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post #1 of 16 Old 11-18-2019, 07:45 PM - Thread Starter
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Montreal: Replacement Preamp Needed? If yes, which ?

Dear All,

I have an Antennas Direct DB8 with a Channel Master CM-7777 30 dB, 1 input, 15V
Inside the condo, the signal is split to three receivers through an Extreme Broadband BDS103HB -5.5dB

Problem
I believe the preamp has died on me because I can't get any signal as of a few days ago.
I checked the PSU and it is reading 14,6V
I checked the current draw and it is reading around 185 mA.




I am not the one who installed the rig on the roof. A professionnal (???) did it as membrane roof, side flashing and bricks were involved.

The antenna is pointed toward 165° (true azimuth) for maximum signals for the channels at about 75 miles from my location (Burlington's towers). Also, the closest channels are at about 20 miles (Montreal's Mont-Royal's towers) and located at 259° (true azimuth).

I used to get those channels: 2.1, 10.1, 6.1, 12.1, 17.1, 15.1, 3.1 (real 20 WCAX), 62.1, 5.1, 33.1, 44.1
I never got WVNY (22.1) but would be happy to.

First
Apart from the PSU voltage reading and the current draw reading, should I do anything else to confirm the CM7777 is dead?

Second
If the CM7777 is dead indeed, can anyone point me at a replacement?

Thanks

Last edited by burette; 11-19-2019 at 11:46 AM.
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post #2 of 16 Old 11-19-2019, 12:36 PM
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Quote:
Originally Posted by burette View Post
Dear All,
I'm not familiar with Canada, but here in the US many stations have to move to different RF channels due to the FCC repack.

Your TVfool post shows stations are pretty strong. As an experiment connect one TV directly to the antenna, bypassing the amp and splitter.

The TVfool database has not been updated in a long time. Checkout the Rabbitears site for update to date transmitter info:
https://www.rabbitears.info/searchmap.php

If stations have moved try recanning your TV.
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post #3 of 16 Old 11-19-2019, 12:44 PM
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Quote:
Originally Posted by burette View Post
First
Apart from the PSU voltage reading and the current draw reading, should I do anything else to confirm the CM7777 is dead?

Yes. Bypass the preamp, remove the power supply and the splitter and see if you can receive any stations. If yes, then the preamp is dead.
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post #4 of 16 Old 11-19-2019, 01:13 PM - Thread Starter
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Quote:
Originally Posted by Tschmidt View Post
I'm not familiar with Canada, but here in the US many stations have to move to different RF channels due to the FCC repack.

Your TVfool post shows stations are pretty strong. As an experiment connect one TV directly to the antenna, bypassing the amp and splitter.

The TVfool database has not been updated in a long time. Checkout the Rabbitears site for update to date transmitter info:

If stations have moved try recanning your TV.
Quote:
Originally Posted by Calaveras View Post
Yes. Bypass the preamp, remove the power supply and the splitter and see if you can receive any stations. If yes, then the preamp is dead.

Both of you: I did check on "rabbitears" but as I had only done three posts at the time of this thread, the system blocked my hyperlink. Now it is my fifth. I will post the rabbitears report as my sixth :-)

Also, are you suggesting that if I just bypass the injector, I should get a strong enough signal, that is the unpowered preamp will not dampen the signal from the antenna ?
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post #5 of 16 Old 11-19-2019, 02:30 PM - Thread Starter
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The preamp being on the roof, I did not get there to bypass it but, I connected a TV directly from the preamp, bypassing the power injector. I can not get any signal.


Here is my "rabbitears" report
https://www.rabbitears.info/searchma...study_id=29107

Last edited by burette; 11-19-2019 at 02:33 PM.
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post #6 of 16 Old 11-19-2019, 02:44 PM
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Quote:
Originally Posted by burette View Post
Also, are you suggesting that if I just bypass the injector, I should get a strong enough signal, that is the unpowered preamp will not dampen the signal from the antenna ?

You must remove the preamp and replace it with a barrel connector. An unpowered preamp is a huge attenuator. You'll receive nothing in most cases.
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post #7 of 16 Old 11-19-2019, 03:04 PM - Thread Starter
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Originally Posted by Calaveras View Post
You must remove the preamp and replace it with a barrel connector. An unpowered preamp is a huge attenuator. You'll receive nothing in most cases.
That's what I tought.


Well, it is winter already. I have to get a ladder and get at the roof to try this. But I will, just to be absolutely sure. Honestly, at this point, I am pretty sure the CM7777 is dead.

So if you have any suggestions on a replacement, I am opened, because I don't want to miss OTA too long.




On the other hand
I read a bit on your page on how to interpret the TVFool report and to my surprise, I realised that maybe I would also need a distribution amp?

