One of the problems I've had with the lights in the main salon is that they are often intermittent. You would turn on the switch and see which lights would come on. In a circuit of 3, usually one or two would fail to light. Sometimes tapping on it would make it light up and sometimes not. When you are talking about 12 volt, 10 watt bulbs, losing even one turns a dimly lit salon into an almost unlit salon.
When I tried to remove one of the bulbs, the end fell off. Well, I guess that might explain why it wasn't working. Swapped in a working bulb and that resolved the problem. The next fixture was more of a puzzle. The bulb looked OK and I tested it with an ohm meter and it said it was OK. I checked the socket and it read 12v (ok, actually 13.6 since the battery charger was on). Yet when I replaced the bulb, no light. Remove the bulb and double check the continuity of the bulb and the voltage and it all seems OK. Hmmm.
I decide to remove the fixture from the ceiling and what I find is rather interesting. The bulb holder socket was pop-riveted to a spade lug and the wire is attached to that. The rivet has loosened and the spade lug just spins around it. That would explain why I could read voltage but there wasn't enough amperage to light the bulb. My guess is that the heat of the bulb slowly causes the plastic to shrink or deform and that results in the loose connection. Guess the designer didn't think that old incandescent bulbs generate heat.
I try using some pliers to tighten the rivet but it doesn't really help. I end up soldering the rivet to the spade to make a better connection. That does the trick. I repeat this procedure with the 5 other lights that are causing the issue and now all the lights work when I turn them on.
While I had the soldering iron out, I decided to give another project a try. The LED replacement bulbs that fit these fixtures can be ordered for about $15 (US) plus shipping or can be found for about $45 each at the local West Marine. The problem is that I wanted to see if LED's would improve lighting a bit before I go buy a bunch of them or even replacement LED fixtures. As it happens, I have the core of a decorative LED house bulb that failed due to a power supply issue. The core had 4 strips of 4 LEDs and I had previously determined each strip operated at 12 volts. So, I removed one of the strips from the core and soldiered it to one of the broken bulbs.
Viola, a home-made festoon LED bulb. The bulbs seem to produce about 30% more light and now that I know how to fix the existing fixtures I think I will keep the existing fixtures and spring for the LED bulbs.