Nokia RM-42 Service Manual page 313

Appendix A: RF Troubleshooting
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
•
(DC offset
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
(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
1.8V). DC voltage level should change if TX channel is changed. VC is lower on lower
"Vinku (N7501) RB_EXT voltage ok?"
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6.5.3 "TX VC in GSM mode
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