BCD996T: Scanner Sensitivity

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EncryptedTech

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I have been using a 996XT for years and I just traded some old portables for a 996T, I noticed that the 996T does not receive as good as the 996XT. So I pulled out the service monitor to play around , most of what i listen to is VHF so I tested on 158.9400 with no Dpl or Pl. The 996XT would break at .32uV -117dBm and the 996T would at -84uV -84dBm. Has anyone ever had these problems? I looked online and the spec of the 996T is the same as the 996XT, is there a "common problem or Fix" to fix this? Any help would be greatly appreciated.
 

JamesO

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33 dB difference!!! Something is broken or wrong here!!! 33 dB is HUGE.

You mention to break the squelch, this may not be a wise test. Also do you have the squelch set consistently at 2 bars on each radio?

Also make sure the 996T does not have the attenuator on.

If the attenuator is not on, at 33 dB difference I would have to guess that the front end or filter matrix has a problem. Most likely caused from a installation where there was too much near field RF energy from a transmitter that damaged something?

Is this just in a specific band or across the board in frequencies?

Also you may want to test with a 1 kHz tone, squelch off to full quieting and see what the difference is between the 2 radios.

I could see maybe a 3-4 dB delta, but 33 dB is not likely a difference in the design but a problem with the radio if the attenuator has not been turned on.

Also compare the radios below 30 MHz, using a Weather channel and in the FM broadcast band. I have seen a lot of the x96 models where there are failures in the filter switching matrix or mixer stages where the radios only work properly in the lower frequency ranges. I did not find the specific switch point, however, I have one radio that will not receive any of the FM broadcast band or above, but seems to be fine in VHF Low Band, it picks up the CB band fine but this is it.

30 dB loss can also be a broken antenna connection. I have seen 30 dB loss when there is no physical/electrical connection on an antenna or feed, so it may be worth checking to make sure the BNC female terminals are not spread due to someone using the wrong center pin or a wire and then maybe open the scanner and check the connection from the BNC connector to the main circuit board. Make sure there is not a cracked or broken solder joint.

Good luck.
 
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FeedForward

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Obviously, that's why someone traded you for the 996. Something in the front end is faulty. Shorted overload diodes (if they exist)? I would guess the first RF amp, or parts directly associated with it. I'd have a closer look.

FF
 

tilt404

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I learned I would never buy a scanner used since I remember so many people used them next to ham radio equipment or had antennas outside right next to the scanner antenna and would transmit on the same bands using high power which could damage the scanner's ability to receive. Its so hard when buying used to know where the scanner has been. : (
 

jackj

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I'm going to disagree with the other experts here. Sounds to me like you have either a mixer or IF problem. An RF amp, if the radio has one, would reduce the 20 db quieting to around 3 µv. A dead IF amp will result in much worse sensitivity. If I'm not mistaken, that radio uses mostly ICs so repair won't be a matter of replacing a transistor. Unless you are experienced with replacing components on modern equipment, leave the trouble shooting and repair to someone who is.
 

EncryptedTech

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Thanks for the replies,! I tested it with a 1k tone and both squelch settings were set to the same. I never noticed an attenuation setting on it, where is it located? I will play more with other Bands when I get home, I only tried VHF because that's mostly what I listen to. The scanner wasn't any place that it could have been damaged by strong signals, it was my uncles, he had it in his garage and wasn't using it anymore. He had it since new and I bugged him for months to make the trade. Ill have to check the soder connection to the board also, I didn't try and open it up yet.

Thanks again
 

wa2sqq

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I bet the radio was used in close proximity to a transmitter. My 996 has been back to uniden twice to replace the front end transistor. They won't provide a service manual so it's virtually impossible to identify the transistor. Very poor design!
 

ko6jw_2

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I killed my 396T with a 6m transmission in close proximity. Sent it back to Uniden for their flat rate repair and it came back as good as new. They also updated the firmware and replaced the display window that had a slight scratch. They do not protect their front ends as commercial or ham radio manufacturers would. Not smart. I'm very careful now.

The built in attenuator will not protect the front end since it is a gain adjustment not a true attenuator.
 

FeedForward

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However, a low pass filter on the front end of the scanner WILL protect it if you are operating on the HF bands.

FF
 

UPMan

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I would think high-pass filter to protect against HF (block HF, pass VHF/UHF+).
 

wa2sqq

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HF not a problem.

However, a low pass filter on the front end of the scanner WILL protect it if you are operating on the HF bands. FF
Never had any problems with HF, and I can run the full legal power. The problem comes up in mobile installations, when you have a 996's antenna installed just a few feet away from a 50W VHF or UHF antenna. This is the exact scenario that exists on thousands of emergency and public service vehicles. In the past year I know of 6 cases. I'm now suggesting other brand scanners or using an older commercial radio that was designed correctly. You would think Uniden would have factored this into their design.
 
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