I replace the Cerenkov signal in the MLU-created T4 efficiency trigger in the left arm with the newly installed S0.
But when I checked the T4 scaler, I saw no rate. Upon further investigation, I discovered that there was no signal from the top PMT (S0A), and because the MLU that creates T4 takes in the AND of S0A and S0B, there was no T4.
I checked the HV, but they had no effect. I checked the signal cables coming off of S0 and replaced the BNC connectors. This seemed to work, as I could now see signal from S0A.
I adjusted the HV to give approximately the same rate from both PMTs, and everything seems to be working now.