Today I went to EY to investigate the issues with the PCALY OFS described in aLog 26229. The first thing I noticed upon approaching the PCAL TX module were a pair of cables (red and yellow) coiled underneath the TX module enclosure.  Upon first inspection, I didn't notice anything else egregious.  Everything was powered on and appeared to be functioning.  I then opened the TX module and verified that the PCAL beam was making it to the OFS PD.  As I was closing the TX enclosure, I noticed that a cable was exiting the TX module at a non-perpendicular angle.  I then noted that this particular cable was the OFS PD cable.  It practically fell into my hand as I went to check it.  I then plugged the cable back in and secured the fixing screws on the connector to the module.  For peace of mind, I then checked ALL cables on the TX module and chassis for security.  Satisfied I had fixed the problem, and unable to log into the workstation adjacent to the PCAL module, I returned to the control room.

While the signals on the PCALY MEDM appeared to be reasonable, Kissel suggested that I turn the 3 PCALY exc. lines back on the verify.  When I did so, I saw that the oscillation monitor railed.  Turning them on one at a time, I could get the first two on, but the third (1kHz, 15000 amp.) line caused a rail.  I gradually stepped up the amplitude of the 1kHz (3rd) line, and saw that at ~4000, it railed.  I began playing with the OFS offset, which was set to 0.56V, and saw that by increasing by a factor of ~3, I could turn on the 1kHz, 15000 amp. line without issue.  Trending the offset, I saw that it changed from 4V to 0.56V on Mar. 23.  I could not find an aLog referencing this change, so I have restored it to 4V. 

After finding an OPS shift aLog referencing Fil and Peter doing table access system work on Mar. 17, I asked Fil if they were doing similar access system work at PCALY on the 18th.  He told me that they had replaced the key switches on the PCAL setup that day.  He also told me that the pair of cables I had noticed coiled under the TX module were for the access system, but had been done further in the past.  Mystery 1: What could have happened to the OFS? SOLVED.  Mystery 2: Why does LIGO always have a loose cable? Unlikely to ever be solved.