As noted on Friday, RLF QPD A segment 3 railed Nov 29th, and has been railed since then.
Filiberto, Kar Meng, Marc and I went to the rack and swapped the two cables for WFS1 + WFS2 on D2000552. The saturaed segment moved with the cable which could indicate the problem is the in vacuum QPD.
The 105kHz segment went quiet at the same time as the problem on the DC segment. This happened on a Saturday, before the chamber was vented. It had been two days of not locking the squeezer or the IFO before this happened.
Edited to add: This is the QPD used for filter cavity length control. We did lock the filter cavity after the segment broke, December 4th.
The attached plot show that the 105kHz channels from Q3 went quiet at the same time that the DC channel saturated, 10:40 pacific time Saturday Nov 29th. Then, on Dec 3/4th, the squeezer and filter cavity were relocked, and the signal on the 105kHz channels for the other quadrants returned but not on the Q3 signals.
This, along with the observation that the saturation didn't change when the QPD was removed (89020) , could point to the transimpedance amplifier being the problem, as that is common to the DC and 105kHz channels. D2000341
