Hi. It is usually caused by the power supply filter electrolytics. They dry out and fail with age. If you haven't had the battery changed, it is a good idea to have it done while the other problem is being fixed.
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If anyone needs a JCM G211 cash register manual, one of the users has it here:
http://the-checkout-tech.com/manuals/cashregisters/JCM_G-211_owners_and_programming_manual.html
James K, June 2014
