jeffrey.kissel@LIGO.ORG - posted 20:25, Tuesday 23 August 2016 - last comment - 10:17, Friday 02 September 2016(29265)
IFO Recovery: IMC's Common Mode Chassis (S1102626) Input 2 has 2 [V] Offset; Temporarily Replaced with Spare (S1102627)
S. Dwyer, J. Kissel, C. Gray After successfully recovering the IMC's VCO and recovering the IMC (29264), we were able to get up through LOCKING_ARMS_GREEN in the lock acquisition sequence. However, we found that ALS COMM failed caused lock losses during the next step (LOCKING_ALS), when the input for IMC length control in its Common Mode Chassis was switched from the IMC's PDH output to the ALS COMM PLL output. The ALS COMM PLL output is connected to IN2 of the IMC chassis that had a new daughter board installed in the star-crossed ISC rack H1-ISC-R1 today (LHO aLOG 29250). After fighting through MEDM screen confusion* at the racks, we found that OUT2 (an analog pickoff pick-off just after the input gain circuit) indicated a ~2.5 [V] offset, even with IN2 terminated with 50 [Ohms]. Suspecting that this symptom was indicative that the input gain circuit (circled in red in MEDM screen capture) was yet another causality of the unfortunate rack power mishap today (LHO aLOG 29253), we've replaced the entire chassis (which lives in U14 of H1-ISC-R1) with a spare we found in the EE shop -- S/N S1102627 (or Board S/N S1102627MC). Notably, this spare does not have one of the new daughter boards on which Chris has worked so hard. We're not suggesting this swap be permanent, but we make the swap for tonight at least, so we can hopefully make forward progress. We suggest that IN2 and/or the input gain stage of SN S1102626 be fixed tomorrow, and the chassis restored so we can employ the new daughter board. Other Details: - Before removing the chassis, we powered down the entire rack using the voltage sequencer around the back at the top of the rack. - After installing the rack, we were sure to have all cables connected appropriately before turning the rack power on again (via the sequencer again). - We added a few labels to the IMC's PDH output and the ALS COMM PLL output cables such that they're easier to follow and reconnect in the future. *MEDM Screen Confusion -- whether IN1 or IN2 is fed into OUT2 of all common mode chassis is selectable on their MEDM screens. For the IMC's common mode board (at least for SN S1102626), the MEDM screen's indication of the status of that switch is exactly backwards. When the screen indicates that IN1 is feeding OUT2, IN2 is feeding OUT2, and vice versa. #facepalm
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With Sheila's help, the OUT 2 switch should now be correct for the MC Common Mode Servo medm (H1IMC_SERVO.adl). This change was committed to the svn.
M. Pirello (reported by J. Kissel from verbal discussion with F. Clara) Marc has inspected the Common Mode Board chassis we've removed (SN S1102626), and indeed found several blown transistors and opamps -- and is not even through the chassis test procedure. Unfortunately, the EE shop needs a restocking of surface mount components before we can make the repairs, but the plan is to shoot for a re-install of this board by next Tuesday (Aug 30th).
Repairs to S1102626 are complete and the chassis has been tested with the 200kHz low pass filter. The chassis performance is similar to the previous test performed September 2011.
When the 200kHz low pass filter is activated we detected a 3mV dc offset which should be noted. The low pass filter works as designed with -3dB gain at 200kHz and rolls off nicely. I have attached files from the testing. File details can be found in the readme.txt included in the zip.
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