You mean it de-senses your scanner. How close is the nearest FM BC transmitter? Without the preamp, who is your loudest signal? Was this a kit or assembled? Try a smaller antenna to make sure the pre-amp is working. You may need some filtering in between because that pre-amp doesn't have any built in.
thanks for your quick reply. The preamp is over 1 year old. The scanner used to pickup more stations and systems and seems as if the preamp overloaded it somehow. I sent the XT back to Uniden for them to tell me it works according to factory specs. Austin Ferret antenna and LMR400 are new.
What you are describing is perfectly normal for placing a preamp in front of most scanners. It could be you were very lucky when it was first used a year ago with not enough local signals to bombard the preamp and cause internally generated IMD which will make paging transmitters, FM/TV broadcast and other signals to appear on frequencies were they don't belong. At the same time your noise floor would be raised covering weaker signals you might have been able to hear before.
You probably have some new strong signals in your area which have pushed the preamp or even the scanner into overload causing these problems. Maybe the new digital TV roll out has lit up some frequencies that now cause problems when mixed with other strong signals.
If you turn on the attenuator in your radio (if it has one) or use an external one with at least as much attenuation as the preamp has gain and the problem goes away that would point to the problem being your scanner is causing the IMD. If the problem persists with the attenuator then the preamp is causing problems.
Usually a preamp is used at the antenna with appropriate band pass filters and this can improve reception by making up for coax loss and the preamp will usually have a lower noise figure than the scanner. But you would need a very high level preamp (expensive) or band pass filter the input of a lesser quality preamp to avoid what you are describing.
PRCguy is right. You're overloading your receiver. If I'm not mistaken, that Ramsey preamp isn't adjustable. I recommend a preamp with a gain adjustment so you can experiment with different settings. I often tell people to use a preamp as a last ditch effort. Using a better antenna and cable comes first. Or if you use a multicoupler, a preamp is almost a must due to the loss in the coupler or filters if used.
I experimented recently with a Stridsberg multicoupler. The one with out the preamp. I had a noticable drop in signals going through the multicoupler. I added an adjustable GRE preamp and what a difference it made. It overcame the loss of the multicoupler and then some.