It seems that a "possible" cause for the missing telephonic function is a factory misplaced transistor V2099 as described here:

http://www.elektroda.pl/rtvforum/topic2095031.html

Translation is more or less:
"...If the phone starts noticeably longer than usually (about one minute more), it means that the V2099 transistor (in photo) is not activated by a signal from the processor CMT_EN. Despite this, the phone passes the test in local mode... "

This is probably picked from a Nokia bulletin.
At the end is said that repair is not possible/suggested - economically speaking i suppose.



Could it be that after a few years of mechanical stress it looses his original uncertain position? ..Who knows?!?

I've noticed that strangely placed thing, in the past. Are you suggesting, that it just desoldered itself? I have 2 motherboards with telephony function problems ATM, and soldering it back again would be much easier than trying to fix modem chip.

OTOH, if it would be related to that transistor, why spring-solution on other components would result in getting telephony functions back, even if only for limited time?

/Estel

@Estel
If you try to repair a circuit with soldering problems without any idea of the exact part of it that has a false contact or a desoldered pin, you can discover that very often applying a pression to some part of the board seems to solve the problem for some time. (also when the point of pressure is at many centimeters of distance from the real problem).

Then, when you discover the real (logic) point of the circuit explaining the wrong behavior, you will be able to see a little, invisible son of XXX with a soldering defect... This is a very common story...

EDIT:
take care, i have not tried to fix a phone with this issue using this information, but i think that if i had an N900 with the telephony function disabled i would try to check that transistor and if misplaced i would at least try to heat it up with an hot air desoldering station.(without move it away)
PAY ATTENTION - NO WARRANTY!

It sounds like a speculation to me. But even if it were true, all it could mean is that the transistor is not getting the signal to open. Which could be for many reasons, including a faulty solder contact under the modem chip.

I'll try to take a closer look (with magnifying glass) on that component on a device with "classical" telephony functions disabled problem - it should be pretty easy to notice is the contact could be broken.

BTW, the article linked is also talking about bluetooth-related problems, and it's not 100% sure if the person citing Nokia's service guidebook (?) was really referring to modem issues (nor where this info originates from, in fact). the funny thing, is that I run into this article some months ago, and discarded it for the very reason.

I don't have high hopes, but still worth checking. Thanks for re-discovering it, xes! Who knows, maybe, just maybe...

/Estel

Could someone point me to the place this transistor is located in? And which out of four is the GSM modem chip?

@dvb

Here attached a cropped image containing the detail previously posted.

Sadly you have to cut a piece of shield to check the transistor...

Hmm. My transistor is in place. Checked by microscope - seems okay. Still, SIM is not recognized, so I assume this transistor is not the point. Any more thoughts on what chip has bad balls?

@dvb
Near that transistor there is the BB5 Baseband "Rapuyama" IC.
If i was in you, i would try some kind of pressure against that IC and if the phone works again, maybe a quick heat up with a hot air desoldering tool could help without a complete rebailing of the bga.

Take care, this would be my attempt if my N900 would be in that condition, i don't want to be responsable of further damages on your device

Hey, no complains, of course!

Do you mean that IC on the above pic, which has 43772I0 label on the first line?

Anyway, I have 3 N900s and two have dead cellular - nothing to loose. I will train on the oldest one, which has no camera/microsd slot working (torn-off cable) and misses three buttons (suffered from heat by my previous attempt to fry the chips. But this time I will try to use a small soldering station and a proper flux, rather than a big heat gun and pine rosin, melted with ethanol...)