Paul,

I do not own a SG3...only an SG1.

But generally, anytime a new machine comes in with ribbon spool movement problems and/or ribbon reversal problems, I do a deep clean of those specific mechanisms with denatured alcohol or lacquer thinner and compressed air blow-out.  This usually makes such problems go away in most instances.

If that does not solve it, I compare left to right side and see if a very small spring is missing on one side. 

And I compare how the ratchet pawls one one side compare to the other side to see how they should engage the finely toothed gears.  Sometimes a bit of minor metal-forming fixes a problem, there.
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