Between October 18 and 24, 2019, Philippe Nguyen accompanied by Ian Hollows investigated the 2.023695 Hz comb at H1 using a portable magnetometer. It was visible in both the SEIRACK and SUSRACK magnetometers at EY (with the peak amplitude at the SUSRACK being about twice that at the SEIRACK) but it was not seen at EX. 

One of the slow controls units at EY had its top panel removed so we had a good view of most if not all the LEDs inside. The ones that were blinking seemed to be blinking quite fast (>12 Hz based on slow-mo video), and close enough in frequency to each other that there was no obvious 2 Hz beating between any pair. We made numerous measurements around the area and found that the comb was stronger when measuring the middle of the three slow controls units than when measuring any other unit in the area (including top and bottom slow controls). The comb was even stronger when measuring the power strip shared by the three slow controls units, compared with measuring near the units or near other power strips. The ranking of comb amplitudes, from strongest to weakest was as follows:

1) Slow controls power strip
2) RF oscillator power strip (right above slow controls power strip)
3) ISC rack power supply (on top of the rack)
3) Slow controls middle unit
4) Slow controls top/bottom units
5) Adjacent racks (TCS/PEM and SUS-C2)
6) Furthest racks (SEI and SUS-C1)

It seemed as if the comb was coming from all three slow controls units. The slow controls power strip does not lead to the power supply that powers the RF oscillators.

The comb briefly disappeared on the morning of October 22, 2019, which coincided with the removal of the entire illuminator chassis from its rack. On October 29, the illuminator chassis was powered off as a test between 09:24-09:50 PDT but the comb was still there. Considering that, and that the comb was present during the May 21, 2019, illuminator failure, we now suspect it only disappears when the entire chassis is removed from the rack. A portable magnetometer was left in operation by the EY SUSRACK. The illuminator chassis was isolated on October 12 and we are currently checking the magnetometer records to identify whether the comb was still present.