Zero counts

8 years agoReply by

As I explained, zero counts means something is defective from the PET's on back towards the front end board. Yes, there is a separate HV power supply assembly found in an aluminum housing that 2 of the PMT cables if in to. The only issue is that it is VERY easy to load down that power supply, even with a DVM, so getting any readings can be problematic. One thing you can do is open the chassis up and make sure that there is not an excessive amount of dirt build up inside of the chassis and on the circuit board. Your LS counter is a "coincidence counter" such that both PET's need to see the light flashes within 20ns of each other to be recognized as a valid count event so one defective tube or socket assembly will prevent it from registering any count activity. I replaced a lot of PMT socket assemblies over the 23 years that I worked on them. I also replaced quite a few front end boards as there was a known defect where one of the surface mount capacitors was installed backwards and eventually failed causing the problem you are seeing. I don't know if your counter has the Singles counting option or not because it can be useful in troubleshooting your problem, too. It is difficult to do much more without eyes on so not sure what else to tell you only that PET's and their sockets were ridiculously expensive even over 5 years ago when I retired and the entire LS product line went obsolete over 7 years ago so how much help you might still get from BCI is problematic, too. Don

8 years agoReply by

Thanks for the info. System is surprisingly clean and dust free.

There are two LEDs on the HV supply. DS1 lights when counting and DS2 flashes briefly then stays out. If I disconnect the cables to the PMTs both LEDs stay on. If I swap the cables DS2 lights, and DS1 does not. So the problem seems to be on the left side: either cable, socket, or PMT. I'm tempted to dig into the detectors and see if I can swap things there amnd/or hopefully see some obvious problem, even if there's not much I can do about it.

How can I tell if I've got the Singles option? Would that be listed in the System Info page?

8 years agoReply by

Check under your special programs and also in a user program where you select your counting mode. You are already on the right track as to diagnosing which side is the problem. Just be careful as the PMT's are sensitive to physical shock and exposure to sunlight and fluorescent lighting. Light shock can literally take a day to calm down. Physical shock can and does break the PMT's. Swap out the connectors coming from the PMT sockets to the front end board and see what happens. This is easier than the next step of physically swapping out the sockets on the tubes. Let me know. Don

8 years agoReply by

The HV supply is working fine now, with both LEDs on steady, and a voltage of about 1100 (measured with HV probe). I've been watching the LEDs on the coincidence/analog board; DS4 lights when it counts, but DS2 doesn't. After swapping things back and forth between Left and Right it sure looks like the pre amp/base is the problem. I've got a line on a replacement, but will try cleaning the old one first. Apparently the left side gets dirty from the fan on that side and can cause problems.

I'm not seeing a singles option anywhere, unfortunately.

8 years agoReply by

I never was able to "fix" a preamp by cleaning it. Replacement is generally what you will be looking at but definitelty give it a try and you are correct in that just about everything on the left side gets messed up because of the air flow although that fan runs so slow that it really did very little cooling and I found more than a few poorly maintained LS counters with non functional fans still functioning properly. One thing you may find that I dealt with on a regular basis is that the closed cell foam around the PMT's has degraded to the point where it really no longer acts to support the tube within the shielding to help keep the tube centered. If you do find that it has been squeezed flat then I recommend that you replace it as this can have an impact upon the overall counting efficiency.

Don

8 years agoReply by

Thanks for the tip about the foam. It's looking a little tired, so if I do get things working I'll be sure to replace it.

8 years agoReply by

If I can think of anything else then I will let you know. It is has been around 5 years since I actually worked on one of these so talking about it is dredging up old memories.

Don

8 years agoReply by

Your advice and help is greatly appreciated. Service engineers who are willing to share information with customers are worth their weight in gold; not all companies allow/encourage it and not all engineers are willing to do it.

Rob

8 years agoReply by

The big difference is that I retired from Beckman Coulter over 5 years ago so I have no horse in that race anymore. Do I have issues with some party service groups? Absolutely, because way too many of them have service people that are not much more than warm bodies telling end users that they know what they are doing. I have and will continue to help those individuals that I can as long as people are courteous and show at least some thankfulness towards someone that is in essence giving them the benefit of 33 years of factory trained service. Those that don't I make note of and ignore but they are the exception. One other thing you might check is the connections on the high voltage plugs for the PMT socket. They used crimp connections on the plug and sometimes soldering them brought things back to life. Just a thought. Don

8 years agoReply by

Are those the connections on the socket side of the board? if so, it looks like a tight squeeze to get in there and solder them, but I can see how that might help.

Rob