Discussion Category:  Scintillation Counters

Zero counts

Trying to resurrect an LS 6500. Got the elevator working and fixed a printer problem. Now I'm getting H# Count rate too low error when counting. If I switch to just plain cpm I get a count rate of zero. Suspect power supply, but everything else seems to work. Is there a separate HV supply for the PMTs? Any way to test that? I've got lots of instrument troubleshooting/repair experience, just no schematics or service manual make it tough to proceed.

SN is 7067355.  It's got the Wyse thin client.

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Asked by

marchem
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avatar placemarkReply by
dpkleessr

Rob, Obviously who you decide for on service is your choice. I do not even know who BCI has that can still work on them as most of the "old dogs" such as myself have retired. As long as you or anyone else takes reasonable precautions then handling of the PMT's is not that big of a deal. If you decide to go the route of replacing the PMT then we will go into more detail. Again, it is not inherently difficult. It just takes a little time and being careful with handling the PMT. In addition to the "light shock" previously talked about, PMT's are especially sensitive to "physical shock" such that they can be broken and you will have no knowledge of that fact until you try to use it. Generally this type of problem would be manifested after dropping a PMT or bumping it with a good amount of force so this falls into the category of just being careful with the handling of a PMT.

Don

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avatar placemarkReply by
LG1

I still think we'll opt for service since it sounds like getting a PMT from 3rd party could be risky, as well as handling them and requiring recalibration, but for my knowledge how would I access the PMT if I wanted to swap? After removing the top cover is it pretty simple to open the lead shielding?

Also, is the DS2 light being on indicating that is the defective side? Or the side that's working? 

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dpkleessr

Rob, there is some benefit in trying that in that by knowing right now which side is showing activity and swapping the tubes around you can see if the activity LED's change with the swapping of the tubes, which is much easier than swapping the sockets around.

Don

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dpkleessr

Forgot to discuss your statement about what to expect from Beckman. First, when I retired 5 1/2 years ago, a single PMT cost over $5,000 and a socket was over $1,000. Ridiculous pricing for parts whose real cost was a small fraction of that price but their pricing just the same. A 3rd party service group may be more cost effective but I have little knowledge of any reputable groups nor there actual reliabilty or capability. I wish that I could help more but I am really out of the loop beyond my own pre retirement experiences.

Don

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marchem

Don, would swapping the left and right PMTs directly be worth trying?  I know it's a hassle to get them out, but that should identify if it's a PMT or socket problem.

Rob

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dpkleessr
No, those are the high voltage anti static probes you are seeing in the picture. The PMT's are found inside the lead shielding found inside of the top cover. Don
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avatar placemarkReply by
LG1

Okay, sounds like something we'll just have Beckman do. Just wanted to make sure I wasn't missing something simple.

For estimating the repair cost, would you agree it sounds like 1 PMT and/or socket is bad but the other good? Also, just for my knowledge, am I identifying the PMTs correctly in this picture? 

https://drive.google.com/file/d/1uBFxbDP2F-Im_j6q9eEGNgTMTIF7_bAJ/view?usp=sharing

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dpkleessr

First, light shock only happens when the PMT's are directly exposed to light such as when you are actually working to replace them and not by just taking any of the covers off of the counter. Next, there are NO major electonic parts that are interoperable between the LS 5800 series and the LS6000/LS6500 series counters. Finally, replacing a PMT is not difgicult at all and takes very little time but it does require careful handling of the PMT itself. The PMT socket replacement is much more involved as it requires taking the center rear panel off in order to get easy access to the cabling which is zip tied along its length and there is a LOT of cabling to deal with. I would be VERY careful purchasing replacement PMT's on line UNLESS they come with some performance guarantee. You will only know that a PMT is good, assuming the PMT socket is functioning properly, when you place it into the counter and power it up and then try to count samples or calibrate the counter and you MUST recalibrate the counter whenever a tube, socket, or front end board are replaced.

Don

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LG1

Okay, so information so far is indicating 1 bad PMT/socket, other PMT and front end board likely okay? 

In your experience, more likely to be the PMT or the socket? And if I were to get my hand on a replacement, how involved is it to switch one of these out? Also, we have an old LS5801 here, any chance parts are exchangeable?

Regarding light shock, does this happen when front cover is off? And is it enough to result in 0 CPM?

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avatar placemarkReply by
dpkleessr

Light shock takes place when the PMT's get exposed to fluorescent or sunlight and it literally can take a day for the tube(s) to calm down enough to get normal counts or even a valid calibration.  It is always best to work with the PMT's under red light conditions.  I had a headlight with red LED's on it that I used when working with the PMT's in a dark room.  Not really a part of your attempts to fux your problems but some perspective on how to safely work with PMT's.

Don

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