6881076C25-D October 28, 2002
Troubleshooting Procedures: VCO Procedures 4-19
4.3.3.3 No or Low Output Power (TX or RX Injection)
Use the test cables listed in the “Service Aids” in the ASTRO Digital Spectra and Digital Spectra Plus
Mobile Radios Basic Service Manual (6881076C20). Measure the power at the synthesizer feedback
port-if it is not within the range specified in the block diagram, troubleshoot the first buffer. If failure is
found in the first buffer, replace the defective component. If no failure is found in the first buffer and
the level out of the VCO (measured with an RF millivoltmeter) is below that indicated in the block
diagram check J straps MP9656-MP9668. If no problem is found with these, replace the entire
carrier board.
If the level at the synthesizer feedback port is within the indicated range, then troubleshoot the
doubler, second buffer, and PIN diode switch.
4.3.3.4 No or Low Modulation
Under standard test conditions with a 1 kHz tone injected and 4.6 kHz (±250 Hz) deviation, there
should be between 500 and 1000 mV present on J601, pin 10 (modulation input). (See the circuit
board overlay for location.) If this level is not present, troubleshoot the audio circuitry. If it is present,
check J601, pin 4 (NEG S.L.). The negative steering line should be -4.0 V (±0.3 V). If this is not
correct, check the negative steering line circuitry on the RF board and/or check R9651 and C9651 on
the carrier board. If no problem is found, check J straps MP9656-MP9668. If no problem is found
with these, replace the entire carrier board.
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