Appendix A: RF Troubleshooting
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A-29
3.1.1.1.5.1 C7550 and C2221 working correctly?
•
Check that components are in place and solder joints are ok
•
Disconnect the power supply from the phone and use an ohmmeter to find out if the VCP2-line is
short-circuited to the ground. If short-circuit is found replace C7550 and C2221. If this does not
help go to the next steps.
3.1.1.1.5.2 Retu ok?
3.1.1.1.5.3 Vinku (N7501) ok?
3.1.1.1.6 Is there RF power in the TX VCO output at all?
•
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.
Quick VCO alive check can be done
also without removing the RF shield. The RF probe should be placed as near the TX VCO output as
possible (Put the head of the probe carefully inside the VCO can through the holes of the shield).
Remember to use low RF Attenuator value in the spectrum analyser with this method.
•
Check if the frequency of the TX VCO is as expected. If the VCO signal is not found try to use wider
span setting. The correct VCO frequency can be found in section “Frequency mappings”. The
output level of the VCO should be about -25 dBm during GSM TX burst.
3.1.1.1.6.1 TX VCO operating voltage VREG2 (VR2) ok?
•
See section
"Vinku (N7501) regulator voltages VREG1, VREG2 ok?"
3.1.1.1.6.2 Replace TX VCO (G7502)
3.1.1.1.7 Is TX VCO RF-signal coming to the Vinku at all?
•
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.
