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  #1 (permalink)  
Old 03-02-2007, 9:52 AM
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Default BCD996T Reception problem or Repair

I have this radio for around 3 weeks, while generally pleased with the overhaul operation I am experiencing a problem. The radio is operated off of a 100' tower with an ICOM Discone antenna and the same one used with my previous 780XLT at this location. The problem with the 996t is either intermod or spurious channel rejection. NOAA 162.550 clear one minute, signal strength drops (most likely do to adj channel interference) and is in audiable the next. Same problem with a number of other VHF frequencies, 155.490, 155.130, 154.845, 155.565. Some of those signals were always somewhat weak, but never plagued by off on operation with the 780xlt.

The core question for UPman, is this characteristic of the design of this radio or should I return it for repair and evaluation? The 780xlt produces superior results on the same antenna, tower, and frequencies.
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  #2 (permalink)  
Old 03-02-2007, 12:28 PM
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Just a thought, have you put an FM Broadcast trap filter in the line? Could very well be plain old desense, quite common with consumer grade gear which isn't really made for really good antennas, being expected to mostly be used with the on-the-box stick antenna.

The biggest culprit is usually FM Broadcast, though any nearby high power signal can affect you.
  #3 (permalink)  
Old 03-02-2007, 5:03 PM
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It's not FM broadcast thats the problem, or you would think the 780xlt would have been plagued also. But when you normaly have 4 bars of signal strength, and it flucuates down to 1 and 2 bars, there is a problem especially on the frequencies I have illustrated. I have never seen intermod drop the carrier freq signal strength. I also notice no Adj Channel or spurious rejection specs in the documentation. What it's looking like is the BCD996T is a less than stellar performer on the VHF high bands, or the unit needs service. What I will do is check the site with an indentical 996t and see if it performs the same way.
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Old 03-02-2007, 5:29 PM
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It's likely just the way it's made. Mine is the same way. I stopped using it on Vhf all together at home except for known stored frequencies with PL od DCS. If I let it run in carrier squelch or search it's completly unusable.
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Old 03-02-2007, 6:10 PM
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I am using it in PL and the interference drops the signal strength meter. What a shame. I have another one on order to confirm my suspicions. Unfortunately one just may end up on Ebay.
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  #6 (permalink)  
Old 03-03-2007, 4:32 PM
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The second BCD996t arrived. Needless to say it's just the way these boxes are engineered. It performs the same (very poorly) on VHF as my original unit does. If your primary listening environment is VHF beware. Unless you plan on spending money for par filters, and traps and hours of tuning! Too bad Uniden can't or won't acknowledge. It seems like the primary focus on this box is digital and 800 trunking while corners were cut in other bands. IMHO it is grossly overpriced for this kind of performence. Flame me if you like, but the truth is if it wasn't for the laundry list of bells and whistles, Uniden has gone backward in performence with this model.
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Old 03-03-2007, 5:14 PM
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Quote:
Originally Posted by RPSINFOMAN
The second BCD996t arrived. Needless to say it's just the way these boxes are engineered. It performs the same (very poorly) on VHF as my original unit does. If your primary listening environment is VHF beware. Unless you plan on spending money for par filters, and traps and hours of tuning! Too bad Uniden can't or won't acknowledge. It seems like the primary focus on this box is digital and 800 trunking while corners were cut in other bands. IMHO it is grossly overpriced for this kind of performence. Flame me if you like, but the truth is if it wasn't for the laundry list of bells and whistles, Uniden has gone backward in performence with this model.
I have no problem listening to what I _need_ to listen to on VHF - but I agree with you totally on the poor VHF performance. I _like_ to DX with my scanner - and that's impossible to do on VHF if you have a TV station, a high power paging transmitter within 10 miles of you (it seems). So many things I could pull out and find interesting on VHF except that any weak signal is either no signal because of the overload or a signal with a paging system and a TV station superimposed on it so that it's uncopyable.

