vor gauge understanding

[gspannelljr]

50025003I have a vor gauge that has 2 needles, a to/from and off flag. It also has a push button on the front that says "0 test 180". Also the push button rotates the compass scale. I have 2 questions, what does the test 180 button do? Second, on the inside, there are 2 contacts that electricly couple the backside of the compass scale to the gauge. What are the contacts for? I have included 2 pictures. Vor1 is the conducting ring that the contacts slide across as the compass is turned. Note the 2 "tracks". The inner track is connected to the green single wire, however the outer track has 4 connections, almost like different points on a potentionmeter. Vor 1(1) shows the inside of the gauge, the back of the compass card, showing the contact points. What the heck is this circuit? Its not lighting, I think. Also, how the heck are gauges lit? I don't see any back lighting. Is it front lighting instead?
Thanks for any help. You guys have been a BIG help. I hope I am able to give back some of my own soon to you.
Glenn

[Mike.Powell]

Glenn
The track with the four connections is, in fact, a special sort of potentiometer. The resistance element is connected as a loop. The four connections are wired to four equidistant points on the element. One pair connected to opposing points is differentially driven with a sin voltage while the other pair is driven with a cosine voltage. The wiper contacts on the compass card sample the voltage on the resistance element, and connect that voltage to the solid track next to the resistance element. The wiper voltage will be of the same frequency as the sin/cos voltages driving the resistance element, but will have a phase that depends on where the wiper contact is positioned. These connections are cabled to the VOR receiver. The VOR information is encoded into the phase of the received signal. The resistance element is part of the circuitry that decodes the VOR signal.

The test button replaces the signal from the VOR radio with a test signal of known phase.

Your VOR head may not have internal lighting. It may depend on an external post lamp on the panel.

[gspannelljr]

Mike, Wow, pretty neat circuit. I guess you figured out I am not very knowleadgeable about VOR. I understand the concept but need to learn more. IE a simulator!
I am comfortable converting the needle movements to a circuit that the sim can output to but not sure how to handle the feedback circuit in question. I will measure its resistance and see if I can use the pot as it. I guess if that works then I can input that information back to FS and let it use that to drive the VOR Needles. Right?