antenna pre-amp replacement - AVS Forum
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post #1 of 22 Old 03-21-2013, 06:16 PM - Thread Starter
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I believe my antenna pre-amp (CM 7777) is failing and I'm looking for a replacement.
About 5 years ago I installed a NOS Radio Shack VHF/UHF antenna on a mast with a Channel Master rotor, pre-amp and distribution amp. I'm fairly rural in Jackson county Michigan and receive channels from Detroit, Lansing & Toledo all are about 40-60 miles distant. Up until about a week ago I could get about 30 channels. Suddenly I'm down to 5 (some with 100 % signal). I've cycled power, re-scanned etc with no change. My 30 channel where a combo of VHF and UHF. The 5 I still get are all VHF channels (7,9,10,11). It's like the UHF section died. Because of the weather I haven't made the climb to check cables and connections but I'm betting the problem is inside the pre-amp that is near the antenna so I'm thinking about its replacement. I'm not too happy with a unit that only lasted 5 years so I'm wondering about alternatives. A couple of searches show a possibility in a Winegard AB8275. Are there others I should also consider? Are there other things I should try? Is 5 years really too short a life for a CM7777? Other thoughts? Thanks for your thoughts/ comments.

Al
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post #2 of 22 Old 03-21-2013, 06:27 PM
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If you have the amp set to "combined", it's pretty unlikely that the amp would fail only on the UHF channels.
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post #3 of 22 Old 03-22-2013, 04:49 AM - Thread Starter
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Well, without knowing about the internals of the box it's tough for me to talk about how it could fail. I'm only looking at the end results. Got an alternate explanation?

Al
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post #4 of 22 Old 03-22-2013, 04:56 AM
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Why not just eliminate the pre-amp altogether & see what you get before buying anything new? How many feet of coax cable between are the antenna & distribution amp?
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post #5 of 22 Old 03-22-2013, 05:47 AM - Thread Starter
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There is 75' of cable, the pre-amp is there because I need it.
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post #6 of 22 Old 03-22-2013, 06:13 AM
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I am assuming you have tried reception with just the distribution amp before? I have used up to 100 feet of RG-6 cable before distribution amps (4 output amplified splitter) & even the most borderline signals are preserved & robust at all 4 outlets.
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post #7 of 22 Old 03-22-2013, 06:27 AM - Thread Starter
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When I installed the system 5 years ago I had more channels/better reception with the pre-amp. When weather permits I can test again without the pre-amp. In the mean time I still have my original questions.
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post #8 of 22 Old 03-22-2013, 07:12 AM
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The only 7777 I am using now is about 6-7 years old with no problems. I was under the impression that UHF & VHF are amplified separately in the 7777, but not sure if just one can fail if in combined mode?

I've used the 8275 before in a fringe situation & was pleased with the results.
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post #9 of 22 Old 03-22-2013, 09:13 AM
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Quote:
I was under the impression that UHF & VHF are amplified separately in the 7777

Only if set to "Separate".
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but not sure if just one can fail if in combined mode?

There is only a single signal path if set to "Combined". If it fails, it will affect all channels. However, failed preamplifiers often turn into attenuators of 20-40dB. If the incoming VHF signals happen to be strong enough to overcome this attenuation but the UHF signals aren't, then your situation can be explained.
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I have used up to 100 feet of RG-6 cable before distribution amps (4 output amplified splitter) & even the most borderline signals are preserved & robust at all 4 outlets.

That will work fine as long as you have adequate signal at the input of the distribution amplifier. Keep in mind that 100' of RG6 will have around 5-6 dB of insertion loss. That insertion loss effectively adds to the noise figure of the amp, so such configuration wouldn't be suggested in a weak signal area where there isn't adequate signal margin.


For the OP, I'd be suspicious of water infiltration into a coax cable or connector.

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post #10 of 22 Old 03-22-2013, 12:06 PM - Thread Starter
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A couple of the strongest signals (steady 100 power) where UHF and are now zero power level. One of the weaker is VHF and still present (mid 50 power) The CM7777 is a 1 input only version and of course was switched to combined. I'm thinking a separate/combined switch failure could also show these symtoms. Yes water infiltration is suspect, especially given freeze/thaw cycles. Can't safely get to the equpment just yet and I don't plan to buy anything until I can do some tests.
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post #11 of 22 Old 03-22-2013, 04:40 PM
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Quote:
The CM7777 is a 1 input only version and of course was switched to combined. I'm thinking a separate/combined switch failure

If you have the (newer) single input version, then there isn't any switch. Only the older (pre-2012) 7777/8 amps had the dual inputs.
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(steady 100 power)

Such meter readings are largely useless when discussing the power level, the only thing an amplifier can influence, of incoming signals.
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post #12 of 22 Old 03-23-2013, 04:27 PM - Thread Starter
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Well, I was looking in the wrong place. Connecting the cable directly to the TV yields all the exprected channels. Coming in thru my Dish Network box limits to channels 13 and under (VHF) FWIW I didn't find this out the easy way. Thanks for your inputs.


Al
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post #13 of 22 Old 03-25-2013, 02:40 PM
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Reading your first post, I thought, holy cow he has the same problem as I. Maybe he found a fix that I can duplicate. Then, I keep on reading until I go to the last post. Crap, I wish mine was that easy.

