Hi, Bob.
Care to tell us what frequencies,antennas and types of feed lines you
have used the tuner on? Have you ever put RF ammeters in the feed line
to see if it's really balanced?

Did you gear drive or chain drive the two coils? Looks like a nice
job!
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To check whether the feed line is truly balanced also depends on the
symmetry of the antenna. Any 'unbalance can best be determined by
clamping a RF current sensor/meter across both feed lines at the same
time, not by measuring the difference between the 2 lines.

Frank GM0CSZ / KN6WH