR1 and CR2 full-wave rectify and C9 filters the 14 vac winding to provide a 5 vdc supply to excite the display diodes. The circuitry of A3 and the DISPLAY control convert the supply to an adjustable square wave by controlling the on-off time of series regulator Q15. At the 50% duty cycle, the square wave frequency is 800 Hz + 20%. When the DISPLAY control wiper is at the +15 vdc end the display is full on, and when the wiper is at the - 15 vdc end the display is full off. (d) Logic Voltage Supply. The 18.5 vac winding of T1 is full-wave rectified by CR3 and CR4 cm the switch-power board, filtered by C5, and regu- lated to +5 vdc by Q5. Q5 is controlled by a differ- ential amplifier consisting of Q8 and Q9 and by transistors Q6 and Q7. The differential amplifier maintains its base voltages at ground potential. As- sume an increase in the base voltage of Q9. Addi- tional current from Q9 flows through R13, de- creasing the bias on Q8. The bias on Q7 increases, causing the bias on Q6 to decrease. The base of Q5 becomes more positive, decreasing the current through Q5. (e) Nine VDC Supply. This supply is lo- cated on the probe control and function switch board. It is powered by one-half of a 37 vac winding which is half-wave rectified by CR12 and filtered by C11. CR13 regulates the dc voltage to 18 vdc and CR14 regulates it to 9 vdc. The 9 vdc is switched by the FUNCTION SELECT switch to power the air- craft indicator check circuit (figs. 1-18 and 1-19), the PROBE CONTROL and probe control compen- sator (MOD-1, fig. FO-1), and the insulation check circuit (fig. 1-23). (f) Plus and Minus 7 VDC Supply. T h is supply, on the calibrator board provides an isolated source of power to drive the % rpm calibrator circuit and the wave-shaping circuitry of A3 on the ta- chometer board. (11) Logic Circuitry. (a) Definition of Logic Levels. Standard positive logic is used. Logic 0 is a low voltage (= 0.4 vdc) and is said to be “false”. Logic 1 is a high voltage (= 2.4 vdc) and is said to be “true”. The logic circuitry excitation voltage is +5 vdc. 1-25