Vatorgator is right , straight forward swap out usually.it sounds like you have the older 211 with the piggy back board ? Just swap the base board , if the fault moves its the board . If not swap out the smaller board , if it moves its that board or or in the software . If you swap both and it doesn't move its a wiring fault , possible short . Providing the KEB board you pit on wasn't faulty also . Chased my tail all over Christmas by replacing a tape head reader board on a 211 with a new one that also had the same intermittent fault as the old one . Deove me crazy .