TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
• U108, U109, or any associated components are replaced or the adjustment has
been changed or has shifted.
PROCEDURE:
The AC error voltage at TP5, the phase-detector output, is adjusted for a minimum
peak-to-peak value with the UUT programmed for INT FM on, with 4-MHz FM
deviation at 168-kHz mod frequency.
1. Program the UUT to SPCL 909, 700 MHz, 4-MHz deviation, and 168-kHz mod
frequency. Turn INT FM on.
2. Set the oscilloscope for 50 mV/division vertical, 2 us/division horizontal, and AC
coupling.
3. Connect the oscilloscope probe to monitor the signal at TP5, using TP15 for the
ground connection.
4. Adjust R116 for a minimum peak-to-peak voltage. The waveform should be less
than 150 mV peak-to-peak.
Loop Gain Adjustment, R167 6C-37.
TEST EQUIPMENT:
• Low frequency synthesized signal generator (LFSSG) Wideband AC voltmeter
(WBVM)
REMARKS:
• The Loop Gain Adjustment is normally required only when U3, Q108, or any
associated components are replaced or when the adjustment has shifted.
• The upper plate cover of the lower module must be installed prior to this
adjustment.
PROCEDURE:
An 800-kHz AC signal is applied to the Sum Loop VCO steering port through TP6.
Loop gain is adjusted via R167 so that the AC voltages at the Sum Loop VCO steering
and phase lock ports are equal.
1. Access R167, TP3, TP4, and TP6 by removing the appropriate plate cover access
plugs.
2. Program the UUT to SPCL 909, 548 MHz, 150-kHz FM deviation. Turn EXT AC
FM on.
3. Program the LFSSG to 800 kHz, 20 mV RMS.
4. Connect the LFSSG output to TP6 via a BNC to clip lead adapter. Connect the
ground clip to the plate cover adjacent to TP6.
6C-43
To Next Page
Loading...
