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A clients Question: - One of our detectors shows a decrease in efficiency with time when counting standards. Before opening the counter, I wondered if you might have suggestions on what to look for. Switching the PMT's would tell me if it is the PMT that has gone bad but I was unable to find instructions on how to do this. Is this something we could do here and if so would it need to be done under specific lighting conditions. There are several possibilities: 1) Re-Plateau the PMT There is a chance that you need to re-plateau the PMT. Those instructions are in the manual. If you can not find the instructions get a copy of my replacement manual from my Web-Site, www.atral.com. When you re-plateau, if you notice that your HV setting is on, or below the knee of the curve, then that may be your problem and increasing your HV slightly will correct this. Be reminded that as your tubes age, you have to set your voltage higher (just above the knee) to keep on the plateau, and your electronic noise increases with increasing HV. When this situation becomes too noisy you have to replace the PMT tube. 2) Switch the PMT's Turn the unit off and let it stand for 5 minutes. This provides safe handling of the components and enables the Hi-Voltage to decay through the bleeder resistors in the pre-amps. You may then unplug the chambers from the unit, using a steady upward pressure pull the chamber up from the unit rotating it like you would use a Mortar & Pestle, with a grinding motion. Of course instead of pushing down, pull up. When the tube comes free, look at the tube base and notice that there is a key-way that needs to match the socket on the pre-amp. Now exchange them, being careful to align the key-way. See if the problem switches with them...Be reminded that you have to switch the voltage dial settings on the HV power supplies, because the HV is adjusted to the plateau value for each tube. Remember you have two identical channels that have a common Low Voltage AC input Power Supply, and this is the first step in switching components. This procedure checks out the PMT...however there may also be a problem in the pre-amp. In the latest units, I have slightly lengthened the cables connecting the pre-amps, letting you swap the pre-amp cables without physically exchanging the pre-amps. If you notice that the "good tube" counts OK in the switched position, after correctly adjusting the HV to that tube, then your whole system is OK and the problem is the PMT that was in question. If you find you have one channel that checks out OK, then it makes sense that the Low Voltage AC input Power Supply, which is common to both channels, is also OK. With two identical channels, using a little common sense and by switching elements, you can figure out just what part, board, tube, pre-amp or HV supply is the culprit. I would suggest that you mark the tubes, boards, pre-amps etc., so that you know what's where, and nothing gets mixed up while switching components, otherwise your whole troubleshooting job crumbles, and you will never know for sure where you started. If I have been made aware of other suggestions, I will post them here for anyone to see. If you have a suggestion you think might help please, E-mail to Applied Techniques Co.
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