No sure what circuit board you are talking about. The cluster comes out in 3 pieces. There are circuit boards on both sides of the speedometer assy. that control the gauges and lighting.

If it is really bad you can just go to the classified area and find a salvage, complete cluster. Lots of people on here that would be willing to help. Just be sure whatever you get (especially the speedo), matches the rear end gearing.

If you are trying to improve the lighting, foil tape, cleaning the prism's and new bulbs make the cluster really bright.

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AJ

1985 300D (SOLD)