4. Connect the probe of a test oscilloscope to the
collector of Q1040 and check for a 50% duty cycle.
d. Check Controlled Oscillator frequency—This part of
the check is only required after repair or replacement of the
Controlled Oscillator board.
1. Connect the Direct Input of the frequency counter
to TP2011 (Fig. 3-54A). Ground pin L on the Offset Mixer
board.
2. Connect an 80 k resistor in series with a 2 k vari
able potentiometer from pin H to ground; then adjust the
variable resistor for a voltage reading of 12.0 ±0.1 V at
pin B.
3. Adjust C1013 (Fig. 3-54A) for a frequency of
25.10 MHz.
4. Now replace the 80 k resistor with a 4 k resistor
and adjust the variable resistor for a reading of 5.75
+ 0.1 V at pin B.
5. Adjust C2011 (Fig. 3-54A) for a frequency of
25.032 MHz.
6 . Repeat sub-parts 3 through 5 until the oscillator
range is 25.100 to 25.032 MHz.
e. Check Strobe Driver—Excessive noise on the display
and intermittent lock are indications that the strobe pulse
out of the Strobe Driver is noisy or low in amplitude. This
can be caused by a mismatch in input or output impedance
to the band-pass filter FL2064. The following procedure is
required if the filter or any component that affects the input
or output impedance match is replaced.
1. With the instrument in phase lock mode, connect a
test oscilloscope probe to TP2087. Note the amplitude of
the 5 MHz strobe signal. Amplitude of the sinusodial
strobe signal is normally 5 to 6 volt peak-to-peak.
2. If the strobe signal amplitude is low and noisy,
change the value of select capacitors C1032 and C2105
to obtain the maximum strobe pulse amplitude at
TP2087. Value of these capacitors range from 3.3 to
27 pF.
REV SEP 1983
3. If the signal amplitude is still low, connect a fre
quency counter probe to TP2015. Frequency must range
from 5.0067 to 5.0188 MHz. Frequency is a function of
the Controlled Oscillator assembly and counter U1022.
f. Error Amplifier Adjustment—This part of the proce
dure sets loop gain and error count break point. This part is
required when either the Phaselock assembly, 1st LO,
Phase Detector, or Error Amplifier is replaced.
Calibration—492/492P Service Vol. 1 (SN B030000 & up)
Adjustment Procedure
1. Set the TIME/DIV to 1 s, FREQUENCY RANGE to
band 2 (1.7—5.5 GHz), PHASELOCK on, FREQUENCY
SPAN/DIV to 50 kHz, and AUTO RESOLUTION on.
2. Pull P3057 (Fig. 3-54B); this turns the strobe to the
Phase Gate on. Turn Loop Gain R3082 fully counter
clockwise. Pull and install P2035 between pins 2 and 3.
3. Connect the test oscilloscope probe to TP3081
(Fig. 3-50B) and trigger the test oscilloscope on the sig
nal at TP2037 (U2048-6) shown in Fig. 3-54B. Set the
Time/Div to 5 ms and Volts/Div to 0.5 V. Note the beat
notes. Beat notes are produced by the difference be
tween strobes from the phaselock (one every 5 MHz) and
the particular frequency the 1st LO is tuned to.
4. Turn Loop Gain R3082 clockwise slowly and note
the amplitude of the beat notes prior to lock. This usually
occurs between 0.5 V and 1.5 V peak to peak. The beat
notes will disappear when lock occurs.
5. Turn Loop Gain R3082 fully clockwise, increase
FREQ SPAN/DIV to MAX, set RESOLUTION
BANDWIDTH to 100 Hz, and TIME/DIV to AUTO.
6 . As the sweep scans across the span note the posi
tion of the smallest beat note. Tune the center FRE
QUENCY to position the frequency dot at this location.
7. Reduce the FREQ SPAN/DIV to 100 MHz. Set
TIME/DIV to 1 s and activate VIEW A if the instrument
has Digital Storage.
8 . Adjust Loop Gain until the beat note amplitude is
1.5 times the amplitude noted in part 4 of this step. If this
does not occur, it is an indication the Phase Gate is
defective.
3-79
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