Broken CM7777 - AVS Forum
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post #1 of 19 Old 10-06-2013, 01:46 PM - Thread Starter
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I have a CM7777 amp that I'm pretty sure isn't working properly (I'm using it in a fringe OTA area with a Winegard HD-8800). ChannelMaster says there's nothing wrong, and that it's too powerful, so I borrowed a 15db amp from a friend of mine. With the 15db amp, I get a great signal. With the CM7777, the signal just disappears. I've also tested it with a 16db attenuator, which should bring the signal to about the same level as the 15db amp, and I still get nothing. I also borrowed a Sedelco DisplayMax 800, and did a couple of full channel scans.

Here's without the amp:


Here's with:


I'm pretty new at this, so I'm not sure what I should see, but that seems like a lot of noise. There's also a huge 15db spike on channel 2, and we don't have a channel 2 around here (and it's a UHF amp).

So, going on the assumption that the amp is bad, I opened it up. The LM7808 regulator measures an output of 8 volts, and the +6v test point measures 7 volts, so those seem to be ok. I am concerned about inductor L8. It looks like it got squished -- at least, I sure don't see any way that that could actually have any inductance, and I suspect that it should look like L7.



Can anyone confirm for me what L8 should look like? If there is indeed a problem with the amp, does anyone have any other ideas of what I can check? I do have access to a signal generator and oscilloscope.

Thanks, everyone.

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post #2 of 19 Old 10-06-2013, 07:09 PM
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If you have the new version of the CM-7777 amp, then it may be too powerful and overloading at 30db gain. The original dual input version at 26db was more favorably recommended. Another good amp is the Kitztech 200 with 24db gain and very low .04 noise figure. I get very good results with that one.
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post #3 of 19 Old 10-06-2013, 07:24 PM - Thread Starter
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Yeah, it is the new version. That's why I also tested it with a 16dB attenuator (after the amp). Unless I'm just completely crazy, that should put the final signal pretty close to the same level as the 15dB amp I'm currently borrowing. The 15dB amp works pretty well, but the 30dB CM7777 - 16dB still completely kills the signal.

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post #4 of 19 Old 10-06-2013, 07:58 PM
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Too strong an input signal can swamp the amplifier. Try putting the attenuator on the input.
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post #5 of 19 Old 10-06-2013, 08:34 PM
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Quote:
Originally Posted by Colm View Post

Too strong an input signal can swamp the amplifier. Try putting the attenuator on the input.
Exactly, the OP needs to post a TV Fool report for a proper diagnosis.
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post #6 of 19 Old 10-07-2013, 07:44 AM - Thread Starter
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Ah, didn't think about the possibility of overloading the input. I purchased the 7777 on the recommendation of a co-worker who works with TV distribution systems, but I'm open to the possibility that it's the wrong amplifier for my purposes. However, if it's possible to get it working using a combination of input and output attenuators, I'd rather do that than have to buy a new amp.

Here's my TV Fool link:
http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d46ae3b8425f2e5

My antenna is an eave-mounted Winegard HD-8800 on a one-story garage. This is how I ultimately plan to split the signal:


Let me know if there's any other information I can provide.

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post #7 of 19 Old 10-07-2013, 07:56 AM
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Probably overloaded.

Install the attenuator at the input to verify as suggested already. You'd likely find that 6-10 dB input attenuation will be optimal.

The CM7778 is a far more appropriate choice for this location.

The 'Tim the Toolman' mantra of "More power!" strikes again. There is only rarely a need for a 30 dB amp (or any with gain more than 20 dB) and that would usually be in very weak signal area with extensive cabling. A low noise (< 2 dB), medium gain (15-20 dB) pre-amp with good overload resistance is the ideal amp for most locations. It would offer the best balance of performance for most folks.
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post #8 of 19 Old 10-07-2013, 07:58 AM - Thread Starter
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I'll borrow a few attenuators, test it, and report back. Thanks!

