This morning I had a quick look at the photodiodes in the ISS photodiode box. Before doing anything
I noticed that the diffracted power chart on the MEDM screen displayed a large (many percent) difference
between the minimum and maximum diffracted power. The REFSIGNAL slider was at -2.02, which is the value
I seemed to remember where it was left last night. PDB is used as the in-loop sensor.
The DC output of PDA was measured with a multimeter to be -11 V. MEDM reported a value of 5.02 V.
Which is inconsistent with the REFSIGNAL setting. At this point ~2.8 W was diffracted. I also noticed
that the output of the quadrant photodiode was quite different from what I remembered the previous day's
reading was (that's just a feeling, not an absolute fact).
I asked Richard to check the DC power supply that powers up the AOM driver. It reported as being fine.
I power cycled the AOM driver but the behaviour did not change. The position of the beam on both PDA and
PDB checked out okay. The AOM driver checked out fine about a month ago when problems with the ISS were
observed before.
At this I would say that the problem is not related to optics, and that there is something intermittent
in the AOM signal chain.
At the moment, the second loop ISS output enable switch is enabled without the first loop mis-behaving. This is different behaviour to how things were as recently as yesterday afternoon. Trend data for some signals suggest it might be worth looking at the op-amps N20, N22, N29, N31, N39, N30, N2 and N34. With perhaps the most likely candidates being N32, N29, N2 and N34. As I recall the board does not have a silkscreen so identification of the chips requires a bit more than a cursory glance.