166 Frequency Synthesizer Fault Finding TB7100 Service Manual© Tait Electronics Limited October 200511.5 Receive VCO and Related Circuitry (UHF 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 UHF radios with a lock error or receive fault, andtherefore with suspect receive VCO and related circuitry. (The minimumand maximum receive frequencies for the different UHF frequency bandsare defined in Table 11.2.) There are six aspects:■ Task 17: check receive VCO■ Task 18: repair PLL feedback■ Task 19: repair receive VCO■ Task 20: check switching to receive mode■ Task 21: repair switching network■ Task 22: check receive buffer amplifierThe measurement points for diagnosing faults in the VCO and relatedcircuitry are summarized in Figure 11.8.Table 11.2 Minimum and maximum receive frequencies for the different UHF frequency bandsFrequency band Receive frequency in MHzMinimum MaximumH5 337 ± 5 441 ± 5H6 378 ± 5 498 ± 5
