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
