eliminated. A complete test is not always required 1000 cps will normally be from 0.3% to 0.5%" This
after repair of an individual section in the 650A, If, exact figure is not critical as long as the distortion
for example, only the output amplifier has been re- measured at the instrument output terminals does not
paired, those steps pertaining to adjustment and cali- exceed 1.0%.
bration of the two oscillators should be omitted.
Correct for excessive low frequency oscillator distor-
4-6A HEATER AND REGULATED POWER tion by lowering the oscillator voltage or by replacing
tubes V11 and/or V12.
SUPPLY VOLTAGES
Adjust heater voltage and output of regulated power 4-6E LOW FREQUENCY OUTPUT DlSTORTlON
supply as instructed in paragraph 4-5.
4-66 PRELIMINA RY LOW FREQUENCY Measure distortion at 30, 1000, and 15,000 cps with
distortion analyzer and a 600 ohm resistive load con-
RESPONSE ADJUSTMENT nected to output terminals. Distortion can be measured
a. Place a 600 ohm resistive load across output term- at 100 KC by means of an elimination filter as instruc-
inals, set FREQUENCY dial to "l", switch to the ted under High Frequency Distortion.
"XlKC" range, and adjust AMPLITUDE control to any
convenient reference level on an external vacuum tube Distortion in instrument output signal when lxned at
voltmeter connected across output terminals. This any point between 10 cps and 100 KC should not ex-
external ac VTVM must have known frequency re- ceed 1.0%.
sponse characteristics and must be accurately cali-
brated. 4-6F CALlBRATlON OF LOW FREQUENCY
OSClLLA TOR
b. Turn FREQUENCY dial to "10" without disturbing
any other control settings. The reading obtained Suggested setups for low frequency oscillator cali-
on the external meter should be the same as the ref- bration and response checks are given in Figure 4-3.
I erence reading established in step &. The output fre-
quency should be 10 KC as indicated by range switch Any high impedance ac voltmeter capable of accurate
and FREQUENCY dial positions. Adjust trimmer voltage measurements from 10 cps to 100 KC can be
capacitors C23 and C24 simultaneously to obtain a used.
10 KC output signal having an amplitude equal to the
reference level of step a. a. Connect a 600 ohm resistive load and frequency
measuring equipment to 650A output terminals.
If both frequency and amplitude requirements cannot See Figure 4-3.
be obtained, adjust to secure the correct amplitude
at a frequency as near to 10 KC as possible. Final b. Switch to the "XlKC" range and set FREQUENCY
frequency calibration will be completed in a later step. dial to"1O"which provides an output signal of 10 KC.
c. Repeat steps g and IJ. c. Adjust frequency calibration at this dial point by
adjusting trimmer capacitors C23 and C24. The
4-6C LOW FREQUENCY OSCILLATOR output amplitude may be disregarded at this time.
OUTPUT VOLTAGE
d. Set FREQUENCY dial to "5" and measure output
Measure ac voltage at junction of R47, R54, and C28 frequency which should be 5 KC. If frequency is
with 650A tuned to 1000 cps. This oscillator voltage off, remove knob directly under dial window and loosen
should be between 18 to 22 volts with approximately but do not remove four exposed Allen screws that
19 volts being optimum in most instruments. secure dial to hub.
Large deviations from this voltage can be corrected e. Set 650A for a 5 KC output signal. Slip frequency
by installing a new lamp for R80. Small deviations dial on hub to obtain a dial reading of "5" and re-
can be corrected by adjusting control R46. A poten- tighten Allen screws.
tiometer can easily be installed for R46 in all instru-
ments with Serial No. 3399 or below as detailed in f. Repeat steps IJ, 5, ci, and e.
paragraph 3-9, Modernization Procedure.
g. Check dial calibration with dial set to "1" and other
If recovery time (indicated by low frequency flutter points between "1" and "10". If output frequency is
I in output signal amplitude) is excessively long when not 1 KC with dial setting of "l", it may be necessary
switching ranges, slightly increase low frequency to change value of 1/2 watt padding resistors in series
oscillator output by adjusting R46. If trouble persists, with R9 and/or R12. (See CAUTION that follows.) If
1 replace oscillator tubes V11 and/or V12. calibration error is not more than 2%, change either
resistor value. If error is more than 2%, change both
4-60 LOW FREQUENCY OSClLLATOR values an equal amount as an unbalanced condition will
DlSTORTlON cause the oscillator to operate at a different level than
the other ranges. Increasing resistor values will de-
Low frequency oscillator distortion measured at junc- crease oscillator frequency with any given tuning
tion of R47, R54, and C28 with instrument tuned to capacitor setting.
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ACEC