Last edited by burette; 11-19-2019 at 04:45 PM. Reason: clarity
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post #8 of 16 Old 11-19-2019, 05:14 PM
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Before adding the barrel connector, remove the power inserter. You don't want to send voltage to your antenna.

I suggest the Antennas Direct Juice preamp.

The new Channel Master V3 preamps look promising, but they have not been proved yet.
https://www.channelmaster.com/Antenn...iers_s/370.htm

If you can not measure it, you can not improve it.
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www.megalithia.com/elect/aerialsite/dttpoorman.html

Last edited by rabbit73; 11-19-2019 at 05:20 PM.
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post #9 of 16 Old 11-19-2019, 05:24 PM - Thread Starter
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Quote:
Originally Posted by rabbit73 View Post
Before adding the barrel connector, remove the power inserter. You don't want to send voltage to your antenna.

I suggest the Antennas Direct Juice preamp.

The new Channel Master V3 preamps look promising, but they have not been proved yet.
https://www.channelmaster.com/Antenn...iers_s/370.htm
When I got the whole rig in 2010 and found that I could not get 22-1(13) ABC, I tought on getting a Research Communications 9263 but they are now out of business.


I red all the thread "Research Communications owner" and IIRC, the "Juice" seems to have reliability issue ?


What about a Kitztech KT-200 ?
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post #10 of 16 Old 11-19-2019, 05:32 PM
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Quote:
Originally Posted by burette View Post
When I got the whole rig in 2010 and found that I could not get 22-1(13) ABC, I tought on getting a Research Communications 9263 but they are now out of business.

I red all the thread "Research Communications owner" and IIRC, the "Juice" seems to have reliability issue ?

What about a Kitztech KT-200 ?
I haven't seen reliability issues for the Juice, but I have seen many reliability issues for the CM7778.

The Research Comm did have reliability issues; the LNA IC was easily damaged by static discharge. This is true of all very low-NF preamps including the Kitz 200, but protective diodes have recently been added. The Juice has a very good warranty.

I doubt that the KT-200 will make a difference with WVNY 13, because the noise level is higher on VHF-High than on UHF, but you are welcome to try.

Otherwise, your choices are now Juice or Channel Master.

If you can not measure it, you can not improve it.
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www.megalithia.com/elect/aerialsite/dttpoorman.html

Last edited by rabbit73; 11-19-2019 at 05:42 PM.
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post #11 of 16 Old 11-19-2019, 06:20 PM
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Quote:
Originally Posted by rabbit73 View Post
I haven't seen reliability issues for the Juice, but I have seen many reliability issues for the CM7778.

The Research Comm did have reliability issues; the LNA IC was easily damaged by static discharge. This is true of all very low-NF preamps including the Kitz 200, but protective diodes have recently been added. The Juice has a very good warranty.

I doubt that the KT-200 will make a difference with WVNY 13, because the noise level is higher on VHF-High than on UHF, but you are welcome to try.

Otherwise, your choices are now Juice or Channel Master.

I'd second the Juice for him. I'm using new KT-200s here and they did not blow up despite all the summer thunderstorms. So far so good.
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post #12 of 16 Old 11-19-2019, 06:28 PM
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Originally Posted by burette View Post
On the other hand
I read a bit on your page on how to interpret the TVFool report and to my surprise, I realised that maybe I would also need a distribution amp?

With a 30 dB gain preamp it's unlikely you'd need a distribution amp. If you go with the Juice (17-18 db gain) a CM3414 distribution amp might be a good idea.
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post #13 of 16 Old 11-19-2019, 06:46 PM - Thread Starter
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Originally Posted by rabbit73 View Post
I haven't seen reliability issues for the Juice, but I have seen many reliability issues for the CM7778.

The Research Comm did have reliability issues; the LNA IC was easily damaged by static discharge. This is true of all very low-NF preamps including the Kitz 200, but protective diodes have recently been added. The Juice has a very good warranty.

I doubt that the KT-200 will make a difference with WVNY 13, because the noise level is higher on VHF-High than on UHF, but you are welcome to try.

Otherwise, your choices are now Juice or Channel Master.
Getting WVNY 13 would just be a bonus, but I definitely would not want to lose other important channels while replacing my CM7777 for another model (I consider the CM7777V3 a new model since the change in design).

As I am in science (chemistry), I would like to understand about radio amplification but unless I really put myself to it (lots of reading), it's kind of a mystery as of now. What I can't get is the effect of gain vs noise figure. My CM7777v2 was listed as 30 dB gain with something like 3 dB noise figure. If I "sizedown" in gain, am I not risking disappointment in losing the towers at 75 miles ?

