Measuring Circuit - TM-11-6660-204-250023

TM 11-6660-204-25

the input to the detector circuit (para 213d).

resistor R209; and the third is through slide-wire

Discharge current through load resistors R203

potentiometer R501B.

and R207 will be proportional to the voltage built

b. Path B is replaced with potentiometer B (fig.

up across the

capacitor combination.

2-10), which represents the slide-wire circuit.

The zero adjust circuit is eliminated. The resist-

e. The voltage across the

capacitor combi-

ance from the wiper arm of potentiometer R202,

nation is directly proportional to the charge (in

through either leg to circuit ground, depends on

coulombs ) stored there which, in turn, is directly

the position of the arm (fig. 210). The sum of

proportional to the rate it receives pulses, because

the currents in path A and path B is practically

each pulse delivers a uniform charge. The dis-

constant, so the current in path B is proportional

charge current at any instant is almost equal to

to the resistance in path A, and is adjusted by

the voltage across the capacity divided by 385

potentiometer R202. The voltage across the slide-

(the resistance of R203) ; this value would not be

wire can be changed by adjusting potentiometer

exact because of the slight effect of the calibrating

R202, the position of the balance point can be

voltage and current involved in resistor R207.

changed with a corresponding change of the re-

These facts all indicate an output voltage that is a

corder pen position. This method is used to align

linear function of the audio frequency of the ini-

t h e reference (calibration) signals with the

tial input signal to the converter, and can be cali-

correct frequency indication on the chart. The

brated on a suitable indicator.

voltage from the slide-wire circuit is fed to the

center tap of the primary of transformer T302

f. The output voltage (that potential which ex-

ists mainly across resistor R207, because resistor

(fig. 2-9) through trolley-wire resistor R501A,

where it is compared with the dc voltage devel-

R203 is comparatively very small) will range

oped by the input signal.

from near 0 to approximately 30 millivolts, which

is again filtered before being applied to the vibrat-

c. The recorder test circuit consists of two re-

ing reed of inverter G301 (para 213).

sistors, R210 and R212, in series with REC.

TEST ADJ. potentiometer R211 and is in parallel

with the slide-wire, The wiper arm of potentiome-

2-12. Measuring Circuit

ter R211, as determined by the resistance values,

(fig. 2-9)

will control 91 to 97 percent of the voltage being

applied to the slide-wire, through the contacts of

The measuring circuit provides a dc voltage of

REC TEST Switch S204 to the vibrator, Poten-

approximately 30 millivolts across slide-wire po-

tiometer R211 is adjusted so the recorder pen will

tentiometer R501B. This is used as a comparison

print at 95 chart divisions.

voltage for the dc signal voltage developed across

load resistor R203. Slide-wire potentiometer

d. The zero adjust circuit (path C, fig. 29) is

R 5 0 1 B and trolley-wire resistor R501A are

similar to the reference voltage circuit explained

mounted on a common shaft in the frequency-time

in b above. A voltage, determined by the position

recorder chassis. The common shaft is mechani-

of the wiper arm of REC. ZERO potentiometer

cally coupled to balancing motor B501, recorder

R208, is obtained across resistor R207. The volt-

pen, and the indicator pointer. The remaining

age across resistor R207 is applied through load

parts of the measuring circuit are mounted on the

resistor R203 to detector circuit G301 and is used

control panel. Zero adjust and recorder test cir-

to balance out the slight voltage from the slide-

cuits are also included in the measuring circuit,

wire circuit when the recorder pen is at zero and

the slide-wire wiper arm contact is about one-half

a. Current from the regulated +300 volts is fed

of one percent of full scale position from the

t o the arm of REF. ADJUST potentiometer,

ground end of the slide-wire resistance.

R202, through R201. The current flowing through

potentiometer R201 is divided into two main cir-

2-13. Detector and Balancing Circuit

cuits (paths A and B, fig. 2-9). (A slight current

(figs. 2-9,2-11, and FO-2)

which flows through path C is considered later. )

Path A is through resistor R206 to ground. Path

The detector circuit detects any difference (or

B is through resistor R205 and then divides into

error) between the millivolts of potential created

three parallel resistive paths to ground. One

by the incoming signal and fed to the vibrating

branch is through a series circuit which consists

reed of G301, and the potential delivered by the

of resistors R210, R212, and REC. TEST ADJ.

wiper arm of the slide-wire potentiometer to the

potentiometer R211; another branch is through

center tap of the primary of transformer T302.