iPod photo hold button acts like an on/off button

Hello, the iPod (iPod photo classic 4th generation 20GB) does not turn on unless the hold button is locked, its quite like an on/off button. To get it on it has to be plugged in because the buttons cant be pressed in lock mode but it does load up properly.

Once on if the lock button is slid into the unlocked position the iPod turns off.

I have never come across this before in a few years of repairing iPods, I have tried a working replacement and the same happens so it looks like a motherboard fault I just don't know where to look at to attempt repair.

Any help is appreciated thanks.