Jim,
I can only guess that the total spring constant of the spring, its
flexures, and the main hinges is too large. I am presently using
hinges that are 0.005" thick and 3/8 inch wide. The free, non-contacting
area where the hinges cross to the frame angles is about 1/16".

As I move the upper contact point of the main leaf spring along the
boom closer to the hinges, adjusting the mass position accordingly, 
I can reach a point of infinite period or instability. The range of
adjustment (moving along the boom) from several seconds to over ten
seconds is only about 1 cm.

Also, the level of the boom affects the period. As the instrument is
tilted back, raising the mass end of the boom, the period lengthens.
This is reducing the gravity vector and direction of the forces at the
hinge.  Obviously, tilt it too far, and it will flop over.

Hope this helps. Maybe someone else has some suggestion.

Regards,
Sean-Thomas

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