With Takahashi-san and Hirata-san.
Summary: after investigation, we identified the problem was in geophone #2. Next week, need to check the in-vacuum connections first and, depending on the result, change the geophone preamplifier.
We began by changing the distributor, and then we changed the power cable and the socket it's connected to in the power strip. In these cases, the switch break-out at the back of the distributor would turn itself off. Finally, we disconnected the geophones cables one by one, and identified the problem was geophone #2.
Then we measured the resistance values between the pins powering the geophones, which are 4, 5 and 9 for VP, VN, PWR_P respectively (per JGW-D1503600-v16 page 8). For geophone #2, we found the resistance between pins 4 and 5 to be about 10 Ohm, whereas for the other two it was open circuit or of the order of many MOhms.
Next week we will investigate the in-vacuum cable first and then consider whether we need to change the preamplifier.