A/C belt is loose (1983 300D)- How to adjust?

The A/C belt on my 1983 300D Mercedes is loose and wobbles wildly. It looks like the parts are all solid, just needs adjusting. I felt the
tension on the other two belts and they are firm, but the A/C belt has quite a bit of give (about a 1/2 inch or so).
Does anyone have experience tightening the A/C belt? Any help in how to do this and also in how tight it should be? It looks difficult to get to. Should it feel like the other two belts' tension?
Thanks, Kate
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