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My be the thermostat, Lawrence - easily checked, and harmlessly bypassed…
Also verify, with a test lamp, that there is power to the rheostat. If power present; there may be a ground issue using instrument body ground, and some instruments are mounted on a sub-panel - which is grounded by a separate wire…
Ground one instrument body to test - and I assume that the instruments themselves work as they should…
Lawrence,
there are three wires going to the rheostat and they may have been mixed up.
If you look at the rheostat shaped like a “U” upside down and the spades on the lower side, the spade far right carries two wires red/white and purple/white, the second from right carries red/white, the third from right carries red/blue. The outer left spade is free.
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The red/blue is power ‘in’ - from fuse #7, Jochen, according to S2 schematics? Red/white seems to serve the lamps - *purple/white may be the opticell?