180 Frequency Synthesizer Fault Finding TB7100 Service Manual© Tait Electronics Limited October 200511.7 VCO and Related Circuitry (VHF Radios)Introduction If there is no fault with the power supplies, the PLL inputs and output, andthe loop filter, check the VCO and related circuitry. The procedures in thissection apply only to VHF radios; the VHF frequency bands are defined inTable 11.4. There are six aspects:■ Task 28: check VCO■ Task 29: repair PLL feedback■ Task 30: repair VCO■ Task 31: check transmit-receive switch■ Task 32: repair switching network■ Task 33: check buffer amplifierThe measurement points for diagnosing faults in the VCO and relatedcircuitry are summarized in Figure 11.11.Table 11.4 Minimum and maximum frequencies for the different VHF frequency bandsFrequency band Frequency in MHzMinimum MaximumB1 84 ± 5 200 ± 5
