Since POP_A and POP_B QPDs didn't show any signal whatsoever when I scanned the beam going into PR2 (alog 91397), I got suspicious about the electronics.
I first went to the field rack (ISC R4), disconnected the cable (CAB_H1: IO-245) that is supposed to go to HAM3 (and to the POP QPDs) from the front panel of the transimpedance amplifier, connected a breakout board to the cable and checked the diode connection from pin 16 (common cathode for QPD1) to pin 1,2,14, 15 (anode for four segments of QPD1) as well as from pin 19 (common cathode for QPD2) to pin 4,5,17 and 18 (anodes for QPD2) using a DVM. There was no connection.
I moved to the D3 flange of HAM3. The IO-245 cable was connected to D3-F7 and that was correct according to T2400150 as well as the redline version of D1002874.
I disconnected the cable from D3-F7, attached the breakout board to D3-F7 and checked the diode connection. Again, no connection.
I moved the breakout board to D3-F6, which is nominally to be used for "LO1 M1 RTSDxxxx" and I was able to see eight diodes between right pins. pin 16 (cathode) to pin 1, 2, 14 and 15 (all anode) showed about 0.4V only when the current goes from anode to cathode, nothing in reverse direction. Same thing for pin 19 to pin 4, 5, 17 and 18.
We don't have LO1 inside HAM3 (if LO1 means a new suspension for BHD or something) for now, so I'm 100% confident that the POP QPD cable is somehow plugged into D3-F6 instead of D3-F7 inside.
I connected the in-air QPD cable to D3-F6. I'll be testing if the QPD channels respond to flashlight/illuminator/laser pointer.
Edit: Tagging SYS
I connected the illuminator power cable to the illuminator in preparation, but when I checked the POP_A and B QPDs at that point both of them were already seeing the IMC beam.
I didn't use flashlight/laser pointer/illuminator. Illuminator power supply cable was left connected. Be careful, even if you turn it on remotely the light will be blocked by the back of the black glass between the illuminator and the viewport (alog 52184).
As a quick note, now that we see beam on the POP QPDs, I tried to center the beam on the QPDs using the straight shot from IM4. However, I wasn't able to move IM4 in any direction in pitch that didn't cause the NSUMs on both QPDs to start dropping- maybe we are clipping somewhere. I was able to pretty well center the beam in yaw on POP A using IM4 trans though.