ER-4915 Samsung electronic cash register poor print quality no codes

I am the owner of a convenience store Fostoria Grocery II, the biggest grocery store in Fostoria, Michigan, because we're only store in Fostoria. I am very comfortable programing the ER-4915 as well as repairing them. Typically of all the used ER-4915 I've owned its the printer that wears out and have several parts machines on hand when parts and repairs are needed. Our main cash register's print quality has becoming very hard to read with the majority of all charters having blanks in their middle sections horizontally. Another way to describe the issue is to imagen this text having white lines thru the middle of each sentence horizontally. Thinking that the print head was wore out, I replaced it with another used print mechanism and get the same results if not worse. I have a repair manual but it did not mention anything that could cause this issue. HELP!!!