Why the Clearstream Juice at 19 dB gain / 1.8 dB NF, and not the
Kitztech KT-200 at 22 dB gain / 1.2 dB NF or the
Tin Lee MA25U77 at 24.5 dB gain / 2.6 dB NF; all these are well below 30 dB gain but at the same time, they show less NF as well.

Again, it's not that I necessarily believe that "bigger is better" but I would just not want to need to go twice or more on the roof during winter at -10°C

Thank you again for your input.

A table I got on the "Research Communications preamp owner" thread.

Quote:
Originally Posted by rabbit73 View Post
[...]

Last edited by burette; 11-19-2019 at 09:13 PM. Reason: cleaning empty spaces
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post #14 of 16 Old 11-19-2019, 09:11 PM - Thread Starter
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Originally Posted by Calaveras View Post
With a 30 dB gain preamp it's unlikely you'd need a distribution amp. If you go with the Juice (17-18 db gain) a CM3414 distribution amp might be a good idea.
I am trying to educate myself and figure what to do with the information on your page and that from "rabbitears" but I am missing something. http://www.aa6g.org/DTV/Noise/noise.html

I have annoted the rabbitears report. The green boxes are the channels I used to get with my CM7777v2 while the red boxes, I could not pull the signal. The channels with "X", are not in the same direction and I don't care. I understand that those are theoritical values assuming the antenna is pointing directly at each signal while in fact, my antenna is pointing toward Burlington/Platsburg at 165° true azimuth.

First, I don't know which signal to use between "Field Strengh" or "Signal Margin". Both have about a delta of about 40 dB between the best channel 2.1 (19) and the worst 44-1 (16). What I think I get correctly is that a 40 dB difference between pointing Montreal vs Burlington means that I would probably overload my receivers if I were to point the antenna toward Montreal using a 30db gain preamp. So I guess that aiming at Burlington (70 miles from the antenna) was the best bet since it worked for 8 years.

But secondly, I am not sure if I should get a lower gain preamp combined with a distribution amp or, keep just an highest possible gain preamp?

Thanks.
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post #15 of 16 Old 11-20-2019, 08:22 AM
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Quote:
Originally Posted by burette View Post
I am trying to educate myself and figure what to do with the information on your page and that from "rabbitears" but I am missing something.

I'm trying to put together the pieces that make up your antenna system from your various posts. You have a DB8 pointed at 165 degrees? A 4 way splitter? 100' of RG6 or less?

First the preamp gain.... From my article you can use the rule of thumb to determine the preamp gain you need:

TV Noise Figure ~6 dB
Splitter loss - 7 dB
100' of RG6 Loss at channel 36 - 4 dB

Total loss + 6-8 dB extra = 23 - 25 dB gain

Anything over 25 dB gain just subtracts from the TV tuner dynamic range and gives you no benefit in reception.

There's another way to do this. Use a CM3414 4-way distribution amp. This amp has a noise figure of about 2 dB and eliminates the splitter loss. Now the preamp gain is:

Dist. Amp Noise Figure - 2 dB
100' of RG6 at channel 36 - 4 dB

Total loss + 6-8 dB = 12 - 14 dB gain

Now you can use a Juice preamp with its lower gain and high overload tolerance.

You can use a KT-200 and get about 2 dB lower noise figure. For most people there won't be any difference because signal strength is not their main problem. Multipath is the most common problem limiting reception because of indoor or attic antennas and/or antennas looking through vegetation.

Your antenna..... A DB8 is a UHF antenna with some degraded high VHF reception. If you want to have any real chance at WVNY you need a real VHF antenna. I guess your DB8 is fixed pointed at 165 deg. You need to point the antenna at the stations you want to receive. Attached is an image of one of my local stations that is perfect line-of-sight. The yellow trace is pointed at 65 degrees right at stand the magenta trace is pointed a 336 deg where most of my stations are. When pointed at it the SNR is 34 dB. When off pointed the station can't be decoded at all because of multipath even though the analyzer shows it to be strong enough.

On the Rabbitears report I'd set Units to dBm instead of dBuV/m. To the best of my understanding dBuV/m is how much signal is intercepted by a 1 meter length antenna in an RF field. It's always 1 meter regardless of the frequency. In this report dBm is the measure of signal power generated by a resonant dipole and is commonly used in many applications. dBuV/m seems to be used mostly by the FCC to describe Field Strength.
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post #16 of 16 Old 11-24-2019, 05:31 PM
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Your stations are MUCH too strong to be using a high gain preamp.
The higher the gain the more interference your tuner has to put up with and the easier the preamp
is overloaded.
The Winegard LNA-200 is a MUCH better choice.
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