Thanks for your reply. I don't have an oscilloscope or spare parts unfortunately. I have found quite a bit of corrosion in 40 pin socket for IOU IC so going to swap the socket out as looks quite poor quality really. Do you know if I can replace standard sockets with gold plated turned pin sockets? The RAM isn't socketed so would probably fit sockets for replacement RAM as well. Found some RAM on eBay which isn't too expensive http://www.ebay.co.uk/itm/4-x-NOS-Samsung-KM4164B-15-16-PIN-DIL-RAM-memory-chip-/200931781697?pt=LH_DefaultDomain_186&hash=item2ec877ac41