[Mondial]

The loctal tubes are notorious for developing dark oxide corrosion on the pins, resulting in intermittent contact with the socket. It is usually very hard to remove.

I have had the best luck with thin strips of 400 grit wet or dry sandpaper. Loop the strip around each pin and pull it back and forth until the pin is shiny and clean. To prevent future corrosion, apply a thin coating of silicone dielectric compound to each pin. The silicone is available in small tubes at auto parts stores and is used for waterproofing electrical connectors and spark plug terminals.