Nokia RM-57 Service Manual page 245

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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.
• Check the level of the TX VCO frequency in T7503 outputs. The level should be about
-30...-35 dBm in both output lines. 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.
3.1.1.1.7.1 Replace balun T7503
3.1.1.1.8 Are TX-IQ signals ok?
These current mode signals are not possible to measure, but are tested with self-tests. So if
•
there is no fail in 2.3. "ST_CDSP_TX_IQ_TEST" these signals should be ok. Otherwise Vinku
(N7501) or RAP3G (D2800) is faulty. Notice that it is not possible to replace RAP3G ASIC.
3.1.1.1.9 Is there TXC-signal coming to Vinku ASIC (N7501)?
• GSM transmitter has to be active before TX control voltage TXC can be measured. Procedure is
explained in section "Transmitter troubleshooting".
• Set TX power level first to the maximum ("5" in GSM850/EGSM900 and "0" in
GSM1800/GSM1900)
• Measurement can be done with an oscilloscope and a probe.
Connect the probe to C7549
•
• Typical TX control voltage TXC timing should look somehow similar to figure
mode (DC Offset 0 V)"
roughly:
GSM850/EGSM900: 1.8 V while TX burst and 0 V otherwise.
-
GSM1800/GSM1900: 1.8 V while TX burst and 0 V otherwise.
-
• Change the TX to the minimum power level ("19" in EGSM and "15" in GSM1800/GSM1900)
(GSM850/EGSM900 TX power level 5) and voltage levels should be
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Appendix A: RF Troubleshooting
4.6.2 "TXC in GSM
A-27