Fixed (not that they were "broken") the unsoldered connectors.

PDH CM board was modified so that it should be identical to X arm.

For some reason we found that the demod output for the PDH was railed at 10V (or was that -10V?). LO and PDH RF were both OK, but the I output as well as IMON were both railed. The demod chassis has two I/Q demod and the unused one was still alive, so we just switched. The broken one is still left there.

Though the only Hartman path optic on the table was a pickoff for that path, I installed another pickoff downstream of the first pickoff and directed the beam to REFLB diode so we have something to monitor.

We started locking the arm again. But the CM board output to VCO would rail really quickly. We might have to enable LSC path to offload the VCO.

When locked, the arm transmission is not maximized, we're probably talking about 10% or so smaller transmission than the true peak. Looking on the oscilloscope, it's clear that the PDH error signal is not symmetric around the 00 carrier transmission when locked. We swapped the RF source with IFR generator and changed the modulation frequency +-30kHz or so (transverse mode spacing is 28.59kHz for Y arm) with a few kHz step at a time. We were able to observe clear steps in the transmission when we changed the modulation frequency, but this was not a big effect, we were never able to fix the asymmetry of the PDH error around the lock point. In the end we changed the oscillator back to the original.

Since it seems like we have a relatively large 20 mode, one thing to try is to improve the mode matching and see what happens.