Took T240s out fo blends for ITMX Isolation; just used L4C with 100mHz blend.  Looked at local sensors.  0.5Hz peak only shows in Y3.  Toggled Gain--no change.  Zero  aha!

I see there is a large offset on this channel -7000cts Y3.  Hit zero button and the signal zeros, no change in the peak, darn.  Cycled power of Trillium interface chassis.  No change in peak.

Hey, that -7000 ct offset is back on Y3...The offset has been present essentially for as long as we've had data.  Swap cables between Trillium 2 & 3, 0.5Hz peak has moved to Corner2.  Oh boy, it looks like the Trillium.  But, we should check the cable.  Pulled cable from chamber and pins at vacuum flange look fine, reseat cable, not change, peak is still present.

Phil does continuity check on cable--there is an open conductor, Phil finds a failed solder joint at the chassis end.  He is repairing now.

The attached plot shows the 0.5Hz peak with resonances, the Y3 was the only local channel with the peak.  The other traces here are when we swapped the cables at the chamber and the peak stays with corner3.

OK looks like that was it.  Second plot below is now with the ISI back in Isolation with the Trilliums in the blend.  Same reference as above.  Top graph shows the peak on Y3 and the current horizontal local channels without any peak.  The bottom graph shows the vertical local T240 channels where we've never seen this.  The middle graph shows the cartisian X channels of asll the ISI sensors with the Optical Levers and the SUS ISI channel.  No trace of the 0.5Hz peak anywhere.  Yeah!

Thanks Jeff, Keita, Phil & Richard, Sebastian!