Becker Mexico Radio Cassette - Security Code not working

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I have a Becker Mexico Radio Cassette in my 1986 BMW. I have the security code for this, and have used it many times after disconnecting the car battery.

This time, after the car spent a few months in a panel shop, the code is NOT working. I suspect a failed component in the memory circuit, perhaps due to the panel shop using a battery charger on the vehicle.

Any suggestions of where to start diagnosing this??
 
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