(c) Standard Day Correction Circuit Schematic

TM 55-4920-401-13&P An investigation of these equations will show that standard day readings are lower than uncorrected readings when the ambienet temperature is above standard day temperature (59° F or 15° C) and higher than uncorrected readings when the ambi- ent temperature is below standard day tem- perature. (c) Standard Day Correction Circuit Schematic. Figure 1-22 is a schematic diagram of the standard day correction circuit. Note that when the STD DAY switch is on, Q17 on the temperature board turns off, removing the “uncorrected” refer- ence divider from the temperature converter, and Q18 turns on. Likewise, Q8 on the tachometer board turns off and Q9 turns on. Note also that when stan- dard day corrections are being made, temperature amplifier A1 becomes referenced to the combined effects of two voltage sources, -9 vdc and the output of A1 on the calibrator board. The AMB TEMP po- tentiometer adjusts the gain of A1 on the calibrator board in relation to ambient temperature. If ambi- ent temperature increases, the gain of A1 is in- creased and causes higher negative reference volt- ages to be generated by A4 on the temperature board and A7 on the tachometer board. Higher ref- erence voltages decrease the reference integration period for a given input signal and consequently create lower readings. The circuitry of A5 and Q10 on the tachometer board increases the slope of the corrected % rpm curve above 49° F (9.4° C) ambient temperature setting. Q10 turns on at the 49° F am- bient temperature setting and parallels resistors R63 and R64. (7) Heater Probe Control Circuit. T h e bestir probe control circuit regulates the applica- tion of power to heater probes and allows them to be heated to the set temperature. (a) When the FUNCTION SELECT switch (fig. FO-1) is turned to HEATER PROBE position, 28 vdc from the switch energizes K1, applying input power to transformer TB1. T1B steps up the voltage to 135 vat. This 135 vac heats the probes and is con- trolled by SCR’s 1B and 2B. (b) It is desirable to apply heater voltage to the heater probes when the voltage is passing through 0 vac in order to prevent the generation of voltage spikes and the accompanying radio fre- quency interference. This is accomplished by con- trolling the inputs of comparator A2 with 18.5 vac (test point 42), which is in phase with the heater probe excitation voltage. (c) Setting the desired probe temperature with the PROBE CONTROL adjusts the dc level at the inverting (-) input of comparator A1. Porbes are heated when this level is more positive than the heater probe thermocouple signal level at the non- inverting (+) input. Added to the dc level at the - input is an exponential voltage of approximately 3 Hz generated by unijunction oscillator Q3. This voltage, having an amplitude equivalent to approxi- mately 10° C, provides the time base for the heater duty cycle when the set temperature is being ap- proached and lessens probe temperature overshoot and undershoot. (d) When the circuit is requesting heat, the output of A1 goes to -15 vdc, back biasing diode CR1. Test point 4.2 (table 4-13) alternates between +0.7 vdc and -1.4 vdc and Q1 turns on when the test point becomes -0.7 vdc or more negative. Q1 assures that Q2 is off at the beginning of the next positive alternation after the output of A1 goes to -15 vdc. When Q2 is off, comparator A2 is allowed to gener- ate trigger pulses in transformer T1A and trigger SCR2A. The + input (test point 42) of A2 momen- tarily is more positive than the - input (test point 44) when the + input passes through the -0.7 vdc level at the leading edge of the positive alternations of 18.5 vac (test point 43). When the + input of A2 exceeds the - input, the output of A2 goes to +15 vdc and generatest a pulse in T1A. See tes point 45 waveform in table 4-13. (e) When a trigger pulse is generated in T1A, conduction through the heater probes con- tinues for one or more complete cycles depending on the heat demand. The trigger pulse from T1A turns on SCR2A. SCR2A triggers SCR2B, a power SCR mounted on a heat sink inside the probe control. Current from T1B flows through the heater probes and SCR2B during the positive half-cycle of heater voltage. Also during the positive half-cycle, C9 charges through CR10, R23, and SCR2B. When the heater voltage passes through 0 vac at the leading edge of the negative alternation, SCR2B turns off. The positive voltage on C9 (approximately 75 vdc) turns on SCR1A through R22 and CR11. SCR1A triggers SCR1B, the other power SCR. SCR1B al- lows current to flow through the probes during the negative half-cycle. During the negative half-cycle, C9 discharges through CR8, CR9, and R23. (8) Insulation Check Circuit. This circuit (fig. 1-23), located in the probe controller, measures the insulation resistance between the aircraft ther- mocouple harness and aircraft ground. The INSU- LATION CHECK meter is the ohmmeter of a cali- brated ohmmeter circuit having two ranges, RX100 and RX1000. The circuit is powered by 9 vdc from a supply on the probe control and function switch 1-21