error 9 while in DFU mode means lost connection between PC and device.... I would try replacing the USB cable first... if the issue persists then it should be an issue related to U2, Q2 or Power Manager, depending on measures taken with multimeter along the related voltage lines... I don't know what your local repairer means for "not worth" to repair... Here in Malta where I operate we manage to fix these issues for a maximum 200eur expense... maybe it could be worth for you or not.... in my opinion when a repairer tells you "not worth" without quoting it's because he's not able to fix it... this to encourage you not to give up and search for alternative repairers.... good luck!
first of all you need to disassemble it and take the board apart. then, depending on the model number (could be x ex. GT-I9500, search on the internet for the proper service manual (or repair manual or troubleshooting). Following the guidelines inside that manual you're on the right way to find the components to test and what IC should be reflown/reballed/replaced.... some microsoldering and testing skills are obviously required in this case... good luck