HP 85662A, Troubleshooting And Repair Manual

Page: 280 / 594

Log Amplifier-Detector, Troubleshooting
If the amplitude of the displayed signals is approximately 10 low (CAL OUTPUT appears
to be -20 in the log mode but correct in the linear display mode, then one of the log
amplifier stages in the Log Amplifier-Filter board or the Log Amplifier-Detector
board is defective. Connect a 10 step attenuator between the CAL OUTPUT and the
spectrum analyzer input. Refer to the Log Amplifier Stage Gain Check note on the service
sheets for the necessary front-panel control settings. When forward biased in log mode, the
Schottky diodes should have voltage drops of 200 to 400
The following procedure will determine if the Linear Gain Steps, and LG20, are
operating properly. The circuitry is located on the Log Amplifier-Filter board
while the LG20 circuitry is located on the Log Amplifier-Detector board. Connect a
10 step attenuator set to 0 between the CAL OUTPUT and the spectrum analyzer
input. Press Instrument Preset and key in the following settings:
(
C E N T E R
. . . . . . . . . . . . . . . . . . . . . . . . .
equal to the CAL OUTPUT frequency
S P A N
) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
LOG SCALE
(
ENTER
(Turns off IF Step Gains)
(
R E F E R E N C E L E V E L
)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCALE LIN
The peak of the displayed signal should be on the top graticule. Adjust the front panel
AMPTD CAL control to place the peak of the signal one division below the top graticule.
If the signal limits at the top of the display, one of the Linear Gain Steps is stuck in the on
state.
Set the 10 step attenuator to 10 and key in
(
REFERENCE LEVEL
)
-70 The peak
of the displayed signal should be one division below the top graticule. If signal limits at the
top of the display, the LG20 circuitry is defective. A signal approximately 3 divisions high
indicates the circuitry is defective.
Set the 10 step attenuator to 20 and key in
(
REFERENCE LEVEL
)
-80 The peak
of the displayed signal should be one division below the top graticule. The circuitry
is defective if the signal limits at the top of the display. The LG20 circuitry is faulty if the
displayed signal is approximately 3 divisions high.
The detector circuit on the Log Amplifier-Detector board is most easily checked with
dc voltages. These are derived using 0 Hz
(
CENTER FREQUENCY
)
and attenuating the CAL
OUTPUT signal. The easiest point to check the Detector output without removing boards is
Connect a DVM to this test point. The voltage at should be V for
a signal displayed on the top graticule. The output will be 0.1 V less for each division down
from the top in either the linear or the 10 log modes when the detector is operating
properly.
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