If the two control LEDs for the battery status, among other things, do not light up, it could be that batteries have leaked at some point, the battery compartment was cleaned but the leaked electrolyte got behind the battery contacts. There is some electronics built in there with contacts that could be damaged.
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Front of the battery contact plate (removed, please ignore the arrow).
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Back, the arrow points to a cooroded contact, damaged by battery electrolyt (rechargeables). Also other parts are affected.
Otherwise, I would check the contacts on the battery holder carefully; if there is oxidation here, the current will no longer flow. The eight AA batteries are connected in series and if one contact fails, the current „sticks“.
You can test the function with a multimeter. With fresh AA batteries, there should be over 12 volts at the battery contacts.
The battery contacts in the battery compartment are harder to reach.
Here I put the head of a cotton swab on a pointed probe to reach the contacts in the battery compartment of a Canon T90. Clean with electronic cleaner, but if there is a solid coating or severe oxidation, the part of an eraser can help. In the worst case (when heavily corroded), the contacts have to be sanded down (destructive), which is not easy to do from the outside.
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