Baffled to find many ground loops, then I remembered that I was baffled by this same thing in all chambers.
Tested some (not all) of the HAM6 ISC cables for ground loop according to T1200131.
Each in-air cable was disconnected from the chassis while still connected to the chamber feed through (other in-air cables were still connected to both the chassis and feed through), and the continuity between the cable shell and the chassis ground was tested using DVM beeper (any continuity=bad).
| cable # | good/bad | |
| OMC DCPDs | 307 | bad. |
| OMC QPDs | 404 | bad |
| OM1/TT1 | 236 | good |
| OM2/TT2 | 237 | bad |
| OM3/TT3 | 238 | bad |
| QPD sled | 232 | good |
| AS_C QPD | 233 | bad |
FYI, for all of the above cables, there was a continuity between pin13 and shell (it's supposed to be like that).
WFS (both DC and RF), OMC PZTs, OMCS and SEI cables were not tested but I'll test at least WFS and OMC PZT.
Anyway, this is not acceptable for the chamber containing our main read out, and should be fixed in chamber.