of the stop on the rear of the panel bearing. When the capacitor is completely
chsed, set the main tuning dial so that the dot to the Left of t135tt on the main 4
tuning dial is exactly under the hairline. Tighten the coupler with the tuning dial
and tuning capacitor in these relative positions, Then tighten the collar of the
stop so that the tuning capacitor rotor cannot strike the frame or stator at either
end of its arc.
After the coupler has been tightenedand the dust cover replaced, it is
desirable to check the calibration of the instrument.
Frequency Calibration --
If a change occurs in the frequency calibration of this instrument after a
long period of use, the cause of this change probably lies in the ageing of the
resistors in the frequency-determining network. These resistors have extremely
good stability, however, and the frequency calibration will remain accurate for
a very long period of time
, Each of the frequency-determining resistors consists of a precision one-
watt resistor in series with a one-$half watt resistor. The latter is selected to
give the instrument a very accurate f-requency calibration. Therefore, adjusting
the frequency calibration requires that the value of these small resistors be
changed as necessary to obtain proper calibration.
I
An oscilloscope, an ac vacuum tube voltmeter, and a secondary fre-
quency standard are needed to adjust the calibration of the Model 200A. The
secondary standard should have sinusoidal output and should provide frequencies
of at least 1 kc and 10 kc. Before recalibrating, allow the Model 200A to heat
foi. thirty minutes or more, Then connect the Mo*del 200A to the vertical deflec-
ting-plate input of the oscilloskope and the secondary frequency standard to the
horizontal deflecting-plate input.
Next, check the calibration of the Model 200A by means of Lissajous
figures on the oscilloscope, determining whether the frequencies generated by
the M.odel 200A are in general higher or lower than the dial calibration. When
making this check, it is necessary that the dust cover be tightly on the instru-
ment as the frequency calibration will change when the dust cover is loose or
off the instrument.
If the instrument is in need of recalibration, the dial calibration will
show a definite trend of error-either higher or lower. than the true output fre-
quency for each range. The various ranges of the instrument do not necessarily
change in the same "directionI1.
If the xl range requires readjusting, change the small resistors R1A and
R4A. If the ouput frequency is higher than the dial calibration, increase the
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ACEC
