Nokia RM-57 Service Manual page 248

Appendix A: RF Troubleshooting
• Check that components are in place and solder joints are ok
• GSM1800 and GSM1900: Disconnect the power supply from the phone and use an ohmmeter
to check that capacitors C7575 and C7577 are not short-circuited. If short-circuit is found
replace the faulty capacitor.
3.1.1.2.3 GSM1800/GSM1900: Is there RF power in the balun (T7502) output at all?
GSM transmitter has to be active before measurements. Procedure is explained in section
•
"Transmitter troubleshooting". Set TX power level to the maximum ("0" in
GSM1800/GSM1900)
• Measurements can be done with a spectrum analyser and an RF probe. Remember to make
correct frequency settings to the spectrum analyser. 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 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.
• GSM1800 or GSM1900: Connect the probe to R7512 input. The RF level should be roughly -
20...-30 dBm.
3.1.1.2.3.1 Replace balun T7502
3.1.1.2.4 GSM1800/GSM1900: Replace attenuator R7512
3.1.1.3 Does GSM PA (N7502) transmit RF-power at all?
GSM transmitter has to be active before measurements. Procedure is explained in section
•
"Transmitter troubleshooting".
Set TX power level to the minimum ("19" in GSM850/EGSM900 and "15" in
•
GSM1800/GSM1900)
• 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 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.
• GSM850/EGSM900: Connect the probe to J7520 (test point). The RF level should be about
-16...-17 dBm.
• GSM1800 or GSM1900: Connect the probe to J7519 (test point). The RF level should be roughly
-29...-30 dBm in both bands.
3.1.1.3.1 GSM PA (N7502) operating voltage ok?
A-30
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