Figure 2-11. Detector and balancing circuit, simplified schematic diagram.

TM 11-6660-204-25

Figure 2-11. Detector and balancing circuit, simplified schematic diagram.

phase shifting, creates a phase difference of ap-

(more positive) than the potential on the wiper

proximately 90 between the output of the servo-

arm of the slide-wire. The vibrating reed of G301

power amplifier and the fixed phase of the line

alternately switches this higher potential between

power. This phase difference is either a lag or a

the two ends of the transformer primary winding.

lead, depending on the polarity of unbalance at

Current flow will ocur during each contact and

the detector input. The direction of motor rotation

the resulting alternating magnetic field in the

depends on the lead or lag relationship between

transformer core will induce an alternating sec-

the two field windings; the motor leads must be

ondary voltage and frequency which is still equal

connected properly so that the motor will rotate in

to the line frequency (nominally 60 CPS).

the right direction to cancel an error voltage.

b. The vibrating reed is polarized in its motion

d. The damping filter in series with the dc sig-

and always hits contact No. 2 for a given polarity

nal applied to the vibrator consists of a three

of voltage across its coil, and contact No, 4 for the

section RC filter network, R333, R334, R335, and

other polarity. The polarity of the hot side of the

C315. Resistor R336 is used to damp the primary

secondary windings of transformer T302 depends

winding of transformer T302 and reduce voltage

on two things: which of the two contacts (No, 2

transients. The damping filter performs two func-

or No. 4) the reed has just hit, and whether the

tions: First, it acts as a filter for the higher pulse

signal voltage on the reed is positive or negative

recurrence frequencies so only the dc portion of

with respect to the wiper arm. Therefore, the

the voltage developed in the output gate circuit is

phase of the secondary voltage depends solely on

applied to the detector circuit. Secondly, it acts as

the polarity of unbalance of the two dc voltages.

a damping circuit to prevent overshooting of the

This phase sensitiveness to the direction of unbal-

recorder balance point (hunting. ) This is an im-

ance turns the motor in whatever direction is nec-

portant feature included in the design of the

tssary to establish a dc balance.

equipment and when this equipment is repaired,

or parts are replaced, the proper values of circuit

c. The balancing motor is a low-power two-

components must be used to avoid lose of the

phase squirrel-cage induction type with two field

damping action. Damping action is explained as

windings. One field winding is powered by the ac

follows: Assume that an unbalance exists between

power line as a fixed phase which cannot be

the dc voltage applied to the detector circuit from

changed except by a change in wiring; the other

the output gate and the dc voltage developed by

is the control field winding. Phase-shifting capaci-

the slide-wire, and that the output gate voltage is

tor C317, in conjunction with some amplifier

2-14