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TM9100 Service Manual Frequency Synthesizer Fault Finding 201

9.6 Transmit 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 system error or transmit fault, and

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

and maximum transmit frequencies for the different UHF frequency bands

are defined in Table 9.3.) There are five aspects:

■ Task 23: check transmit VCO

■ Task 24: repair PLL feedback

■ Task 25: repair transmit VCO

■ Task 26: check switching to transmit mode

■ Task 27: check transmit buffer amplifier

The measurement points for diagnosing faults in the VCO and related

circuitry are summarized in Figure 9.8.

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

Frequency band Transmit frequency in MHz

Minimum Maximum

H5 371 ± 5 492 ± 5

H6 419 ± 5 545 ± 5

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