Maintenance—492/492P Service Vol. 1 (SN B030000 & up)
e. Rotate the CENTER FREQUENCY control counter
clockwise until the analyzer stops tuning, then rotate the
control clockwise to center a pair of markers on the display.
Adjust the REF LEVEL as needed.
f. Note the spacing of the markers to the nearest 0.5
minor divison. Enter this data on a graph of spacing versus
marker pair number.
g. Tune the frequency up to the next pair of markers and
center on the display. Note the spacing and again plot on
the graph.
h. Repeat this measurement process for each suc
cessive marker pair until the end of the tuning range is
reached. (This requires approximately 38 measurements.)
The finished plot depicts the tuning sensitivity variation over
the tuning range of the 2nd LO. Aberrations or ripple in the
plot with a period of two to four points and amplitude of
about one minor division peak-to-peak are due to discrete
gain breakpoints in the shaping circuit. A single large dis
continuity (approximately 2.0 minor divisions peak-to-peak)
indicates a defective shaper resistor or diode.
i. Note the mean slope of the plot. Disconnect one end of
R1070 and carefully measure its resistance with the digital
meter. Record this value on the tuning plot.
j. Increase the value of R1070 approximately 10% if the
plot has positive slope. Decrease the value if the slope is
negative. Record the new value and reconnect the
potentiometer.
k. Make a new plot as outlined in parts b through g and
compare the slope of the new plot against the previous plot.
Interpolate or extrapolate from the two slopes and the resis
tance values to determine the resistance necessary to ob
tain zero average slope.
I. If the plot is convex (upward curvature toward the
ends), decrease the resistance of R1068. Increase the value
if the curvature is downward. The procedure for this adjust
ment is similar to that described for R1070 in parts h and i.
Change the resistance of R1068 approximately 10%; then
rerun the plot and interpolate the resistance value until you
have minimized curvature. This adjustment may affect the
slope because of interaction; therefore, it may be necessary
to repeat the adjustment of R1070. When R1068 and
R1070 are properly adjusted, the tuning plot should ap
proach a straight line with zero slope. Peak-to-peak vari
ation across the plot should not exceed two minor divisions,
including breakpoint ripple. Shape of the plot is not impor
tant if the total excursions do not exceed two minor divi
sions (p-p). An S-shaped plot is not uncommon.
m. Measure the resistance of the potentiometers for
R1068 and R1070 and replace with standard value 1%,
1/8 W resistors. Do not replace R2063 at this time.
6. Set the Center Frequency of the 14-22 MHz Oscillator
a. Switch the FREQ SPAN/DIV to 100 kHz or more;
then connect a frequency counter to the 18 MHz output port
P224.
b. Select the jumper tap combination as shown in Fig.
4-23, so the center frequency is within 500 kHz of 18 MHz.
7. Check and Calibrate Tune Sensitivity
a. Set the FREQ SPAN/DIV to 100 kHz then to 0. Verify
that the tune voltage at TP4044 on the Center Frequency
Control board is 0 V, ± 50 mV.
b. Measure the 14-22 MHz Oscillator frequency at the
18 MHz output port (P224) to the nearest 10 kHz and
record the frequency.
c. Monitor the voltage at TP4044 on the Center Fre
quency Control board and turn the FREQUENCY control
clockwise until the voltage stops changing.
d. Adjust the Fine Tune Range, R4040, on the Center
Frequency Control board for a voltage of —9.38 V at
TP4044.
e. Adjust the Gain resistor, R2063, on the 14-22 MHz
Phaselock board so the counter frequency is 2.25 MHz
greater than the number recorded in part b.
f. Remove the potentiometer for R2063 and measure its
resistance; then replace it with the nearest standard 1%,
118 W resistor.
8. Conclusion
a. Replace the housing lid with its 14 screws.
b. Tighten the screws sequentially, starting from the
center of the lid and progressing toward the corners to pre
vent gaps between the lid and the housing. Use care to not
strip the screws as you tighten them.
4-34
REV JUN 1983
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