Nokia phones with weak signal?

[iDk]

hi. i just wanna ask which part of nokia GSM phone (like 3310) should i have to check if its signal strength is very weak? lets assume that all voltages were all normal. is it at Rx or Tx side got problem?

thanks

 

[FANT]

Usually to work on the mobile phones you MUST have:
1) Schematic diagram;
2) Good experience;
3) Mobile tes instruments.

Anyway, let's try to solve the problem.
a) If the signal strenght is weak, but is possible to receive the call, this means that the I/Q demodulator and the Local oscillator are working properly;
b) Check the pin diodes used to switch the antenna in RX/TX mode;
c) Check the transistor polarization on the receiver side;
d) try to find on internet the data sheet of the components from the marking; a good site is: **broken link removed**
e) try to find a spectrum analyzer and try to follow the signal path.
If you not have specific instrumentation is difficult to go further.

P.S. A last and very important suggestion :
First of all take a very good lens ( at least 10X ) and verify ACCURATELY all the solders; for my experience most of the fails in the mobiles is due to bad connections or cold joints - spend a lot of time in this last work !!!!!

Mandi

 

[FANT]

I don't know why this is happened, but the phrase :
First of all take a very good lens ( at least 1 ) and verify ACCURATELY

means :

First of all take a very good lens ( at least 10 * ; ten times ) and verify ACCURATELY

Sorry for the mistake.

Mandi

 

[flatulent]

There could be another problem. Your handset sends information to the node site about how it is receiving so the transmitter at the node can adjust power for you. If there is some defect in the IC that sends the wrong signal strength number back you could get a low power transmission. One example is if the signal strength word has the MSB stuck high. This will make the transmitter ramp down to minimum power.

 

[iDk]

thanks. i only got 20mhz oscilloscope, 1ghz frequency counter and 10x/20x microscope. sorry i'm not familiar yet on some terminology with regards to RF. i'm just starting to learn with RF on how the signal flow or how the transciever works and its components. and i'm trying to understand it.

 

[iDk]

@FANT:

i got level 3 service manual for some nokia phones. if the Rx value of a weak signal phone is -110 compared to a good phone with Rx value of -80 does this mean that i should have to concentrate in troubleshooting on its Rx side?


btw, does a bad non polarized SMD capacitor when being heated does it changes its value or it became good again for a short period of time?

 

[FANT]

If you are sure about the test point, try to find the trouble before the test point.
Take care that the signal level on the IF path can change very much, depending on the field received.
About the capacitor, if you heat the capacitor and the phone is working means that the trouble is around there, change the capacitor.
The capacitor can be good after the heating only if before was a cold joint on one of the two terminals.
I have never seen a bad capacitor became good heating it !

The measurement following the levels reported on the service manual are valid only if the phone is connected on the test bench, otherwise can change very much.

Mandi

 

[KIG]

Hi IDK,
I have read your problem,
Before to destroy your mobile , check that the problem come from
the receiver part , it exist code (call GSM string) you enter that give you the receive signal from other cell in th network. As you said good mobile have a sensitivity aroud -108 dBm (-110 for the best) , the GSM standard require -102 dBm but generally the network stop the communication around (-80/-90). So I don't think that the problem come from the receiver part .Check if your antenna is well connected and try to find GSM string that give you the receive d signol from th BTS.

Bye

 

[Leosedf]

mobiles is my job.
80% is the power amplifier problem and the antenna switch
then you have to check ccont hagar and make energy management calibration and TX/RX calibration. but first change the PA and ant.sw

 

[rayan]

Hi,
your problem seems to come from your PA or your antenna switch. But,verify first that the different signals driven your PA (ramp....)and your antenna switch are Ok
rayan ;-)

 

[vfone]

For this particularly problem on 3310, I am pretty sure you have a bad RX SAW filter. I had the same problem on Nokia 3310. When you say that you heat a cap and sensitivity is going better confirm this. Actually you are heating the SAW filter indirectly through the PCB.
I think was a bad batch of SAW’s that became bad in time, increasing the Insertion Loss and indirectly the Noise Figure of the LNA.
regards

 

[TropFish]

I don't think you can rule out your Tx at the moment, but it's good idea to check your Rx first since you got weak signal. Fix this first!

 

[Leosedf]

Many phones also have only corrupted data. Use Wintesla to check RSSI etc.