... it was not possible to get the cavity locked in green. So we decided to do the first measurement with infrared locked only.
When we sat down to align the Yarm to the green, the green light was happy to flash in the cavity, but wouldn't lock, even after Jan had tweaked the mirrors such that we were flashing the TEM00 mode. When we went down to the end to investigate, the Universal PDH box was saturating both negative and positive. Turning down the gain knob all the way to zero didn't change anything, so I put it back to 52.5. Curiously, when we unplugged the Servo OUT monitor cable (which was presumably going to the rack to be acquired), the saturation happened much less frequently. I think (but I need to look at the PDH box schematic) that that's just a monitor, so I don't know why that had to do with anything, but it was repeatable - plug cable in, almost constant saturation....unplug cable, almost no saturation.
Also, even with the cable unplugged, the light wouldn't flash in the cavity. When I blocked the beam going to the green REFL PD (used for the PDH signal), the light would flash.
Moral of the story - I'm confused. I'm going to look up the PDH box schematic before going back down there to investigate.