J. Kissel, K. Izumi

Summary -- PCALY optical follower servo (OFS) has been saturated since the Saturday, Sep 10th 2016 power outage (see LHO aLOG 29592). Recovery required a quick OFF/ON cycle of the OFS loop from the MEDM overview, which is now complete. All lines are now confirmed back in the spectrum.

The story:

Kiwamu noticed that the 331 Hz calibration line was missing from the wall spectrum once we powered up/reduced the noise enough to see it at the expected amplitude. We looked into the MEDM control screen and found the excitations still happily going out to the DAC. We checked and confirmed that the 1083 Hz line was also missing in the DARM spectrum, implying something was wrong in analog land. Finally, I noticed that the OFS error signal, normally displayed on the PCAL overview (H1:CAL-PCALY_OFS_PD_OUTMON) looked abnormally low and quiet. 

After trending for the past 6 days, seeing the sudden change of behavior at 19:30:48 UTC (12:30p PDT) on Sep 10th. and a little bit of aLOG scouring, I put two-and-two together that the power outage caused the failure. Knowing that turning the OFS OFF then ON is a common "guess-and-check" method of fixing problems with PCAL, I performed this time honored tradition. Normal PCAL functionality was restored immediately after re-closing the loop. Easy fix!

Perhaps we need a PCAL line monitor like we have for bounce, roll, and violin modes, so we can have DIAG_MAIN set up an alarm once we're at sufficient DARM noise levels.