Tektronix 491 Maintenance Manual page 118

Fig. 6-33. Adjusting avalanche voltage.
just below the state of free running avalanche. Free running
avalanche, appears as an RF burst signal between the 1 MHz
pulses as shown in Fig. 6-33C; or with the INT REF FREQ
control in the OFF position, the free running avalanche
transistor signal will feed through to the 1 MHz MARKERS
OUT connector, and appear on the test oscilloscope as a
2 MHz burst. If free running avalanche does not occur, turn
the adjustment fully clockwise.
6-30
F i g . 6 - 3 4 . L o c a t i o n o f L 1 1 0 4 a n d 1 1 1 0 8 , R 1 1 5 0 a n d R 1 1 4 0 .
e. Switch the band selector to band C. Turn the INT REF
FREQ control on.
f. Push the LOCK CHECK button. Check for beat signal
displays as the FINE RF CENTER FREQ control is rotated
through its range.
27. Adjust 1 MHz REF FREQ Range
a. Equipment set up is given in step 26.
b . A p p l y t h e o u t p u t f r o m t h e 1 M H z M A R K E R S O U T
signal through a 10× attenuator to J120 on the honeycomb
assembly as follows: Connect a GR to Sealectro adapter
(such as a P6040 probe cable), a GR to BNC adapter and a
20 dB attenuator, (see Fig. 6-32) between the 1 MHz MARK-
ERS OUT connector and J120. Disconnect the Sealectro
connector from J120.
c. Set the DISPERSION to 100 kHz/Div and position a
1 MHz marker to the screen center with the IF CENTER
FREQ control.
d. Adjust L1108 (Fig. 6-34) for a positive oscillator start
when the INT REF FREQ control is turned from its OFF to
ON position. The signal position on the screen should be
consistent as the INT REF FREQ control is switched from
OFF to the initial ON position. (The control must not be
turned past the initial ON position or the oscillator frequency
will be changed.]
e. Rotate the INT REF FREQ control through its range
Check the total frequency shift of the internal reference
1 MHz marker.
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