Nokia RM-57 Service Manual page 261

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3.1.3.1.2 TX VCO control voltage VC ok?
• GSM transmitter has to be active before TX VCO control voltage VC can be measured.
Procedure is explained in section "Transmitter troubleshooting".
• Measurement can be done with an oscilloscope and a probe.
• Connect the probe to R7519.
• Typical TX VCO control voltage VC should look somehow similar to figure
mode (DC offset
lower channels and higher if higher channel numbers are used.
3.1.3.1.2.1 VCP2-voltage ok?
GSM transmitter has to be active before VCP2 voltage can be measured. Procedure is
•
explained in section "Transmitter troubleshooting".
Measurement can be done with an oscilloscope and a probe.
•
• Connect the probe to C2221 (or C7550).
• VCP2 voltage should be about 4.75 V.
3.1.3.1.2.1.1 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.
3.1.3.1.2.1.2 Retu ok?
3.1.3.1.2.1.3 Vinku (N7501) ok?
3.1.3.1.2.2 Vinku (N7501) RB_EXT voltage ok?
• See section
3.1.3.1.2.3 Balun T7503 ok?
• 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
1.8V). DC voltage level should change if TX channel is changed. VC is lower on
"Vinku (N7501) RB_EXT voltage ok?"
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Appendix A: RF Troubleshooting
6.5.3 "TX VC in GSM
A-43