TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
PROCEDURE:
The Sum Loop VCO steering voltage is adjusted to +26V DC with the Sum Loop VCO
steering DAC set to full scale.
1. Program the UUT to SPCL 909.
2. Program the UUT to SPCL 943. This special function programs all DACs to full
scale.
3. Connect the DVM to measure the voltage between TP3 and TP15 (ground).
4. Adjust R112 for +26.00V ± .02V.
5. Program the UUT for SPCL 00. This clears all Special Functions.
Buffer Gain Match Adjustment, R121 6C-35.
TEST EQUIPMENT:
•
DVM
REMARKS:
The buffer gain match adjustment is normally required only when U106 or any
associated components are replaced or when the adjustment has shifted.
PROCEDURE:
The SUMAUDIO buffer amplifier is adjusted for equal gain in the inverting and
non-inverting modes.
1. Program the UUT to SPCL 909, 800 MHz, 4-MHz FM deviation, and 1-kHz mod
frequency. Turn INT FM on.
2. Connect the DVM to measure the AC voltage between TP2 and TP15 (ground).
3. Note the DVM reading.
4. Program the UUT to 700 MHz.
5. Adjust R121 for a DVM reading equal to that noted in step 3, ± 5 mV.
FM Null Adjustment, R116 6C-36.
TEST EQUIPMENT:
• Oscilloscope
REMARKS:
The FM null adjustment is required under the following conditions:
• The A12 Sum Loop PCA has been replaced or the A14 FM PCA has been replaced
or repaired.
6C-42
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