J. Warner, J. Kissel, M. Pirello
2025-04-06 Power outage: LHO:83753

Among the things that did not recover nicely from the 2025-04-06 power outage was the aLIGO BSC ISI Interface Chassis (D1002432) for Corner 1 of the ISI ITMY. Page 1 of the wiring diagram for a generic BSC SEI (HPI and ISI) system D0901301 shows that ST1 and ST2 CPS, ST1 L4Cs, and ST2 GS13s of a given corner (Corner 1, Corner 2, and Corner 3) are all read out by the same chassis. In the instance of this drawing / rack for ISI ITMY is SEI-C4, and this corner 1 chassis lives in SEI-C4 U40.

This morning, Jim identified that this particular interface chassis failed -- after hearing that Tony tried to re-isolate the platform repeated last night and couldn't -- by opening the ISI ITMY overview screen and found that all of these corner one sensors' signals were reading out a "noisy zero" i.e. 0 +/- a few counts. See attached time-machine screenshot, where the corner 1 sensors are higlighted with yellow arrows showing less than 2 counts.

Jim, Marc, and I went out to the CER SEI-C4 and found that power cycling the chassis would not help.
There happened to be a spare chassis resting on top of the existing corner 1 chassis, so we just made the swap.
We suspect that chassis power regulator didn't survive the current surge of the rack's DC power supplies losing power and coming back (perhaps unequally), but we'll do a post-mortem later.

Taken out Interface chassis S1201320.
Replaced with Interface chassis S1203892.

The attached images show the top of the SEI-C4 rack BEFORE and AFTER.