Something is really only marginaly stable.
Attached is an example of low frequency oscillation of IMC WFS DC measured by using a DB15 break out board on the front panel of the DC interface (i.e. the breakout board is inserted between WFS head and the WFS DC interface).
This is easily triggered by disconnecting the WFS DC cable from the front panel of the WFS DC interface and then plugging in again. In this case it was WFSA, segment 4 (i.e. between pin1 and pin9, why is this not the segment 1, I don't know), and it's oscillating at about 100Hz. When this happens, all the quadrants show similar symptom, and it doesn't go away spontaneously. Sometimes this goes away by wiggling the cable or touching some of the pins on the breakout board.
It's not exactly easy to make things accidentally oscillate at such a low frequency, I'm suspicious about power problem, maybe weak grounding of big capacitors e.g. power capacitor on the WFS head.
Interestingly, when the breakout board is not there, when you look at the fact channels for WFS DC, the oscillation dies down on its own within 10 or 20 seconds for WFSA.
I was curious to do the same measurement for WFSB, and with the breakout board it does something similar (second picture, in this specific case it was slower). Without breakout board, I didn't see any craziness. Also, when this was going on, the +-18V power that are passed through to the WFSB head were also showing something similar (third picture, in this case the WFSB was oscillating faster than when the second picture was taken) . I haven't measured WFSA power (see the entry attached to this one).
When they're not oscillating, things look OK-ish in that nothing is outrageously wrong.
And the WFS DC interface board broke in the middle of the above measurement for WFSB.
Circuit breaker switch of the board was triggered, I disconnected WFSA and WFSB from the board and switched the board on again, +15V LED came back but -15V didn't.
I brought the DC interface chassis back to the EE shop, and Filiberto thinks that one of the FETs on the protection board inside the chassis (the one that provides -18V to the -15V voltage regulator as well as to the WFS heads) is dead.
Since we don't have spares, I removed the WFS DC interface that was originally intended for IFO REFL WFS (i.e. HAM1) and put it in place of IOO WFS DC interface. We are not going to use REFL WFS for a long time.
Anyway, after we replaced the WFS DC interface, the DC power came back but we still don't know if there is any damage in the WFS heads themselves.
We noticed very large dark offsets after the new WFS interface was installed. Looking more closely, we saw the same type of oscillations as with the other board. This time, we couldn't stop the bad behavior by unplugging/touching/rebooting, so we turned off the WFS interface board for tonight.