A replacement of a PMT or repairs of the electronic amplification chain can be accomplished by removal of that specific PMT-Base combination. Turn the HV off on all PMTs and remove the rubber hood covering the base and housing interface region. Now remove the three small screws attaching the base to the integral housing. Note that the base can only be secured to the housing in one specific orientation.
Carefully slide out the base with the PMT and -metal shield mounted as one unit. Remove the elastomeric ring positioned between the PMT and the -metal shield. Loosen the nut securing the -metal shield to the base and carefully apply upward force on the shield while someone else is holding onto the base. This will remove the PMT and the -metal shield from the socket and base, respectively.
Replacement of the PMT requires experience because it has to be done with the -metal shield installed in, but not secured to, the base. The PMT pins need to be aligned with the socket pins in a specific geometry, thus, the insertion has to be done by feel and experience. Once the PMT is inserted in the socket, the -metal shield is secured the base with the nut. Make sure the shield protrudes past the photocathode as much as the tapered design allows. Carefully insert the elastomeric ring between the PMT rim and the -metal shield. This ring supports the PMT and prevents it from sliding out of the pins during movement; it also helps seal the interior of the counter from the outside environment and reduces CO2 leakage rate. Reverse the process for installation.