Nokia 6680 RM-36 Service Manual page 292

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not as expected then replace T7503. VCO shield has to be removed before measurement.
Remember to solder the shield back after the phone repairing.
3.1.3.1.2.4 Components near TX VCO ok?
C7571, R7519, R7523, C7573 and C7568 working correctly?
• Check that components are in place and solder joints are ok
• Disconnect the power supply from the phone and check resistors resistance values with an
ohmmeter.
• Use an ohmmeter to check also that capacitors are not short-circuited
3.1.3.1.2.5 Replace Vinku (N7501) or TX VCO (G7502) or both
3.1.3.1.3 Replace TX VCO G7502
3.1.3.2 Is TX VCO signal level in the T7503 output high enough?
• GSM transmitter has to be active before TX VCO's output level can be measured. Procedure is
explained in section "Transmitter troubleshooting".
• Measurements can be done with a spectrum analyser and an RF probe. Spectrum analyser centre
frequency should be set according the used TX channel (see section "Frequency mappings").
• Spectrum analyser RBW and VBW = 1 MHz, Span = 0, sweep time 1 ms. Notice that GSM
transmission has pulsed nature and VCO output power should be measured during TX burst
(triggering needed). Another possibility is to use following settings: RBW = VBW = 1 MHz, Span
200 kHz and sweep time at least 2.5 seconds.
• Check the level of the VCO frequency in T7503 outputs. The level should be about -30...-35 dBm in
both output lines during GSM TX burst. If the signal level is correct in the input (about -25 dBm)
but output level is not as expected then replace T7503. VCO shield has to be removed before
measurement. Remember to solder the shield back after the phone repairing.
3.1.3.2.1 TX VCO G7502 output level high enough?
• GSM transmitter has to be active before TX VCO's output frequency and output level can be
measured. Procedure is explained in section "Transmitter troubleshooting".
• Measurements can be done with a spectrum analyser and an RF probe. Spectrum analyser centre
frequency should be set according the used TX channel (see section "Frequency mappings").
• Spectrum analyser RBW and VBW = 1 MHz, Span = 0, sweep time 1 ms. Notice that GSM
transmission has pulsed nature and VCO output power should be measured during TX burst
(triggering needed). Another possibility is to use following settings: RBW = VBW = 1 MHz, Span
200 kHz and sweep time at least 2.5 seconds.
• Connect the RF probe to the T7503 input. VCO shield has to be removed before measurement.
Remember to solder the shield back after the phone repairing.
• Check if the frequency of the TX VCO is as expected. If the VCO signal is not found try to use wider
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Appendix A: RF Troubleshooting