Ok I have tried and a different test and working WiFi Board which made no difference (wasted £30.00). I have tried a working RF Board and still no solution. I am next going to reflow the Realtek RTL 8151GNM Ethernet Controller chip - wish me luck :/
Remember for Sony to send you a new one you will need the purchase receipt - well, if you live in the UK you will. Bought one sealed in its box with this problem from a Christmas fund raiser at a local church and Sony do not want to know.
'''I repair these thing for part of my living and am seeing system power off problems and 'flash pop' will not power on at all issues that do not seem to relate to the standard top screen . speaker ribbon problem. The p6 connector is a pain o the 3sd XL and they are very easy to damage. Parts for the 3ds XL are hard to come by and expensive. Any ideas anyone? cheers Dan'''