New CM7777 preamp kills all channels

[hillpc]

Large Radio Shack VU-210XR combo antenna, 25 feet up, pointing directly into an oak tree less than 2 feet away, still gets fairly decent reception, but pixellating now and then for even the 3rd signal down the radar table (attached). There's a rotator on it. I have a 4way splitter in the line, one oulet capped, one to an FM receiver, one to an HDTV, and one to a cheap digital/analog converter for a second TV.

I recently purchased a CM7777 preamp and it kills all the stations dead. Zero signal level on every channel according to both the HDTV and the d/a converter box. This is curious, since I'd had an older CM7777 on the antenna for years without problems. The antenna and old CM7777 was put up about 20 years ago. I took off the 7777 about a year ago when it seemed to die and kill all the channels. Just bought a new one to try to get rid of the pixellation, maybe get some more channels.

I've put the 7777 preamp at the bottom of the antenna lead just for troubleshooting purposes, before the 60 feet or so of additional run and the power injector and the splitter. The channelmaster customer service people said this should be OK and also told me that I could put a 2way splitter (-3.5 dB) in the line into the preamp from the antenna in case the preamp was overdriving the signal. Didn't help, neither did a 0 - 20 dB adjustable attenuator. I'm baffled now. This setup worked at one time and now I get nothing if I put in the preamp. Even a station so weak it's nothing but pixellated all the time goes completelt dead. Is it possible it's bad out of the box? This is actually a 2nd one in a week because I thought the first was bad out of the box.

[GroundUrMast]

The CM-7777 is a 'fringe' to 'deep fringe' preamplifier. Your signal conditions are not 'fringe'.

Sadly, the Channel Master specifications do not provide a maximum input value that would allow your or I to calculate how close to overload your signals plus antenna gain would cause the CM-7777 to operate at.

BUT, I doubt you will solve the reception reliability problem with any amplifier. If you have an oak tree in the way, and you are already using high-gain directional antenna. You don't have a problem with low signal levels, you almost certainly have a problem with 'foliage induced multipath interference'. The solution is, find a mounting option that gets the antenna above or around the tree(s)... or get a chainsaw.

Back to the idea of a preamp... perhaps, if you have many TVs connected, a distribution amplifier might be indicated... But the conditions shown in your report jump off the page saying 'No preamplifier needed'. The CM-7777 has been redesigned... it's entirely possible the 'new and improved' version is less tolerant of strong signals.

Even the Channel Master ad copy cautions against using the CM-7777 in conditions like yours: http://www.channelmasterstore.com/v/...4jan12_web.pdf

As a test put all of the , preamp , the preamp the power supply the power injector . on the antenna coax before any splitters.

Some splitter pass power through the splitter some do not.

Even so , the tree is going to move , or the antenna is going to move.

Do not use the preamp.

For 1 Tv connected use no splitter.

For 2 Tv's connected use a 2 way splitter.

For 4 Tv's connected use a Channel Master CM 3414 distribution amplifier.

[hillpc]

Thanks for replying. This all makes sense, but I have some more questions to help me better understand your points.

On the signal strength displays on the HDTV and the converter box, wouldn't I see something other than zero if I were getting too much amplification from the CM7777?

I'd hoped that putting -20dB into the line somewhere with the attenuator would allow me to troubleshoot this issue but it didn't do anything to help, at least when I put it into the input to the amp, and on the line to HDTV after the splitter. I didn't try it between the power injector and the splitter. The CM7778 (for moderate signal areas) has 16 dB of gain instead of 30; I had hoped to simulate this level of amplification (and even less).

What's the difference between a distibution amp and one of these CM7777/7778 preamps? The Channel Master ad copy suggested by GroundUrMast seems to indicate that the preamps could be used to reduce the detrimental effect of splitters.

Yes, the preamp and its power supply is before the splitter.

Taking out the splitter completely and running only one TV only helps a little, with no preamp.

The trees have grown in over 20 years here. I'm not sure cutting down that one oak would help all that much; I think I'd have to get a tower to get over the 70 foot tall trees just 40 feet farther away in the desired direction. My mother-in-law has an unused one, but even though I'm a hardcore do-it-yourselfer, This might not be worth the safety risk, at least not without some very tall bucket truck. Not to mention all the time and effort.

[GroundUrMast]

Your description makes me lean toward suspecting an out-of-box failure or some sort of misreading of the installation instructions. Even if the 'new and improved' CM-7777 is less able to handle strong signals, the 20 dB attenuator should have been enough to get you into a workable range.

You are absolutely correct, a preamplifier's primary roll is to overcome the loss in cable and splitters that are connected to the amplifier output. They may also negate a portion of the internal noise of the tuner. After adding up the losses and considering the noise figure of the tuner, excess preamp gain is of no benefit.

The VU-210XR claimed 11 dBd gain in the UHF band. If true, you should be able to connect 50' of RG-6, a 4-way splitter and then another 50' of RG-6 (a total of about 14 dB loss) and end up at a tuner with a NF of 8 dB... at a Net-Noise-Margin of 15 dB when aiming at a signal listed at a NM of 26 dB in the air. (A Net-NM of 15 implies that the signal is 15 dB stronger than the minimum needed for reception. You would expect to be getting reliable reception at this and higher levels.) So that translates to all the signals down to the mid range of those highlighted in yellow on your report. (This is why I don't think a preamplifier is indicated, nor is it the solution to your problem.)

The trees are the culprit.

If you can't solve the tree problem, and want to try an amplifier, a high input unit such as the CM-7778, HDP-269 or my favorite, the CPA-19 have a better tolerance for strong signal levels. (I'm not recommending you spend the money, because I don't think any amplifier will address the real problem.)

Connect a --> Known to be good looong coax <--> to the antenna and run the coax through a open door or window direct to the Tv , no splitters , no preamps , no nothing , direct to one Tv.

What is reception like now??

Now with the antenna connected to the looooong coax move the antenna to locations where there is the least amount of objects in the direction of reception.

What is reception like now??