| Ask as many questions as you like, it's the only way to find these things out and I do the same. There are 2 inductors within the circle I've drawn, they are the ones that need to be checked, and they should read short-circuit when measured. They pass the 5V USB supply directly to two MOSFETs which feed it on to the PM IC and elsewhere. I have heard of the MOSFETs failing as well, which I think gives the same symptoms as you have now.
The top current-sense resistor should read short as well, so that's okay. Test the inductors on the resistance (ohm) range of a multimeter. They're very small, so a bit tricky to get the probes into place, but can be done.
I did change the PM IC myself, but it's quite a job. My family trade is electronics repair, and as such, although I no longer work in the field, I still have soldering stations, a hot-air station, etc., and so used what I've got to do the job. Main difficulty is removing the old IC, as they use black epoxy underfill to help adhere the IC's body to the board, giving it extra strength against board flex. Also, lead-free solder has been used which has a higher melting temperature than traditional lead solder, so removing the IC without a PCB preheater means using a very high temperature to melt the solder, and applying upward pressure the whole time because the glue is weakened by heat but still holds on, meaning you can't tell by feel when the solder has melted, but must just apply pressure till eventually the glue bond breaks and the chip flips up.
Then the area must be cleaned of epoxy and old solder using lots of flux, desoldering braid and gentle but firm pressure with the iron as you move the braid around. New IC must be re-balled using a stencil which I bought from China (bought 10 with aim to sell the other 9), and that can then be soldered into place using hot-air station at a lower temperature if you use leaded solder paste, as I did.
It's a small IC with very fine pitch solder pads on it, and that makes alignment a careful process, but it went alright for me. Main problem was as detailed above, which seems resolved after 5 weeks of being left alone. I don't understand the mechanisms involved since I'd left the battery disconnected for days beforehand, but something happened and now it's working and charging normally, and most importantly holding its charge. Before the repair, not only would it not charge, but when I fitted a new battery, it discharged from 85% to dead in one hour.
The IC was also bought from China after a week of emailing lots of different sellers and eventually getting the best price for the IC and shipping to the UK. I think the total price was £25 or thereabouts, but I'd have to check as the payment was in dollars. I bought two in case of me damaging one, again with the aim of selling the 2nd one if it wasn't required.
First though, try to find out if anything else is wrong. I don't know if PM IC failure can cause the phone to not be recognised by iTunes, mine didn't have that trouble, but failure will occur in different ways, depending which section of the IC is actually destroyed, as it has many functions. There is a post on here titled something like 3GS PMIC voltage hints. Find it and use the voltage reference points as a comparison with yours when the USB connector is plugged in. You may find voltages are missing, and the location of those missing voltages can then be tracked back to see what the likely cause is. Post the information on here, and then I or others can check the schematics and help. |