Adjustment Of Hum Balance; Calibration Of Voltmeter - HP 206A Operating And Servicing Manual

1.
With the generator s e t to deliver 1,000 cycles, connect a high-
impedance vacuum tube voltmeter to pin 4 of tube V3, and
m e a s u r e the input to the selective amplifier.
2.
Connect the voltmeter to pin 8 of tube V5, and m e a s u r e the
amplifier output. The gain through the amplifier should be
approximately 24 db (+. 5 db).
3.
If amplifier gain i s too low o r too high, adjust variable r e s i s t o r
R96 (Fig. 4) to obtain approximately a 24 db gain.
N O T E
If with a gain of 24 db the waveform a t
the OUTPUT terminals shows clipping,
lower the gain of the amplifier until clip-
ping stops.
ADJUSTMENT OF HUM BALANCE
Variable r e s i s t o r R79 (Fig. 4) provides an adjustment for mini-
mizing the beat between the oscillator and the power supply f r e
-
quency. Either of two methods may be used for the adjustment.
1.
If a distortion analyzer such a s a Hewlett-Packard Model 330B
o r 330C i s available, adjust r e s i s t o r R79 f o r minimum d i s t o r -
tion in the output waveform with the instrument generating 5 to
20 cycles above o r below the power line frequency.
2.
If a distortion analyzer i s not available, output amplitude v a r i -
ation (60-cycle beat) may be observed on a sensitive high-im-
pedance vacuum tube voltmeter, and r e s i s t o r R79 adjusted f o r
minimum variation i n output amplitude with the instrument
tuned to a frequency near that of the power line. F o r example,
where the power line frequency i s 60 cycles, the Model 206A
should be tuned with the FREQUENCY dial for maximum beat
(approximately 58 cycles), and then r e s i s t o r R79 should be
adjusted f o r minimum beat.
4-10 CALIBRATION OF VOLTMETER
Calibration of the control panel voltmeter i s s e t a t the factory, and
unless one of the c r y s t a l diodes in the rectifier circuit r e q u i r e s r e -
placement o r the voltmeter should be smashed, i t i s unlikely that
the voltmeter will r e q u i r e recalibration during the life of the Model
206A. To recalibrate the voltmeter, a comparison m e t e r standard-
ized f o r 4 volts a t 100 cycles i s required.