Any chance that you have or had a transmitter operating near the scanner.
I was given 6 x Uniden 996's that were used by a search and rescue group with antennas that were installed within 4 feet of a 20 watt VHF hi band comm radio on vehicles and one in a aircraft with the TACTICAL FM radio operating at 10 watts.
All RF preamps in the scanners were damaged just after they were installed by someon else besides me.
All antennas were checked and found to be VHF quarter waves on the FM comm radios and tri band VHF/UHF/800 Mhz on the scanners and all were connected properly to their respective units but still all scanners receiver preamps were damaged.
The cessna C208 was using a Comant VHF TSO'd aviation, 148-174 MHz antenna, for both the scanner and for TACTICAL 10 watt FM unit
The scanners were scanning a considerable amount of the VHF band along with some UHF and some 800 MHz trunking systems at the same time while the SAR FM TACT radios were operating near 155.805 MHz.
The 20 watt VHF radios are all set to be transmitting at 20 watts or +43 dBm of RF output power into the quarter wave antennas while the Cessna radios power was set to 10 watts into a Comant 148-174 MHz VHF aviation antenna.
Even if the antennas had been spaced at one wavelength, in the middle of the VHF band lets say ~ approximately 6.5 feet apart, the path loss would only be 22 db and the front end of the scanners would still see +43dBm-22 dB ( antenna spacing/isolation) or around +21 dBm of power ( just over 100 milliwatts) which the receiver preamps may be able to take but still I wouldn't feel comfortable even at one wavelength of spacings.
Since all of these installations, including the Cessna C208 used by another agency not a SAR group, they still used antennas spaced at 4 to~ 3.25 feet the isolation number was only 18 to 20 db ( isolation was measured in all installations) this stilll equals +43dBm - (18 to 20db) or +23 to +25 dBm possibly going into the front end of the scanners if the scanners were tuned to the same VHF high band as those that the TAC FM radios were transmitting near.
seeing that +23 dBm is 200 milliwatts going into the adjacent receiver, operating in the same band, and this is not a comfortable number for the installations in which these radios are used.
As a result I replaced the damaged receiver active devices and installed back to back schottky diodes across the receivers input which so far has eliminated the issue.
I also had a similar issue with some of our SAR dog tracking collars when operated near our VHF mobile radios.
The dog collars operate at 151.825 MHz and the SAR radio were on 155.805 and the dog collars receiver along with some of the handlers GPS receiving units had damage to the VHF receivers front end amplifiers.
Adding back to back Schottky diodes resolved this issue also in the collars and in the handheld GPS units VHF receivers.
Looking back at past history, I had a similar issue with another SAR group several years ago in Arizona where their 60 watt Vertex FTL1011 low band mobiles were damaging a VHF high band FM tactical radios receiver amplifier.
The VHFhigh band radios ( Vertex FTL2011's) were being used for both ham and SAR 150 MHz use.
These installs in 4X4 vehicles also had closely spaced antennas on the rear and even though they were in different bands VHF hi and VHF low, the use of a 5/8 wave VHF high band antenna which is also an effective VHF low band quarter wave antenna was an issue.
It was discovered that the 3rd harmonic of the FTL1011 low band transceiver, while even though into a 50 ohm load it met a -58 dBc spec on harmonics out to the 7th order, a 3rd harmonic was barreling thru the VHF high band 5/8th wave whip which was being used as a VHF low band 1/4 wave antenna resulting in very little isolation thereby causing receiver damage to the FTL2011 VHF high band receiver amplifiers.
The solution was to add a 3rd harmonic stub/notch to the FTL1011 transceivers output prior to the VHF antenna and back to back schottky diodes to all of the receivers input just in case.
