Yesterday, seismic restarted testing HAM-ISI in the staging building. It seems that one of the horizontal GS13 is malfunctioning. The symptoms are similar to those we see when the proof mass is stuck (no low frequency response but high frequency response looks OK). After hitting the GS13 with a wrench and handling it, the mass is still stuck. I have attached Powerspectra of the 3 horizontal geophones when the ISI is locked (H3 geophone is the bad one).
The horizontal GS-13 POD SN 68 was opened and inspected. The flexure rods were not broken and the proof mass was moving freely when the geophone was leveled. One kinematic foot or the crossbar was probably not tightened down allowing the geophone to tilt inside the leveled can. Consequently the mass proof was touching the stops.
Kinematic feet of the horizontal GS13POD SN 68 were readjusted and the crossbar tightened up. After adding the can and reinstalling the GS13 in the HAM-ISI, a powerspectrum (attachment) confirmed that the geophone is functioning properly.