I spent some time the other day trying to diagnose the problem with the Y Arm universal PDH box (S/N 17), which Katrin has been unable to use for locking the green beam. As far as I can tell, there is nothing wrong with the box itself (though the weird TF behavior that Katrin noticed was not initially reproducible, so its cause may still be there).
I did notice that I was unable to generate a PDH error signal using the universal box. In this configuration, a summing circuit is needed to add the PZT modulation signal (fmod = 178875 Hz) in along with the feedback signal. To do this, Katrin was using a slightly different version of the passive summing box that Kiwamu built for the X Arm. I read this entry to understand how it is supposed to work and noticed that the "expected transfer functions" were not what the circuit actually does. I have talked to Kiwamu about it and he found that he posted the wrong TFs (he has the right ones on his computer). As you can see from the plot below, there is extra low passing that severely attenuates the modulation path to the PZT. In addition, there is a phase shift of ~-60 degrees, which is bad.
To combat this, I propose we simply change the resistor in the modulation path from 1M to 10k. This leaves the feedback path TF unchanged, and changes the mod path into a sort of bandpass filter for the modulation frequency. The fact that the phase is near zero at fmod means we don't have to come up with some way to phase shift the signal for demodulation. The attenuation level of ~-36 dB is also convenient: The ZAD-8 mixer wants 7 dBm, so, 10 dBm (FG) - 3 dB (splitter) - 36 dB (sumbox) = -25 dBm ~ 12 mV. This is roughly the desired PZT voltage level.