Figure 1-20. Partial Schematic of Tach Converter.

TM 55-4920-401-13&P 1-17) sets the temperature to which heater probes are heated. Eighteen and one-half vac from a sec- ondary winding of transformer T1 in the trimmer is half-wave rectified by CR12 on the probe control and function switch board. This rectified voltage is filtered by C11 and C12 and voltage regulated to 9 vdc by R27, R28, CR13, and CR14. The 9 vdc passes through deck 3 of the FUNCTION SELECT switch when it is in HEATER PROBE position and excites the PROBE CONTROL potentiometer. Current from the PROBE CONTROL wiper passes through R19 and R20 on the probe control circuit. The volt- age developed across R20 represents the heater probe set temperature and is applied directly to the input filter of the temperature indicator, (4) % RPM Indicator. The following is a de- scription of circuitry that is unique to the % RPM indicator. The indicator measures the rotational speed (% rpm) of the low pressure (N1) and high pressure (N2) compressors. The selected tachometer signal is applied through C17 and R30 on the ta- chometer board to the inverting input of amplifier A3. Connected to the noninverting input is the com- mon signal return. Diodes CR2 and CR3 clamp the input signal level to 0.7 vdc. The output of A3 is ad- justed to a symmetrical square wave by R26. Nega- tive half-cycles of the input signal cause optical coupler OC1 to conduct and turn on Q13. Plus 15 vdc from Q13 turns on Q12 (fig. 1-20) during the nega- tive half-cycle. During the positive half-cycle, Q13 turns off and -15 vdc through R37 turns on Q11. When Q12 turns on, C19 is charged by +9 vdc through two precision resistance paths. One path contains R47 and the other path contains R48, R49, and R50. When Q12 turns off andQ11 turns on, C19 discharges through two precision resistance paths. One path contains R48 and the other path contains R47, R49, and R50. Since resistors R47 and R48 are equal (3.7K ), the resistance path containing R50 is equal for both charge and discharge current. Both the charge and discharge currents develop equal positive pulses across resistor R50. These pulses charge filter capacitors C20 and C21 at the input of amplifier A4. The gain of A4 is adjusted with R44 and R45 and the zero calibration point is adjusted with R41 when the % RPM indicator CAL ADJ pot is set to the center of its range. Figure 1-20. Partial Schematic of Tach Converter. 1-19