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TM8100/TM8200 Service Manual Frequency Synthesizer Fault Finding 203

9.5 Receive VCO and Related Circuitry (UHF Radios)

Introduction If there is no fault with the power supplies, the PLL inputs and output, and

the loop filter, check the VCO and related circuitry. The procedures in this

section apply only to UHF radios with a lock error or receive fault, and

therefore with suspect receive VCO and related circuitry. (The minimum

and maximum receive frequencies for the different UHF frequency bands

are defined in Table 9.3.) 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 amplifier.

The measurement points for diagnosing faults in the VCO and related

circuitry are summarized in Figure 9.9.

Table 9.3 Minimum and maximum receive frequencies for the different UHF frequency bands

Frequency band Receive frequency in MHz

Minimum Maximum

H5 337 ± 5441 ± 5

H6/H7 378 ± 5498 ± 5

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