Bought a milk crate full of Guide lights at the big Cub auction in Des Moines earlier this month. Before the auction, I spied two rear combo lights in the crate; one with the top bullet light and the other with an internal jewel light. By the time the auction rolled around and I bought the crate, somehow the rear combo light had jumped out of the crate and landed in some other pile, which I didn't get to take home. Hey, it happens.
Got out the internal jewel light to see what was going on with it. It was dirty. Some PO had relegated it to barn life because the knob switch didn't turn and perhaps the lens was broken. This came without a lens and the rim was dented. Otherwise, it was mostly intact.
Disassembly begins by taking the rim off and inspecting the interior. Most of the wiring here was fine, although I had to use a Dremel to cut off the 8-32 screw connecting the main power supply.
Next was repairing the rim. I had made a walnut disc some years back that is the same size as the rim. Inserted the disc and using a hammer, I tapped the sheetmetal back to shape, tightening the clamping screw to take up the slack as I went. It's not perfect, but better than it was.
The set screw was removed from the knob (don't drop it), the knob removes, then the locking nut from the shaft collar. The whole assembly drops out the bottom through the interior of the light.
Upon closer inspection, the switch appears to be in good external shape, but as I said before, the knob shaft is corroded and stuck inside the collar preventing it from rotating.
By carefully bending the 3 prongs on the back of the switch, the back is removed revealing the interior.
The terminal labeled "B" is where the main power is connected and "R" and "L" are separate wires to each lamp. As you can see, there is an upper disc (connected to the shaft) and a lower disc having 4 connectors. When the switch is functioning properly, the shaft rotates the upper disc that allows a copper connector to land on only 2 of the 4 connectors on the lower disc (either B & L, or B & R), allowing power to go to either the main lamp or the jewel lamp. The copper connector has 2 tabs that go into the upper disc keeping it in place. Once I cleaned everything, I still had the problem of getting the shaft separated from the upper disc housing (aluminum) and collar (brass). Having no good way to hold this while trying to get pressure on the end of the shaft to drive it through, I replaced it back in the housing with the locking nut and gently tapped on the end of the shaft. I'll also say that I had used a good quantity of 50/50 acetone and ATM fluid and some Seafoam penetrant for a few days prior to this.
My patience was rewarded with success. Knob switch fully disassembled.
Once cleaned with a combination of wire brushes, fine sandpaper, alcohol and blown dry with compressed air, the switch is reassembled and the 3 prongs gently bent into place. The light is reassembled. I wasn't happy with the condition of the lens reflector, so I used some aluminum foil, sprayed the back with adhesive, and stuck it on. Yeah, it looks cheap. Not pretty, but more reflective than before.
NAPA sells jewel bulbs (LMP-51, 6V), so I fitted it up with an extra lens (on hand) and a rubber gasket (made from a windshield wiper blade) and it all works as designed.
Other than the cost of light bulb, I really didn't buy anything, just disassembly and cleaning. Nice way to spend a few evenings during winter. Hope this encourages you to give it a try.
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