Nokia RM-57 Service Manual page 250

Appendix A: RF Troubleshooting
• GSM1800 or GSM1900:
Connect the probe to C7561 or C7556. Notice: C7556 is a non-assembled component so the
-
probe should be connected to the pad that can be still found from the PWB.
Typical full TX power bias currents (Icont_31 and Icont_32) should look somehow similar to
-
figure 6.5.5 when measured with an oscilloscope and a probe. Check both currents.
3.1.1.3.2.1 Vinku (N7501) RB_EXT voltage ok?
• See section
3.1.1.3.2.2 Are capacitors in GSM PA (N7502) bias lines working correctly?
EGSM: Icont_21 missing – Replace Vinku
Icont_22 missing – C7545 short-circuited?
GSM1800/GSM1900: Icont_31 missing – C7556 short-circuited?
• Check that components are in place and solder joints are ok
• Disconnect the power supply from the phone and use an ohmmeter to check that capacitors
are not short-circuited. If short-circuit is found replace capacitors mentioned above. If this
does not help go to the next step.
3.1.1.3.2.3 Replace Vinku (N7501)
3.1.1.3.3 Replace PA (N7502)
3.1.1.4 Are control voltages VC1, VC2 and VC3 coming correctly to the antenna switch (Z7503)?
• Use "RF Controls" window in Phoenix test software to activate the GSM transmitter and to
select the wanted GSM band. Procedure is explained in section "Transmitter troubleshooting".
GSM RX activation is described in section "GSM RX chain activation for manual
measurements".
• Use an oscilloscope and probe to find out if antenna switch control lines are working
according to table shown below. "Hi" means that there is 2.4 – 2.8 V control voltage level in
the corresponding control line. "Lo" means levels 0 – 0.2 V. Remember to trigger the
oscilloscope because control voltages VC1, VC2 and VC3 are pulsed
• Connect the probe to correct test points to measure VC1, VC2 and VC3 voltages (check test
point locations from section "Test point locations"). Notice: these test points are PWB pads
for three non-assembled capacitors.
• Note! EGSM_RX and EGSM_TX voltages are the same for GSM850.
A-32
"Vinku (N7501) RB_EXT voltage ok?"
Icont_32 missing – C7561 short-circuited?
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