Problem with backlight, or is it something elseMy daughter's 4s got wet. I opened it up (thanks to ifixit), cleaned the small amount of corrosion around the connectors near...
Answer to: Problem with backlight, or is it something elseFound the problem and now everything works fine!! FL4, which connects one of the backlight signals to connector J4, had actually failed. I decided the part was too small for me to replace so I ran a piece of enamelled copper wire from TP22 to C134. This was the easy bit as I then discovered that pin 4 of J4 had fractured. This is the second connection to the backlight. I did think of replacing J4 but again decided the soldering was too much for me without a hot air soldering station. Eventually I ran another wire, this time from TP21 to the display ribbon cable that mates with J4 pin 4. This means that the motherboard is no longer removable without desoldering but it's cheaper than throwing away an iPhone! So, a bit of a bodge but it all works. Thanks for the comments from others. By the way, if you want schematics of iPhone motherboards just go to 'service-schematics/ru'. Brilliant!