Balancing

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This is pretty hard as unbalance in a decent quality platter is very small so You would ned to balance dynamicaly.
You can try to put the platter in a lathe, if central shaft is part of platter attach a bit of shrink tubing to shaft to avoid dents in shaft.
If platter is free from shaft then use a simple shaft from a scraped tt and thread the tip so You can lock platter to shaft with a nut.
Now You can check static balance with lathe unengaged from drive, and also find any geometrical unbalance turning it at slow speed. If lathe is of good precisison and shaft integral with platter You can adjust smaller errors.
 
A large flat platter can be statically balanced by using a larger ball
bearing in the centre hole and noting where the edge comes to rest.

If you get consistent results usually just proves the
centre hole has been drilled in the wrong place ;) .

Dynamic balancing I presume is much harder.

For a thorens :

If you attach the whole platter sub platter mat etc to
a carefully levelled motor via a flexible drive - rubber
tube from shaft to the bearing spindle and spin it at
speed you will get an edge float to to the inherent
precision of the edge with any dynamic imbalance
superimposed.

You could probably just do this with the normal bearing
and investigate if their is an ideal position for the platter
on the subplatter in terms of edge float.

You only need a slight tilt of the main bearing to show
the static imbalance, if you use thin oil in the bearing.

A tilted platter oscillating to its final position is not a good sign ;).

:) sreten.
 
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