996 Sensitivity and front end RF damage

[nydxa]

In the past two months I've run across two 996's that has visibly low RF sensitivity, especially in the 850-900 mhz band. Conincidently, both radios were mobile installations where two-way 450 mhz radios were in use.

1. Has anyone experienced damage to a scanner's front end, resulting in poor sensitivity, the result of being operated in close proximity to a transmitter?

2. In one of the above two radios, "poor sensitivity" was diagnosed after noting the signal strength on one radio was showing 1/2 the signal srength on the 996's "s-meter". Do 996's have an s-meter adjustment that may have been mis adjusted?

 

[mikewazowski]

In the past two months I've run across two 996's that has visibly low RF sensitivity, especially in the 850-900 mhz band. Conincidently, both radios were mobile installations where two-way 450 mhz radios were in use.

1. Has anyone experienced damage to a scanner's front end, resulting in poor sensitivity, the result of being operated in close proximity to a transmitter?

Happens quite often. The transmitting antenna should be placed as far away as possible from the scanner's antenna. I've personally had 2 radios affected this way.

2. In one of the above two radios, "poor sensitivity" was diagnosed after noting the signal strength on one radio was showing 1/2 the signal srength on the 996's "s-meter". Do 996's have an s-meter adjustment that may have been mis adjusted?

How can adjusting the calibration of the s-meter affect the sensitivity of the unit unless you're relying on the s-meter as an indication that the scanner has weak rf receive?

How do the scanners *sound* compared to other scanners? If the signals they receive are weaker by *ear* then you've got problems with their rf circuitry.

However, I don't believe there's an s-meter calibration function. It's probably factory set.

 

[nydxa]

reply...

Regarding the "s-meter" question, I fully understand that such an adjustment has nothing to do with actual sensitivity. I asked, because I've also seen identical scanners side-by-side (using the same antenna) display different signal levels. Usually most receivers have a caliberation procedure where a signal at a given frequency and level is applied and the meter adjusted to a specified reading, often "S9".

Given the above it "might" suggest that the sensitivity of Uniden scanners is not as consistent as one might assume.

Regarding the possibility of high RF damaging the front end, it gets you to wonder, considering how many Uniden scanners are installed in police / public service vehicles.

 

[mikewazowski]

Given the above it "might" suggest that the sensitivity of Uniden scanners is not as consistent as one might assume.

Or that the s-meter is really just an extremely rough measure of signal received and really shouldn't be taken as gospel.

Regarding the possibility of high RF damaging the front end, it gets you to wonder, considering how many Uniden scanners are installed in police / public service vehicles.

Quite a few and usually by competent two way radio shops that are away of proper spacing between antennas.

I had about 2 feet of spacing between a 50w vhf mobile and that wasn't good enough. Once I got my 396T back, I moved the scanner down to an antenna on the right front of my truck while keeping the 50w mobile on the cab of the truck. Haven't had a problem since.

 

[Skypilot007]

I use 796D and a 45W UHF mobile radio and the 796D has been uneffected so far. The antennae are both trunk lip mounted about 30 inches apart. The UHF radio does completely block reception on the 796D in certain bands when transmitting but it has not seemed to effect the reception. I know it can't be good for the scanner.

 

[topgun260]

996T Repair

Mike,

Did you send your 996T back to Uniden for repair? If so how much was it to get it repaired? I have one that needs to go in for this problem.

 

[nydxa]

Mine was under warranty

Mine was just inside the warranty period, so it was fixed at no charge. I believe it would fall under the flat rate repair they have. The price will be reasonable, but they generally take up to 1 month to get it back to you.

 

[DannB]

Yeah Me Too

Same happend to my 396xt My ham radio did the damage, forgot i had it on high power and transmitted ...Uniden fixed it for 60 bucks works like new now..My fault it happened..

 

[nydxa]

Poor design

To be honest, it's a really poor design. How many of these might be installed in a police car or emergency vehicle? It should be able to withstand the high RF. My Motorola, Icom, Kenwood and Alinco radios have no problem tolerating each other.

 

[PHOENIX_SCANNER]

To be honest, it's a really poor design. How many of these might be installed in a police car or emergency vehicle? It should be able to withstand the high RF. My Motorola, Icom, Kenwood and Alinco radios have no problem tolerating each other.

