Tektronix 491, Maintenance Manual

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c. Check—The amplitude of the SAW OUT signal, should
measure between 70 and 90 mV.
17. Check Unblinking Waveform
o. Equipment setup is given in step 16.
b. Connect the
1×
probe from the test oscilloscope to
pin K of the Horizontal Display circuit board.
c. Check unblinking waveform. Amplitude should mea-
sure between 8.0 V and 9.0 V, typically 9.0 V (see Fig. 6-16).
d. Remove the
1×
p r o b e .
Sweep Circuit
voltage is too high it will produce a display similar to an
amplitude modulated signal, with sidebands 100 kHz to 180
kHz from the IF feedthrough signal. If this type of display is
present reduce the RF amplitude voltage to eliminate the
side bands.
g. Return the DISPERSION RANGE and DISPERSION selec-
tors to the 10 MHz/div positions. Switch the VERTICAL DIS-
PLAY selector to LIN. Remove the probe from pin P of the
honeycomb connector.
19. Adjust Center Frequency Range
o
a. Test equipment setup is given in step 18.
18. Adjust RF Amplitude
o
a. Test equipment setup is given in Fig. 6-14.
b. Set the TIME/DIV selector to 20 ms. Apply a calibrated
200 MHz signal from the Time-Mark Generator (2nd harmonic
of 10 ns sine wave) through a 20 dB attenuator to band B
RF INPUT connector. Switch the RF INPUT selector to band
B. Set the TIME/DIV to 20 ms.
c. Adjust the GAIN control for a displayed IF signal ampli-
tude of 6 divisions. Tune the RF CENTER FREQUENCY control
for minimum converted signal interference.
d. Establish zero volt reference level on the test oscillo-
scope by connecting the probe to chassis ground on the Type
491, then connect the probe to pin P of the square pin con-
nector for the honeycomb assembly. See Fig. 6-17.
Fig. 6-18. Sweeper circuit adjustments.
NOTE
The IF CENTER FREQ controls and the IF CENTER
FREQ-CAL adjustment must be centered (000).
b. Adjust the Center Freq Range R251 (Fig. 6-18) for mini-
mum IF signal shift as the DISPERSION selector is rotated
through the 10 MHz to the .2 MHz positions. The DISPER-
SION RANGE switch must be in the MHz/DIV position for
this adjustment.
c. Return the DISPERSION selector to the 10 MHz position.
20. Adjust Sweep Center
o
F i g . 6 - 1 7 . L o c a t i o n o f p i n P o n t h e h o n e y c o m b s q u a r e p i n c o n -
nector.
e. Adjust the RF Ampl R290 (see Fig. 6-18) for —0.85 volts
±0.1 volt of trace deflection on the test oscilloscope.
f. Switch the VERTICAL DISPLAY switch to LOG position
and check the display with 100 kHz dispersion. If the RF Ampl
a. Equipment setup is given in step 19.
b. Adjust the Horizontal POSITION control to center the
sweep. (¼ of a division sweep extension from either end
of the graticule.)
c. Adjust the Sweep Center R203 (Fig. 6-18) to position
the 200 MHz signal at the graticule center horizontal line.
6 - 1 9