Detchar, please tell us if the 1.66Hz comb is back.
We changed the OM2 heater driver configuration from what was described in alog 72061.
We used a breakout board with jumpers to connect all OM2 thermistor readback pins (pin 9, 10, 11, 12, 22, 23, 24, 25) to Beckhoff at the back of the driver chassis. Nothing else (even the DB25 shell on the chassis) is connected to Beckhoff.
Heater voltage inputs (pin 6 for positive and 19 for negative) are connected to the portable voltage reference powered by a DC power supply to provide 7.15V.
BTW, somebody powered the OM2 heater off at some point in time, i.e. OM2 has been cold for some time but we don't know exactly how long.
When we went to the rack, half of the power supply terminal (which we normally use for 9V batteries) was disconnected (1st picture), and there was no power to the heater. Baffling. FYI, if it's not clear, the power terminal should look like in the second picture.
Somebody should have snagged cables hard enough, and didn't even bother to check.
Next time you do it, since reconnecting is NOT good enough, read alog 72286 to learn how to set the voltage reference to 7.15V and turn off the auto-turn-off function, then do it, and tell me you did it. I promise I will thank you.
Thermistors are working.
There is an OSEM pitch shift of OM2 at the end of the maintenance period 3 weeks ago (Aug 29)
Having turned the heater back on will likely affect our calibration. It's not a bad thing, but it is something to be aware of.
Indeed it now seems that there is a ~5Mpc difference in the range calculations between the front-ends (SNSW) and GDS (SNSC) compared to our last observation time.
It looks like this has brought back the 1.66 Hz comb. Attached is an averaged spectrum for 6 hours of recent data (Sept 20 UTC 0:00 to 6:00); the comb is the peaked structure marked with yellow triangles around 280 Hz. (You can also see some peaks in the production Fscans from the previous day, but it's clearer here.)
To see if one of the Beckhoff terminals for thermistors is kaput, I disconnected thermistor 2 (pins 9, 11, 22 and 24) from Beckhoff at the back of the heater driver chassis.
For a short while the Beckhoff cable itself was disconnected but the cable was connected back to the breakout board at the back of the driver chassis by 20:05:00 UTC.
Thermistor 1 is still connected. Heater driver input is still receiving voltage from the voltage reference.
I checked a 3-hour span starting at 04:00 UTC today (Sept 21) and found something unusual. There is a similar structure peaked near 280 Hz, but the frequency spacing is different. These peaks lie on integer multiples of 1.1086 Hz, not 1.6611 Hz. Plot attached.
Detchar, please see if there's any change in 1.66Hz comb.
At around 21:25 UTC, I disconnected OM2 thermistor 1 (pins 10, 12, 23, 25 of the cable at the back of the driver chassis) from Beckhoff and connected thermistor 2 (pins 9, 11, 22, 24).
Checked 6 hours of data starting at 04:00 UTC Sept 22. The comb structure persists with spacing 1.1086 Hz.
Electrical grounding of the beckhoff systems has been modified as a result of this investigation -- see LHO:73233.
Corroborating Daniel's statement that the OM2 heater power supply was disrupted on Tuesday Aug 29th 2023 (LHO:72970), I've zoomed in on the pitch *OSEM* signals for both (a) when the suspected time of power distruption (first attachment), and (b) when the power and function of OM2 was restored (second attachment). One can see that upon power restoration and resuming the HOT configuration of TSAMS on 2023-09-19 (time (a) above), OM2 pitch *decreases* by 190 [urad] over the course of ~1 hour, with a characteristic "thermal time constant" exponential shape to the displacement evolution. Then, heading back to 2023-08-29, we can see a similar shaped event that causes OM2 pitch to *increase* by 160 [urad] over the course of ~40 minutes (time (b) above). Then at 40 minutes IFO recovery from maintenance begins, and we see the OM2 pitch *sliders* adjusted to account for the new alignment, as had been done several times before with the OM2 ON vs. OFF state changes. I take this to be consistent with: The OM2 TSAMS heater was inadvertently turned OFF and COLD on 2023-08-29 at 18:42 UTC (11:42 PDT), and The OM2 TSAMS heater was restored turned ON and HOT on 2023-09-19 18:14 UTC (11:14 PDT).
