Pretty sure this will be your problem.... Clicky
It doesn't seem to be a common problem with this set so it would be worth buying one from ebay and fitting it yourself.
There aren't many repair shops nowadays that will successfully repair them, they tend to tell you it would be cheaper to buy a new set.
If you want to take the plunge and give it a try heres one on ebay, tested and working with a 14 day money back gauruntee for £27... Clicky
The parts on these pcb's cannot be replaced as they are so small, there will not be any available new either so if you take it to a repair shop they will get one from ebay, charge you extra for the part and then charge around £40-£50 labour.
Thse are easy to fit, you just need to be carefull when removing the ribbons from the old pcb.