instrument light problem

[Frank]

My '65 PA-30 still has the original rheostat for the instrument lights, and it normally works fine. I switched to Nulites a while back, which shouldn't matter for this question.

A couple nights ago my instrument lights went to full bright. I had not just adjusted them, they just went to max bright with no pilot input. Turning the dimmer switch back and forth had no effect. The following landing wasn't bad, but there was a bump, and the lights went totally out at that moment. Adjusting the dimmer again, the lights operated normally... I had apparently left the dimmer in the full dim position when attempting to adjust the lights earlier.

So apparently there's a loose connection, which came back together with the slight bump of landing. I pulled the rheostat and did find a poor solder joint on one side of it, which of course I fixed. But I don't see how, if that connection had been bad, the lights would have gone to bright... they should have gone off, seems to me.

So the question is, what possible malfunction could cause the lights to go full bright? And then return to normal?

[Michael Rath]

Frank,

You must have stumped everyone!! After I had my electrical melt down and I was laying under the panel looking things over I noticed the dimmer. My non scientific observation is that the old original dimmer has a metal coil around it. When you adjust the dimmer it slides a metal arm around the coil. The more you turn it the more voltage it gets. You might want to check and see that the little metal arm on the back of the dimmer switch is not lose or that you don't have any corrosion on the coil. You might have to drop the breaker panel to get to it.

MR

[Frank]

Thanks for the reply... does seem everyone's stumped! I had the dimmer apart, and am pretty sure there's no corrosion, etc.

Interestingly, after fixing the bad solder joint, it did exactly the same thing again on a subsequent night flight. (Only this time I didn't leave the dimmer in full dim position, so on landing it just went a little dimmer than full bright).

[Michael Rath]

Let us know if you find a fix.

[VANBLADEREN]

Frank, you indicated that your plane was a 1965 twin which I assume the serial number is less than 30-1102. Of interest that your lights receive full power and the ground is varied to achieve the brightness. Since ring lights are completely isolated, it should not make any difference connecting to the dimmer system. The rheostat as you found is a wire wound which has been a little problematic with age. You can check the health of the pot with an ohm meter. If it is steady over the range of the pot, then the pot is ok. If not, consider replacing wit h a MAX DIM 9100-001B pot. It is STC'd. You would have to revise the wiring to the instrument lighting a little since it varies the power out and assumes a good ground. It lists for$260, with screw in connectors.

If you pot is good, then there are some bad connections somewhere. Check the ground from the pot.

Regards

jvb