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Check all connections. With power Off, check them with a voltmeter set to Ohms, and placing the problems firmly and steadily upon the cleanest firmest measuring points available.
Then, you may have to replace that selector switch - with time and age the contacts inside may have degenerated esp. at the position of pin 5.
(Or, doubtfully there might be a need of cleaning the internal switch contacts, perhaps with electric contact cleaner spray, provided that there are no sensitive components (within spray splatter reach of the pin 5 contact) anywhere near it.)
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