SERVO AMPLIFIER 1A5

11-5820-921-40-1

2-92. Opening and then grounding the positive motor line, either SERVO IN A or SERVO IN B, makes

it possible to start the motor in the reverse direction by changing the polarity on the other input signal line,

It also provides a rotor position sense signal, LMAX GND from 1A8A3S1 and LMIN GND from 1A8A3S2,

to the tune control and sequencing circuits, depending upon which limit switch is actuated. The LMAX

GND signal goes to Tune Control PWB Assembly 1A4 (see paragraph 2-1 18). The LMIN GND signal goes

to System Control PWB Assembly 1A5 as a PA FAULT (see paragraph 2-65).

2-93. SERVO AMPLIFIER 1A5 (See figure 3-69)

2-94. The servo amplifier has four distinct modes of operation and depends upon control signals from

other circuits of the servo system.

2-95. SERVO DISABLED CONDITION

2-96. When the SERVO ENABLE (ground) signal is not present, pin 8 of 1A5 is at +19.0 Vdc. This causes

the grounding of both servo amplifier output lines (pins 1 and 5 of 1A5) as shown in table 2-2.

TABLE 2-2. SERVO DISABLE CONDITION

TRANSISTOR

SIGNAL AND LOCATION

CONDITION

CHANNEL

1A5Q10

1A5Q4

1A5Q7

1A5Q8

1A5Q9

SERVO IN A (Pi-l)

OFF

GND THRU 1A5Q10

1A5Q11

1A5Q12

1A5Q13

1A5Q14

SERVO IN B (P 1 -5)

1A5Q6

OFF

GND THRU 1A5Q14

2-97. This condition is present when the power amplifier is TUNED and READY to operate. Grounding

the servo amplifier outputs prevents activation of servo motor 1A8A3B1 by spurious signals. The SERVO

ENABLE (ground) signal is provided automatically by Tune Control PWB Assembly 1A4, as the TUNE

(ground) signal, when the power amplifier must be retuned.

2-98. SERVO LMAXFORCE CONDITION

2-99. At the beginning of the TUNE CYCLE, the polarity of the servo amplifier outputs are such that

the SERVO IN B signal is grounded through transistor 1A5Q14 and the SERVO IN A signal is +19 Vdc

through transistor 1A5Q9, This forces torque motor 1A8A3B1 to tune the coil wiper of 1A8A5L1 toward

the LMAX position. To accomplish this, a positive (about + 10V) LMAX FORCE signal is applied to pin 6

of 1A5 from Tune Control PWB Assembly 1A4. Table 2-3 shows the effect of the positive LMAXFORCE

signal in the servo amplifier.

2-100. SERVO LMIN FORCE CONDITION

2-101. At the start of the final tuning cycle (LMAX FORCE cycle completed), the servo amplifier is forced

to turn the rotor of coil 1A8A5L1 toward the minimum inductance (and the correct tune setting) position.

This is necessary because the initial outputs from the phase discriminator are not high enough to control

the servo amplifier. The LMIN FORCE signal (about +10V) from the tune control circuits has the

following effects on the servo amplifier circuits shown in table 2-4.

2-23