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Old 03-03-2007, 7:08 PM
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Thanks for the info guys. I sometimes think about putting a 996 on my want list, but often wonder if it will perform better than the 396. Not to hijack the thread, but the 396 is terrible in the upper UHF band (460's and 490's PS frequencies).

Dewey
  #9 (permalink)  
Old 03-11-2007, 8:22 PM
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The radio is operated off of a 100' tower with an ICOM Discone antenna
------------
If you are in a metro area with significant radio activity, this is a guaranteed certainty of causing yourself desense problems on a scanner. Scanners are consumer grade gear, not commercial, and they are not built to operate with excessive signal levels.

Any time you see signal strength dropping in an 'on off' sort of way, rather than a slow variance that might be propagation, you're seeing your unit desensing on strong signals nearby.

Using a discone makes this much worse, since it's pulling in not only the scanner stuff you want, but FM band, TV Band, and everything else, all equally. Your specific problem could be a nearby pager, or several of them, or any number of combinations of high power signals, all adding up to 'help me, help me!' from your front end when they get too strong.

Do you get the same wandering signal levels on the weather transmission if you use a back-of-radio antenna? Or on other signals you can hear with one? If not, you've identified desense from 'too good an antenna'. The only cure is to find out just what's causing the desense, and filtering it out. That, of course, can be a heck of a challenge to actually do, though.

Oop, probably should mention for the technically less experienced, 'Desense' means your radio's front end, the RF amplifier section, is recieving so high a signal level on some frequency, whether or not it's the one tuned to, that it's forcing the automatic gain circuits to cut back the sensitivity of the amplifier to protect it from overload causing intermods, etc. The only way to keep it from happening is to not give the front end too large a signal strength to cope with. Commercial gear is highly shielded, more expensively designed, and the antennas are tuned for the intended frequency, with protective filters in the feed line if high signal levels are expected. They usually are, with commercial repeaters, since they 'live' in the same box as probably dozens of OTHER high power repeaters, often on the same bands as they are. Mountaintops are HELL on recievers.

Last edited by SkipSanders; 03-11-2007 at 8:27 PM..
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Old 03-12-2007, 7:06 AM
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Well there are only so many ways to say this. The same antennas (and I have multiple not just the discone) used on the BC780xlt and old Pro 2006 and they perform flawlessly!!! The 996t (I have two of them) are not as sensitive as the 780. The problem is the poor IF rejection, and spurious image rejection, and thats just the way they were engineered. The 780 and Pro2006 have much better front ends. I am not dumb founded when it comes to radios and electronics. Been a amatuer operator for 20+ years. The hallmark of a great radio is one than is able to discern and reject garbage while being able to pull in weak signals. For 5 bills one would expect much better!! They should have used the front end from the 780 and designed the new interface around that, then you would have a hell of radio. The 996t feature set is what sets it apart from the rest. It's RF performence sets it back making it only a good performer overhaul.

Last edited by RPSINFOMAN; 03-12-2007 at 7:20 AM..
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Old 03-12-2007, 11:36 AM
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Have you tried using the attenuator?
  #12 (permalink)  
Old 03-12-2007, 12:32 PM
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Quote:
Originally Posted by GreatLakes
Have you tried using the attenuator?
Yes, attenuates signal too much. I am looking at some other filtering options for the VHF band since this is were the problem lies. No doubt a mix of strong pager signals, and whatever else is out there, but as I stated before the other radios are immune from this problem. UHF and 800 are not a problem, but the UHF bands don't hear as well as the 780 either. I am not going to reverse engineer the damm thing, just wait for the next model to come along with better IF filtering perhaps. Since VHF is my primary listen, I may use the 780 for that (superior) and use the 996t for 800 and P25 which is really what it is tuned up for.
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Old 03-12-2007, 12:53 PM
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There's no question that the BC780-XLT is an outstanding scanner! That explains why people continue to pay high prices for used ones.