My setup is PreAmp Channel Master CM7777 UHF/VHF to Amp 8-Port Bi-Directional to TV. In late Fall, after the weather turned cold, I quit getting the weak signals, the two CBS channels. I tried a direct preamp to TV, it didn't help. It used to work since 2008. Now I'm missing NCAA games. This is my March Madness. I can't get on the roof because of snow and it's 25 degree colder than normal.
I may have a bad preamp..................Will take a look at it in the Spring..............whenever that will be.

http://transition.fcc.gov/mb/engineering/maps/
55960

Callsign Network Virtual Channel Band
Click on callsign for detail
Moderate Signal KAAL ABC 6-1 UHF
Moderate Signal KSMQ PBS 15-1 UHF
Moderate Signal KTTC NBC 10-1 Hi-V
Moderate Signal KXLT FOX 47-1 UHF
Weak Signal KIMT CBS 3-1 UHF
Weak Signal WKBT CBS 8-1 Hi-V

Weak Signal KYIN PBS 24-1 UHF
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post #14 of 22 Old 03-25-2013, 06:11 PM
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Prany:

Using coordinates, 44.131165, -92.531693, for Oronoco MN 55960 your tvfool report looks like this:

http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d1dda49c83a0e92

Your two CBS stations are 2Edge signals and fairly weak, which means they will probably not be 100% reliable.

I expected to see one of your translator transmitters carry CBS, but I don't see it listed as a subchannel:
http://www.rabbitears.info/search.php?request=zip_search&zipcode=55960&miles=60&address=&lat=&lon=&dbtype=dBm&height=

What type of antenna are you using, how high is it, and is it aimed at one of the CBS stations?

If you can do a tvfool report using your exact coordinates and antenna height, it would be more accurate than the generic Oronoco report.

This thread might help you with your preamp:
CM7777 preamp failure question
http://www.avsforum.com/t/1336632/cm7777-preamp-failure-question

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post #15 of 22 Old 03-25-2013, 06:24 PM - Thread Starter
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I feel your pain. All my mucking around swapping cables and pre-amps and raising and lowering the antenna caused me to loose a few precent of all the signals so now I have even fewer channels even though I'm back to the original setup. Not real sure I want to swap out the defective dish network box either. If I do I loose a ton of stuff I've recorded and not watched yet plus last time I had the option to swap one they sent a box that had some noise issues. Complex systems generate complex issues.

For your situation : is it possible that the stations dropped their output power?
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post #16 of 22 Old 03-26-2013, 11:49 AM
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Thanks Rabbit73,
"What type of antenna are you using, how high is it, and is it aimed at one of the CBS stations?"
I'm using Channel Master 4228 UHF antenna aiming directly at CBS 3-1.
Antenna is sitting approx. 6' above the peak of the roof of a two story house (maybe 30 feet above ground). House elevation is the problem since blinded by trees and a hill.

I will read up on the preamp-failure-question forum.
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post #17 of 22 Old 03-26-2013, 02:39 PM
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If you end up replacing it, I'm very pleased with my Antennas Direct CPA19. Low noise and features overload protection for strong signals. For some reason it's not listed on their site at the moment, but it's available in a bunch of places.
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post #18 of 22 Old 03-26-2013, 05:17 PM
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Prany:
Quote:
I'm using Channel Master 4228 UHF antenna aiming directly at CBS 3-1.
Since CBS 3.1 is on real CH42, the 4228 is a good choice.
Quote:
Antenna is sitting approx. 6' above the peak of the roof of a two story house (maybe 30 feet above ground).
That's good.
Quote:
House elevation is the problem since blinded by trees and a hill.
That makes reception more difficult, but since you previously had good reception I suspect the preamp or coax downlead unless there is a problem with the KIMT transmitter.
Quote:
It used to work since 2008.
If you antenna has been up that long, it might be a bad balun or connection to the 4228.

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post #19 of 22 Old 03-26-2013, 07:47 PM - Thread Starter
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Originally Posted by Cabal0 View Post

If you end up replacing it, I'm very pleased with my Antennas Direct CPA19. Low noise and features overload protection for strong signals. For some reason it's not listed on their site at the moment, but it's available in a bunch of places.


I see the spec shows this amp only goes to 700mHz. Normal UHF goes to 890mHz. Is the upper part of that area no longer used?

Al
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post #20 of 22 Old 03-26-2013, 11:34 PM
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Originally Posted by Cabal0 View Post

If you end up replacing it, I'm very pleased with my Antennas Direct CPA19. Low noise and features overload protection for strong signals. For some reason it's not listed on their site at the moment, but it's available in a bunch of places.
I believe they may be replacing that preamp with a new model in the coming months. You could contact Antennas Direct for more details.
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post #21 of 22 Old 03-27-2013, 03:38 AM
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Originally Posted by yzfoot View Post

I see the spec shows this amp only goes to 700mHz. Normal UHF goes to 890mHz. Is the upper part of that area no longer used?

Al
The highest UHF channel (51) occupies 692-698 mHz. 4g phone service occupies much of the former spectrum used by channels 52-69.
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post #22 of 22 Old 03-27-2013, 05:31 AM
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The amp works well beyond 1 GHz. I guess they chose to limit the documentation of its performance to the frequency spectrum that is in use for TV service.
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