m. Switch back to 10 kHz RESOLUTION BANDWIDTH
and recheck centering.
n. Switch the RESOLUTION BANDWIDTH to 1 MHz
and FREQ SPAN/DIV to 500 kHz; then adjust the 1 MHz
filter response and centering with C1033 and C1026 (Fig.
3-44).
o. Return the FREQ SPAN/DIV to 5 kHz and RESOLU
TION BANDWIDTH to 10 kHz and center the signal on
screen.
Calibration—492/492P Service Vol. 1 (SN B030000 & up)
Adjustment Procedure
p. Adjust the 10 kHz filter with C2037 (Fig. 3-44) for best
filter shape.
q. If the instrument has Option 03, switch the RESOLU
TION BANDWIDTH to 100 Hz and the FREQ SPAN/DIV to
500 Hz. Adjust the 100 Hz filter with C1030 and C1035 (Fig.
3-44) for amplitude and waveshape.
r. Disconnect the cable between P683 and the Log Am
plifier input (J621). Reconnect P683 to J683 and connect
the output from the VR#2 to the Log Amplifier input (J621).
The signal should now pass through both VR#1 and VR#2
to the input of the Log Amplifier.
s. Check the waveshape, bandwidth, and centering of all
filters. Center the 100 kHz and 1 MHz filter response around
the 10 kHz filter. If necessary, make only fine or minor ad
justments. Figure 3-45 shows typical response shapes.
t. Check filter leveling using the 100 kHz filter as the ref
erence amplitude. Adjust all filters to the 100 kHz level as
follows:
Filter
Adjust
Location
1 MHz
R1065 VR#2
10 kHz
R3035
VR#2
1 kHz
R3025 VR#2
100 Hz (Option 03)
R3015 VR#2
Locations of the adjustments are shown in Figs. 3-42
and 3-44.
11. Presetting the Variable Resolution Gain and
Band Leveling
NOTE
The Log Amplifier must be calibrated before adjusting
any VR gain settings. Log Amplifier calibration can be
verified by applying a OdBm, 10 MHz signal to the
input (J621) of the Log Amplifier and checking for full
screen display with a —20 dBm REF LEVEL.
The Post VR Gain, R2038 (Fig. 3-42), is normally pre
set by removing the VR#2 module cover and applying
a —21 dBm, 10 MHz signal to pin JJ. Adjust for a full
screen display with a REF LEVEL of -3 0 dBm. Re
place the cover before proceeding with the other gain
adjustments. Band gain adjustments, other than band
1, must be done after tracking has been calibrated
and flatness checked. If the range of any band gain
adjustments is insufficient, add a diode between the
output from U3023 and the base of Q2049, as shown
on the schematic diagram for Variable Resolution
No. 2.
a. Test equipment is shown in Fig. 3-41. Install VR#2
module on an extender board as shown in Fig. 3-42. Set the
front panel-controls as follows:
REF LEVEL -3 0 dBm
MIN RF ATTEN 0 dB
FREQ SPAN/DIV 1 MHz
RESOLUTION BANDWIDTH 100 kHz
VERT DISPLAY 2 dB/DIV
b. As described in the preceding note, the gain of the
Post VR Amplifier should be 21 dB for best signal-to-noise
ratio through the VR stages. If any maintenance has been
performed on this stage, perform the following steps.
1. Remove the cover for the VR#2 module. Discon
nect the jumper connector to the input of the Post VR
Amplifier (pin JJ).
2. Apply a 10 MHz, -21 dBm signal from a 50 Ω sig
nal generator source to pin JJ of the amplifier.
3. Adjust Gain R2038 for a full screen display.
4. Remove the signal from the input to the Post VR
Amplifier and replace the jumper between pins JJ of the
2nd Filter Select output and the input to the Post VR
Amplifier. Replace the cover for the VR#2 module.
3-66
REV NOV 1982
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