Calibrating Wien Bridge Circuit

TM 11-6625-25514/TO 33A113170-1

q. Adjust NOISE GAIN control R5 to produce

follows:

a meter reading of exactly +20 dB.

a. Set the BALANCE control to its midrange

position.

718. Calibrating Wien Bridge Circuit

b. Set the frequency RANGE switch to X10.

Adjust the bridge circuit as follows:

c. S e t t h e A F - R F s e l e c t o r s w i t c h t o A F

a. Set BALANCE control R23 to its midrange

(TS-723A/U).

position.

d. Set the meter range switch to +20 DB.

b. Set the frequency RANGE switch to X10.

e . Set the function switch to DISTORTION.

c. S e t t h e A F - R F s e l e c t o r s w i t c h t o A F

f. A d j u s t t h e s i g n a l g e n e r a t o r t o 1 , 0 0 0 H z .

(TS-723A/U).

Check the signal generator output with the

d. Set the signal INPUT control to MAX.

frequency counter.

e. Set the meter range switch to +20 DB.

g. Adjust the FREQUENCY tuning control

f. Set the f u n c t i o n s w i t c h t o D I S T O R T I O N .

knobs to obtain a meter indication of zero on the

Check to see that the FREQUENCY tuning dial is

top meter scale. Decrease the meter range switch

not set near 100.

s e t t i n g to -30 DB. Adjust the FREQUENCY

g . Connect the signal generator to the AF

tuning control knobs to obtain the maximum left-

INPUT binding posts. Adjust the signal genera-

hand meter deflection of the meter pointer. The

tor for 1,000 Hz and increase its amplitude until

m e t e r pointer must deflect to the left of -10

t h e spectrum analyzer meter indicates exactly

DECIBELS (equivalent to 60 dB down from the

+20 dB.

o r i g i n a l +20 DB reference setting),

h. Turn the FREQUENCY tuning dial at

h. Securely hold the capacitor drive shaft i n

exactly 100; this is equivalent to 1,000 Hz.

this position with a pair of pliers, gripping the

i. The meter pointer should deflect to zero on

drive shaft collar on the inside end of the shaft.

the top scale. Decrease the setting of the meter

Remove the upper FREQUENCY tuning control

range switch one step at a time. The meter should

knobs by loosening the two setscrews. Slip the

return to zero on the top scale with each setting of

F R E Q U E N C Y tuning dial around its hub until

the switch. In the -30 DB position of the switch,

the 100 mark lines up exactly with the line down

t h e meter pointer should be to the left of -10

the center of the FREQUENCY tuning dial

DECIBELS (equivalent to 60 dB down from the

w i n d o w . Tighten the four Allen-head screws in

o r i g i n a l reference setting of +20 DB). If the

the collar of the frequency dial shaft by inserting

p r o p e r meter indications are observed, do not

an Allen-head wrench through the shaft hole in

make any adjustments to the bridge circuit. If the

t h e front of the front panel. Replace the upper

proper meter indications are not observed, pro-

FREQUENCY tuning control knob.

ceed to j below,

720. Recalibrating FREQUENCY Tuning

j. Set the meter range switch to the position

w h e r e the best null indication was obtained ( i

Dial

When variable capacitor C13 is replaced, it may

above). Minutely adjust trimmer C12 and balance

b e necessary to recalibrate the FREQUENCY

setting control R89 until the proper meter

t u n i n g dial. The old FREQUENCY tuning dial

d e f l e c t i o n is obtained. W o r k b a c k a n d f o r t h

may be reused by turning it over, or a new blank

between those two controls as required. Do not

c h a n g e the setting of the FREQUENCY tuning

dial may be requisitioned. Recalibrate the

F R E Q U E N C Y tuning dial as follows:

dial.

k. If the proper meter deflection cannot be

a. Replace the old FREQUENCY tuning dial

obtained (j above), replace switch assembly S3

with a new blank dial or turn the old dial over.

and repeat j above.

R e m o v e the FREQUENCY tuning dial window.

Using the FREQUENCY tuning dial window as a

NOTE

template, cut a piece of thin metal identical with

The 12 resistors attached to switch

t h a t of the FREQUENCY tuning dial window.

a s s e m b l y S3 are factory-selected and

Scribe the metal with a centerline on the FRE-

cannot be replaced individually,

Q U E N C Y tuning dial window. Measure three-

719. Adjustment of FREQUENCY Tuning

eights of an inch from the edge of the plate where

Dial

the mounting holes are located and scribe a line at

W h e n the capacitor drive assembly has been

right angles to the centerline. Cut along this line

disassembled, or if the screws that retain the dial

to the centerline. Cut down the centerline to the

s h a f t have become loosened through vibration,

right-angle line and remove the rectangular piece

t h e FREQUENCY tuning dial may shift out of

of metal. Attach the notched metal plate to the

position. Return the dial to its proper position as

face of the mounting chassis in place of the FRE-

7-18