It all comes down to dollars and cents. Put a (relatively cheap) scanner close to a high power transmit antenna and the front end will be damaged, seen it myself.

 

[Daniel_Boone]

The bottom line is that an RF in close proximity to the receive of the scanner is going to affect it.

Most good radios has a circuit that limits how much power the front end can see.

The 900 MHZ part - the higher in frequency you go - the more loss there is in the cable and the more the antenna has to match the center frequency you desire to receive.
So you can't use a universal scanner antenna if you desire to receive 800 - 900 mhz...

The one problem that I have had with the Uniden BC 890 XLT is that I had both antennas connected by way of the switch to the radio and the scanner and the switch does not have enough isolation inside of the switch....

 

[cowand]

I've sent my 396XT in to the Depot twice for this problem. I was not sure at first, but figured it out after the second trip. I was keying up a VHF XTS5000 in an XTVA, which the RF passes through a 55 watt amp. So basically I was keying up a 55 watt mobile. The antennas are about 18-20 inches away from each other. Both depot trips were under warranty, with about a one month wait each time.

In the past, I hooked up a Radio Shack Pro-96 to the exact same coax with no problems. I noticed that the receive audio muted on the Pro-96, but I never fried it. I also hooked the Pro-96 up when I had my Tahoe, and had no problems. In the Tahoe, I had a 55 watt VHF Maxtrac and used it a lot with the scanner antenna about 18-20 inches away.

So the Pro-96 would handle it, and the 396XT would not. I wouldn't test it again. Thought you all might benefit from this.

 

[benjaminfs733]

Not sure if I can cross reference posts but I just posted some of this here before I found this thread:

http://forums.radioreference.com/un...ty-signal-strength-after-20-30-minutes-3.html

I had this 996 mounted in my cruiser and and worked great for years. Changed jobs and re-installed it in a car with a 400 MHZ Digital radio, 50W. It worked fine for a few months and then I began to noticed that I could not receive a strong signal on our county's 800 analog trunked system. I could still receive 400 Digital and other non 800 channels just fine.

Jump forward to a few months later, I have a new job (again) am now in Louisville (new city) listing to an 800 Digital trunked system and am experience poor reception.

Could I have fried my font end by using such a powerful UHF so close?
Can it be repaired?
If yes, what does Uniden charge to do so?
Any help would be great.

 

[nydxa]

Anyone have a schematic of 996?

Very possible. Does anyone know whereto get a schematic of a 996's front end? Adding some overload protection is not that difficult.

Sure, Uniden can repair it but it may only happen again. I can't believe they never planned for this. 796 does not have this problem.

 

[mancow]

This can alleviate the issues.

Ramsey Electronics RFS1 Mini Kits RF Sensed T-R Relay Kit Ramsey Electronics RFS1 RFS-1 RFS 1

 

[benjaminfs733]

Sure, Uniden can repair it but it may only happen again.

Thanks for the response nydxa. Doubt it will happen to me again as I wont have the scanner near another transmitter, but it is a problem that I would not expect from this quality of a scanner. Thanks for the info.

 

[benjaminfs733]

mancow, is that a replacement for an internally damaged part in the 996? Is it something that someone with moderate experience with electronics and soldering experience can replace if so? Thanks.

 

[magicjellybeans]

996 and RF

I have my 2820H and my 996 antennas VERY close and I have no issues with the 996.Oh yeah and I talk alot (long keys) on the 2820.
I would try a full reset and reprogram her.

 

[jim202]

The simple solution to blowing the front end out of any receiver is to install a pair of hot carrier diodes across the input of the first amp in the receiver to ground. You might have to cut the circuit board and add a series resistor of say 51 Ohms in series with the gate to limit the current through the diodes.

I did this on a commercial General Electric public safety radio after the front end got blown twice by a high powered UHF radar system where I use to work. Also did it to a couple of other Motorola radios in my vehicle while I was at it. Never had another blown front end again.

 

[Gator596]

Ilegal Voltage?

When I have a 396xt plugged into the lighter socket of a vehicle equiped with a Motorola mobile and transmit on it the scanner "wigs out" and displays "illegal voltage". Same thing happens if I transmit on a xts3000 portable. NOTHING happens if I am running the scanner on batteries. What is going on here, and am I damaging the scanner?