Hi Jim:

I completed a modified Lehman prototype just in time to catch the 
September 30 quake in Mexico.  I was test driving it, even though I had 
not yet installed the damper.  I got a very clear signal of both the P 
and S waves, somewhat to my surprise.

My unit uses a 7 pound pendulum mass. The important point is that the 
top node (where the wire exits the upright post) is slightly off-set 
from an imaginary vertical line through the knife-edge of the boom, the 
off-set being toward the mass.  You may look at it as if the top of the 
vertical post is tilted slightly toward the weight.  This off-set should 
be on the order of about 5 mm.  Without this off-set, the pendulum will 
not be able to find a null point, where it will come to rest.  If the 
off-set is in the opposite direction, the boom will try to swing in the 
wrong direction.  It takes a bit of fiddling - and patience - to find 
the right tilt, and I would recommend using three leveling screws for 
the base instead of shims.  I use three 5/16 wing bolts.

Also, remember that the larger the off-set, the sooner the boom will 
come to a rest, but the less sensitive the pendulum becomes.

Toby Thams

Fullerton, Calif.

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