Chapter 6 79
Maintenance
Replacing the Photomultiplier Tube
The PMT should not need to be replaced frequently if it is handled correctly. If the
output signal begins to deteriorate, increase the PMT voltage. If the required PMT
voltage becomes excessively high or if the type of PMT needs to be changed, then
use the following procedure.
1. Using WinPulse, set the PMT voltage and ignitor current to zero. Disconnect
the PMT HV cable and the PMT signal cable from the Detector Controller.
2. Turn off the Detector Controller and allow the detector to cool to room
temperature.
3. Detach the PMT housing assembly (Part #286096) from the PMT mounting
by loosening the PMT thumbscrew (Part #177065) and gently pulling off the
housing. Rotate the housing back and forth if necessary to remove it. Open the
PMT housing assembly by unscrewing the PMT housing from the cap coun-
terclockwise (see Figure 6.3).
4. Place the PMT housing assembly onto a clean, lint-free lab tissue, ensuring
that none of the contents fall out.
5. Carefully detach the PMT (Part #284364, #290049, or #290106) from the
PMT socket and place onto a clean, lint-free tissue or in an optical storage
container.
6. Insert the new PMT into the keyed PMT socket. Reinsert the PMT into the
body of the PMT housing. Screw the cap (with attached PMT) back onto the
PMT housing body. If the filter rattles when the cap has been fully screwed
on, an additional Viton O-ring (Part #288084) is required to take up excess
space between the PMT and the filter. If this is the case, remove the PMT
again, install an additional Viton O-ring, and reassemble the PMT assembly.
Note: Do not use excessive force when tightening the PMT housing together.
PMTs can be easily damaged by overtightening the housing assembly.
7. Verify that the PMT Viton O-ring, which causes the PMT housing assembly
to fit snugly onto the PMT mounting, is in place and not damaged. Replace if
necessary. Twist the PMT housing assembly back and forth onto the PMT
mounting and reattach it using the thumbscrew.
8. Route the PMT cables to avoid high thermal zones, sharp edges, or places
where the cable could be pinched or sheared.
9. Connect the PMT signal cable and the PMT HV cable to their respective
connectors on the back of the Detector Controller.
10. Using WinPulse, reset the PMT voltage to its appropriate voltage (e.g.
nominally 600 V).
11. Optimize the PMT voltage. See step 12 in “Starting the PFPD System” in
Chapter 5.
!
CAUTION:
The PMT optical
parts are easily
damaged and
may have to be
replaced if they
fall out of the
PMT housing.
!
CAUTION:
Do not use
excessive force
when tightening
the PMT housing
together. PMTs
can be easily
damaged by
overtightening
the housing
assembly.
!
WARNING:
Handling the
PFPD while it is
hot can cause
serious burns.
!
CAUTION:
If the PMT
voltage appears
to be unable to
accept any value
other than zero
volts, check for
light leaks.
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