Ice build up and fridge at 0dC at min. Candy CCFF29/13

I had to defrost the evaporator three weeks ago to remove excessive ice build up. I initally suspected the the freezer door had been left open but alas the problem has not gone away and the unit runs almost continuously. So, it looks as though there is a problem with the defrost cycle can someone advise the cost of a new PCB and what else should I check ❓ A service sheet would be very usefull 😉 The pcb seems to use a single custom controller chip 🙁 but the remaining components appear standard. One strange thing is is I press the ‘fast freezer’ button when the compressor and fan are running they immediately stop only to restart a short time later.

Rey