Fig. 1 Cross-section view. This drawing is not to scale but shows how the ring clamp functions. Platters specifically made to work with the clamp have an outer diameter that the felt pads on the ID of the clamp slide onto nicely - making that diameter do the centering of the clamp, not the LP edge. The platter also has a bit of a 45 degree guide slope or chamfer. My mod was to add 4 blocks to my undersize platter to make it function like more current platters.

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