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post #9 of 19 Old 10-07-2013, 06:46 PM - Thread Starter
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Here's my full channel scan with an 8dB attenuator before the CM7777 input:


So, the amp is obviously working, and the input is being overloaded, like Colm and ProjectSHO89 were theorizing. With that, all of my actual TV channels are peaking at -10dB. There's a huge spike on channel 60, though, and my (albeit rather uneducated) guess is that it's the cause of the overload. Doing some research, it looks like Verizon 4G LTE uses the same frequency. And hey, there's a Verizon tower with 4G less than a mile from my antenna. It appears that if I filter channels 60+ before the input to the amp, I probably won't need any attenuation, and that should get my TV signals up near the ideal 0dB. Would that be correct?

Thanks again for everyone's help.

-Brent

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post #10 of 19 Old 10-07-2013, 07:19 PM
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Signals don't have to be exactly that. Levels between -20 dBmv and + 30 dBmV at the input pf any tuner is fine.

I saw that Radio Shack sells a 4G/LTE filter on their website , online only. About 10 bucks plus shipping, so figure around $17-18 shipped.

IIRC, VZW uses 746-75? and ATT uses 736 to 746 (or something close to that).
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post #11 of 19 Old 10-07-2013, 09:07 PM - Thread Starter
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Oooh, thanks for the tip, projectSHO. I wouldn't have even thought to check RadioShack; I'm too used to going straight to Amazon. I've heard good things about shoprunner from a friend of mine, so I signed up for a free trial of that, so free 2-day shipping.

As far as input values... I was told to aim for anything from -15dB to +15dB. But 0dB is the reference, so it's just such a nice number for my OCD. biggrin.gif

I'll report back in a couple days when the low-pass filter comes in, but I'm thinking this is solved, thanks to all of your collective great advice.

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post #12 of 19 Old 10-08-2013, 12:35 AM
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Specs for 4G/LTE Filter from Radio shack website:
http://www.radioshack.com/product/index.jsp?productId=12934420&filterName=Category
Insertion loss 1dB between 50-600MHz
Insertion loss 3.5dB between 601-698MHz
Return loss 12dB between 5-698MHz
Rejection 50dB between 740-1000MHz

NOTE: NO SPECS FOR FILTER ROLL-OFF CHARACTERISTICS IN THE CRITICAL 698-740 MHz REGION!!!!!

Note the 3.5 dB Insertion Loss for Ch36-51, which can seriously degrade sensitivity on HALF the UHF Band.
Unless you KNOW there is a 4G/LTE Tower within a few miles of your location, you probably DON'T need it.

I addressed effectiveness of various Low Pass Filters against LTE's 700 MHz Band here:
http://www.digitalhome.ca/forum/showthread.php?p=1388395
Radio Shack may be the least expensive, but it isn't very effective if an ATT 4G/LTE is close and you need a TRULY effective filter. Verizon 4G/LTE is in a higher band and hence is not as much of a problem. Fol. includes info on how to determine if you are close to a 4G/LTE Tower (there may be other, better methods):
http://www.digitalhome.ca/forum/showthread.php?p=1693522.

UPDATE: Last year, 717-728 MHz was allocated as a DOWNLINK (Tower Broadcast) Supplemental Data Link (SDL), which should be available "soon" [likely already in selected "test" areas]. It will increase data rates in new HSDPA capable devices. SDL will become the PRIMARY interference source from 4G/LTE towers....and will be difficult to attenuate with a simple Low Pass Filter:
http://www.4gamericas.org/documents/4G%20Americas-Benefits%20of%20Digital%20Dividend-September_2012.pdf
http://www.qualcomm.com/media/documents/benefits-hspa-supplemental-downlink
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post #13 of 19 Old 10-08-2013, 04:34 AM
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Do yourself a favor & just get a more appropriate pre-amp that will get the job done & won't require any signal robbing band aids. The RCA TVPRAMP1R is difficult to overload & would be my suggestion for best bang for the buck.
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post #14 of 19 Old 10-08-2013, 05:25 AM
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Don't waste your energy and time trying to hit that specific number. ATSC signals have an ~80 dB dynamic range (-85 to -5 dBm, add 48.8 to convert to dBmV).

