Tektronix 492 Service Manual page 145

Maintenance—492/492P Service Vol. 1 (SN B030000 & up)
b. Apply 5 markers from a time mark generator to the
m s
RF INPUT and tune one of the markers to center screen.
Reduce the FREQ SPAN/DIV to 50 kHz. Adjust the REF
LEVEL for a signal level of 2 to 3 divisions.
c. Turn the FREQUENCY control counterclockwise until
the center frequency stops tuning; then turn clockwise until
a marker signal is one major division from the left edge of
the graticule. Another signal, one major division from the
right edge, should also be displayed. Spacing between the
left and right markers should be approximately eight
divisions.
d. Note the spacing between the left and right marker
signals to the nearest 0.5 minor division.
e. Turn the FREQUENCY control clockwise, to increase
center frequency, until the next marker signal is one division
in from the left edge and again note the spacing between
this marker and the marker near the right edge.
f. Continue this process of tuning up in frequency until
the center frequency stops tuning, noting the signal spacing
at each check point.
4-32
g. If the total spacing variation is two minor divisions or
less, the linearity is satisfactory and you can proceed to part
6 (Check Center frequency of the 14-22 MHz Oscillator). If
the error was more than two minor divisions, continue with
part 4 (Coarse Linearity Adjustment).
4. Coarse Linearity Adjustment
The shaping circuit has a wide range of adjustment to
compensate for variations in the tuning varactor character­
istics. This range of adjustability, however, makes the circuit
calibration tedious. This coarse adjustment procedure
presets the component values to make it easier to perform
the fine adjustments.
a. Replace select resistors, R2063 (Gain) with a 10 kfi
potentiometer, R1070 (2nd Order Shape) with a 5 kii poten­
tiometer, and R1068 (3rd Order Shaper) with 2 kn potenti­
ometer (see Fig. 4-22). Center all potentiometers. Install a
wire jumper between T1077 and T1075 in the normal posi­
tion as illustrated in Fig. 4-23. Increase the sweep sensitivity
by connecting a 750 Ω resistor from the 2nd LO Sweep in­
put (at C222) to the standoff at the junction of R2063 and
pin 2 of U2063.