Marc P, Chris W
As per Work Permit #6104, we added a 200 kHz lowpass filter (D1600314) to the fast path of the Common Mode Servo Board (D040180), located in the LVEA ISC-R1 rack. The serial number of the servo board is S1102626.
When reconnecting the power to the Common Mode Servo Board, the 17V supply was connected first (should connect 24V first or use the sequencer), which resulted in blowing diodes D4 and D5 at the power input (see D0901846). At the time, we saw LEDs of other boards on the rack turning off. The diodes were replaced and the current drawn by the Common Mode Servo Board was confirmed to be the same as drawn by an equivalent spare board. The board was then replaced in ISC-R1 rack, this time using the sequencer switch to power cycle the whole rack.
For reasons unknown it seems that the first channel on the modecleaner IQ demodulator broke today. The I output gives a fixed 300mV offset. We switched to the second channel (spare).
I also removed 2 TNC-to-BNC adapters, 2 BNC cables and a BNC barrel which were used to connect the I output of the demodulator with the IMC board input. This mashup was replaced with a proper TNC cable.
An FRS ticket has been opened for this event. See https://services.ligo-la.caltech.edu/FRS/show_bug.cgi?id=6084
The IMC VCO had a couple of blown OpAmps that were replaced. The IMC is locking again and the MC_F spectrum looks OK.