If it doesn't matter as to your actual reception (channels received and view-ability) , leave the flippin' attenuator on there and be done with it already.
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post #15 of 19 Old 10-08-2013, 06:54 AM - Thread Starter
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Quote:
Originally Posted by holl_ands View Post

Unless you KNOW there is a 4G/LTE Tower within a few miles of your location, you probably DON'T need it.

I know for a fact that I am 9/10 of a mile away from a Verizon 4G/LTE tower. I also have a very strong signal on channel 60, which is 746-752MHz, and no signal at all on channels 58-59, which is where ATT would show up.
Quote:
Originally Posted by ProjectSHO89 View Post

Don't waste your energy and time trying to hit that specific number. ATSC signals have an ~80 dB dynamic range (-85 to -5 dBm, add 48.8 to convert to dBmV).

If it doesn't matter as to your actual reception (channels received and view-ability) , leave the flippin' attenuator on there and be done with it already.

Despite my comments, I'm actually not taking any extra effort to try and hit 0dB. I'm fine with anything in the acceptable range, I'd just like to get the signal as good as I can now, so that when it snows, rains, or I add other splitters (I've been testing without one of the splitters in place), it doesn't degrade too much. The attenuators I was using were borrowed, and not mine. So I need to buy something -- either a new amp, an attenuator, or a filter. The filter is about the same price as an attenuator (with shipping), but I don't want to spend as much as I'd need to to get a new amp, so that seems to be the sweet spot for me.

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post #16 of 19 Old 10-08-2013, 10:52 AM
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A four-way splitter with a few F-terminators will yield about 7.5-8 dB attenuation. You might have them in your supply box already or can pick them up cheaply.

I just wanted to caution you not to let your self-assessed "OCD" get the better of the project. wink.gif
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post #17 of 19 Old 10-09-2013, 01:38 PM
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A 2-Way Splitter with only 3.5-4 dB of Loss MAY be all the Loss you need on the Preamp input....because for each 1 dB of Loss, there is 3 dB of reduction in 3rd Order Intermod noise levels, hence 3.5 dB Loss yields 10.5 dB of IMD reduction. And since it's more likely you can get you have 2-Way Splitters already (may need to "borrow" from behind your existing system), you could also try putting TWO of them in tandem for 7-8 dB of Loss, yielding 21+ dB of IMD reduction.
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post #18 of 19 Old 10-12-2013, 02:31 PM - Thread Starter
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The 4G LTE filter almost completely took care of the problem. I'm also leaving the amp right before my first splitter, instead of on the mast, in order to get a couple dB of loss from the cable. If I need a little extra, I'll toss in a splitter before the amp. Interesting info about the intermodulation noise and reduction. I'd never heard of that before; sounds like it has to do with sideband noise as a result of non-linear amplification? And if I'm reading and understanding correctly, the closer the amplification transistors are to saturation, the more IMD noise there is?

At any rate, I'm currently pulling more channels than I have before, and at good signal quality.

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post #19 of 19 Old 10-12-2013, 03:27 PM
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Explains IMD in Preamps (generating signals on top of desired signal), desired goal to maximize SFDR (Spurious Free Dynamic Range), TUNER tolerance to IMD and more re 4G/LTE Interference:
http://www.avsforum.com/t/381623/the-official-avs-antenna-and-related-hardware-topic/2730#post_6163454
http://www.avsforum.com/t/381623/the-official-avs-antenna-and-related-hardware-topic/3000#post_6290253
http://www.avsforum.com/t/1456477/parma-oh-trying-to-pull-1-out-of-the-weeds#post_22927689

When trying to receive weak signals subject to IMD from very strong local signals, Preamps operated close to maximized SFDR levels will support a Dynamic Range in the range of 55-70 dB, depending on the Preamp (and sometimes you need to insert a small amount of LOSS on the Preamp's input] You can see from your TVFool results how much Dynamic Range you NEED...perhaps adding another 10+ dB to allow for fading signals levels.

This means the STRONG signal levels need to be kept 20-40 dB BELOW the point of TRUE "Overload", i.e. 1 dB Gain Compression Point, Saturation and /or frequent clipping [see my signature line below for links]:
http://photos.imageevent.com/holl_ands/files/ota/DTV%20Preamp%20Signal%20Overload%20Calculator%20-%20RevM.xls.pdf
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