TB7100 Service Manual Frequency Synthesizer Fault Finding 175
© Tait Electronics Limited October 2005
11.6 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 11.3.) 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 11.8.
Table 11.3 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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