Maintenance—492/492P Service Vol. 1 (SN B030000 & up)
3. Adjust the tuning slugs for best response shape, cen
tered around the reference. Ensure bandwidth (6 dB down)
is 1 MHz.
4. Check resolution bandwidth accuracy over the range
of the RESOLUTION BANDWIDTH selector to ensure that
bandwidth is within specification.
829 MHz Converter Maintenance
Some circuit boards in this assembly contain critical
length printed elements. When damaged, these elements
are usually not repairable; therefore, the circuit board must
be replaced. Even though replacement boards are
precalibrated and repair can be accomplished by replacing
the board, we recommend sending the instrument or assem
bly to your Tektronix Service Center for repair.
The 829 MHz bandpass filter in the IF section, and the
719 MHz LO in the LO section, require adjustment only if
the board has been damaged or active components (transis
tor or varactor) have been replaced. The following describes
preparation for service and replacement procedures. The
first two steps describe how to gain access to either the LO
or the IF section; the remaining steps describe adjustment
procedure for each section.
1. To gain access to the LO section
a. Switch POWER off; use a 5/64 Allen wrench to loos
en and remove the cover screws.
b. Remove the cover.
c. Refer to step 3 for adjustment procedure.
2. To gain access to the IF section
a. Switch POWER off; use a 5/16 inch wrench to discon
nect and remove all coaxial connectors to the 829 MHz
converter.
b. Unscrew and remove the six mounting screws,
unplug the input power connector P4050, then remove the
829 MHz converter assembly.
c. Turn the assembly over and remove the cover for the
IF section.
d. To troubleshoot or calibrate the circuits, set the as
sembly at a location so the input power plug P4050 can be
reconnected to the Mother board. Be sure to observe plug
orientation (pin 1 to pin 1 ).
e. Refer to step 4 for adjustment procedure.
3. 719 MHz Oscillator Range Adjustment
a. Adjustment requires the following test equipment:
A frequency counter with a frequency range to 1 GHz
(nine digit readout), sensitivity of 20 mV rms prescale,
15 mV rms direct (such as TEKTRONIX DC 508A); a digital
voltmeter with a 3.5 digit readout (such as TEKTRONIX
DM 502A); test leads for the DVM, a 50 Ω coaxial cable with
bnc connectors (Tektronix part number 012-0482-00) and a
sma male-to-bnc female adapter (Tektronix part number
015-1018-00).
b. The 2nd LO range is from 714.5 MHz to 723.5 MHz
(with the cover off). 719 MHz is the optimum center frequen
cy. Frequency of the oscillator is controlled by the Tune
Volts from the 25 MHz Phaselock circuit (located at
TP1011) which varies from +5 V (low end) to +11.9 V (high
end) with +6.75 to +7.5 V as the limits for operation at
719 MHz. Set the digital voltmeter to measure 12 V then
connect it between TP1011 (Fig. 4-12) and ground.
c. Disconnect the 100 MHz reference from the 3rd Con
verter by unplugging P235 (Fig. 4-12). The oscillator should
go to its upper limit and the voltmeter indicate about 11.9 V.
d. Connect the 75 MHz—1000 MHz input of the fre
quency counter through a 50 Ω coaxial cable to the front-
panel 2nd LO OUTPUT connector.
e. The 719 MHz oscillator frequency is a function of the
length of the printed 1/4 wavelength transmission line. Minor
adjustments to the oscillator frequency are made by
shortening the U-shaped transmission line stub, off the main
line. Graduation marks (see Fig. 4-12) along the side of the
stub provide a guide to calculate frequency correction. Each
minor mark from the end or cut across the stub, represents
an approximate change of 2 MHz.
Check the frequency by noting the reading on the fre
quency counter. If above 723.900 MHz, the stub must be
lengthened. Solder a bridge across the cut and recheck fre
quency. Nominal frequency for an uncut stub is 710 MHz.
f. Shorten the line so the frequency is near
723.500 MHz. For example: The frequency difference be-
REV JUN 1983
4-21
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