Hi, I got my sbt-11a a couple weeks ago, finished installation and it worked fine. I go to test it today and it reads 3k cpm with no radioactive sources nearby. I had is sitting in my garage for a while. Is it possible the freezing temperatures break it? When I shake the tube it sounds like there is something broken inwside it. But I heard this when I first got it.
It could be the tube problem. You need to make sure the tube voltage is correct. If the voltage exceeded the upper limit of the tube. It may damage the tube.