I have extra photomultipliers to sell. If you are not receiving a signal with any instrument that contains a PMT, it might seem like an obvious part to change to fix the problem but please do not rush to do this. PMTs are highly reliable if cared for, I read one fluorometer service manual that stated they have 100000 hour life expectancies and that all other possible options should be ruled out before replacing the PMT. I have had some that had lower signals then others and it may be worth changing one for this reason. Please be very careful with the software, if the signal is red, pull the lid off quickly. The shutter will activate and the PMT will be protected. These PMTs are very fragile and can easily be damaged by excessive light. Technically you are even supposed to wrap them in cloth if uninstalled as room light can apparently damage them even when they are not powered on.