Yesterday afternoon the lock of the north arm became unstable, due to the common oscillation of the payload pendulum mode at 415 mHz. This is a known problem of the arm lock, likely induced by the radiation pressure,  and stabilized by the use of local dampers of that mode, acting on both F7_TX.

Yesterday the NE_F7 damper stopped working, due to something happened on F7 vertical LVDT #3 (fig 1). Looking better at the signal, one can notice that its response to the vertical motion changed sign and amplitude (fig 2), as the modulation/demodulation phase was rotated by 180+-45 deg.

This morning the problem has been 'fixed' adjusting the gain of that sensor before the d.o.f.'s reconstruction. The origin of the problem has not been understood, which means that it could happen again.