I am now able to engage the ISS second loop.  Keita turned on the chassis earlier today.  It seems that one of our various computer changes over the last ~10 months caused a snafu with the SDF, so that a bunch of the channels I needed weren't initialized, and didn't have a setpoint.  I loaded up an old safe.snap, and reverted the secondloop channels to that time. 

I was taking some clues from alog 58845 and also looked at alog 46425, so used the first safe.snap that I found that was near that alog 58845 time.  But, since that alog mentioned that a gain was moved into a filter bank, I did not revert the two channels that were related to that gain (the FM being engaged, and I hand-changed the gain).  See screenshot for the snap time and the channels I reverted.

The CTRL output on the far top right of the second loop screen had a giant number in it (32k) so I put a value into ISS-THIRDLOOP_OUTPUT_OFFSET on the bottom left of the second loop screen to make it closer to zero when the second loop was not on.

After that, I followed the hand-engage instructions from alog 58845 to turn on the second loop, although I did the 'hold' first (so I inverted steps 1 and 2).  Since that worked, I tried a few times to use the IMC guardian, and it is able to turn the second loop ISS on and off.  Excellent. 

There's a bit of a strong transient, since the old ISS liked to have the diffracted power right around 2.5%.  Right now, there is so much fluctuation in the power of the laser system that we've been keeping the diffracted power above 3%, so that the first loop doesn't run out of actuation range.  The first loop is having to work much harder now than in O3, so we should investigate that. If the first loop just is doing what it needs to be doing, then we should look at convincing the second loop that 3% diffracted power is the new normal.

I stop for now, assured that we can actually use our ISS, even if it's not yet optimized (I see the DC power out of the IMC quiet down when the second loop is on, so it's not doing nothing!).