Grove Enterprises carries PAR filters, which work very well, if the interfering signals fall within the range of the filter. I've actually seen an installation where one guy daisy chained three PAR filters of different frequencies, in line to eliminate interference. You would have thought the induced loss would have killed the signal he was monitoring, but in fact it fixed the problem and worked very well!

Grove Enterprises, PAR Filters:
http://www.grove-ent.com/filters.html

Have you tried the radio on a different antenna? Considering your antenna, at that height, is a magnet for just about everything, maybe something more moderate would actually work better on that particular scanner.
  #14 (permalink)  
Old 03-12-2007, 3:17 PM
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Quote:
Originally Posted by GreatLakes
Grove Enterprises carries PAR filters, which work very well, if the interfering signals fall within the range of the filter. I've actually seen an installation where one guy daisy chained three PAR filters of different frequencies, in line to eliminate interference. You would have thought the induced loss would have killed the signal he was monitoring, but in fact it fixed the problem and worked very well!

Have you tried the radio on a different antenna? Considering your antenna, at that height, is a magnet for just about everything, maybe something more moderate would actually work better on that particular scanner.
Thanks for the par filter info. I have tried the radio on different antennas. There are several on that tower. One is actually cut for VHF (Antenna Specialist MON series) but provides 3db of gain, something I don't want. It however performs poorly in the 800 range.
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Old 03-12-2007, 3:24 PM
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That PAR-152 looks like the ticket. Since the offending signal seems like 152.4800 paging system.
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Old 03-16-2007, 7:03 AM
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PAR VHFSYM152HT has been installed. Absoulutely an impressive device with no or very minimal insersion loss. There is no degradation of signal on any of the bands and has cured the problem. I did however lose one bar of signal strength on one of my 800 TRS, but thats minimal compared to the benefits. I highly reccommend Par Electronics. This device was custom built and shipped to me in two days. Very high quality product and excellent customer service. It's too bad it added $80.00 to overhaul cost of 996t for a problem that should have been corrected with it's overhaul design.
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Old 03-16-2007, 8:25 AM
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Quote:
Originally Posted by RPSINFOMAN
PAR VHFSYM152HT has been installed. Absoulutely an impressive device with no or very minimal insersion loss. There is no degradation of signal on any of the bands and has cured the problem. I did however lose one bar of signal strength on one of my 800 TRS, but thats minimal compared to the benefits. I highly reccommend Par Electronics. This device was custom built and shipped to me in two days. Very high quality product and excellent customer service. It's too bad it added $80.00 to overhaul cost of 996t for a problem that should have been corrected with it's overhaul design.
Glad that solved the problem for you. I too suffered from the same type of interference from VHF paging signals for many years. I finally bought a PAR filter, and it helped tremendously. Then the paging transmitter that had been causing me the headaches for so long was apparently taken off the air, or moved. Good riddance!
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Old 03-17-2007, 4:21 AM
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Quote:
Originally Posted by RPSINFOMAN
It's too bad it added $80.00 to overhaul cost of 996t for a problem that should have been corrected with it's overhaul design.
I think it's safe to assume most people are not using disones that are 100 feet up!

Your installation is not typical, so why do you expect only typical problems?

Electronics are not built the same as they used to be... what IS?

Glad the filter solved the problem.


Joe M.
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Old 03-17-2007, 11:18 AM
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Quote:
Originally Posted by Voyager
I think it's safe to assume most people are not using disones that are 100 feet up!

Your installation is not typical, so why do you expect only typical problems?

Electronics are not built the same as they used to be... what IS?

Glad the filter solved the problem.


Joe M.
It's certainly no question electronics are not built the same, and whats typical is hightly subjective. However, what I do expect out of a $500 radio is an equal or BETTER performing piece of equipment especially from the same company! If the 780 can outperform in the same installation setting for half the cost, I have every right to make those assersions. The IF filtering in the BCD996t is seriously flawed.
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Old 03-18-2007, 8:47 PM
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How does one determine which filter(s) is needed, that is what freq is the problem?
 

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