It takes about 5 minutes to remove the brushes from the alternator, I would check them first. I had a similar experience. If you have the original Becker radio then the only way to reset it is with a reset tool, there are no codes as in later years. The dealer or a Becker service facility have this special tool.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor