: Signal Amplifiers (Amps, Preamps, Distro Amps) - See Chart in Post #1
2011-09-16, 12:43 PM
Well i think i might have figured out what the problem with my vhf reception is from this thread http://www.digitalhome.ca/forum/showthread.php?t=107706&page=38 post 564, ya so i think my vhf portion of my cm 7777 preamp is burnt out. I put up my old vhf antenna that always worked and i now recieve nothing tryed three different baluns nothing, i figure it must be the preamp, when i was changing antennas and stuff i never unplugged the preamp and so i'm thinking maybe it shorted out? Does anyone know how to tell or test a 7777 to see if the vhf portion is working or not? Thanks
2011-09-16, 01:14 PM
brantford bill, Someone of course needs to ask you the very basic question which could explain why you think your cm7777 vhf input may be burnt out.\
Did you set the Separates/Combined setting inside the 7777 to Separates ?
2011-09-16, 02:19 PM
if you read my post and the posts in the link you'll see my experiences recently with my vhf antenna's, and of course the inputs are on seperate, and i've check the coax, and tryed different baluns, and now went back to an antenna that i know worked.... it's all just frustrating and it has now lead me to believe that the preamp may have some how blown? on the vhf side. Just wondering if anyone knew how to test it, maybe with an ohm meter or something?
2011-09-16, 02:29 PM
As an extra test, did you try to set the 7777 back to Combined, and merge your UHF and VHF antennas with a UVSJ before it gets into the Combined input ? (just for testing, i don't mean this to be a fix)
2011-09-17, 12:57 PM
Thanks Canuck and Holl Ands for the info! It really helped clarify things!
Follow up question:
I just came up from the basement where I saw where everything is. I do not have to hook the antenna to the old cable input location. I can get by with a 50' to 60' run from the Stealth Hawk straight down to the basement and then split to the TV's. Most will be will be within 30'. I still think I need an amp but, based on what you said, maybe one not as strong as the 7777, especially with the shorter run to the basement.
Both of you said I had several really strong stations and suggested the 7777 amp might amplify too much. Again, my TV fool:
Channel Master preamp 7777 = +26 db
Channel Master preamp 7778 = +23 db
Channel Master one port distribution amp 3410 = +15 db
I am tempted to get the lower power 3410 and install it it in the attic 20' from the antenna which will be on the roof.
Does this make sense? Again, thanks for your help.
2011-09-17, 05:20 PM
The above URL has been shortened...with embedded dots....
When you copy/paste, you have to go back to the original:
C-M doesn't provide ANY specs for 3410, other than 15 dB Gain.
Since it is also intended as a Cable Drop Amp, it's probably not too bad wrt Overload...maybe....
C-M makes 16 dB Gain & W-G makes 19 dB Gain Preamps that have very good Overload specs:
Chart is found in Post#1 in this thread.
PS: W-G HDP-269 specs now say 150,000 uV max input (vice 350,000), hence Max Input
drops to -20.0 dBm, so it isn't that much better than these higher Gain Preamps.
Since TVFool assumes Omni, 0 dB Gain Antenna with NO Losses, be sure to adjust
for Gain of the Antenna on the strongest channels, minus Losses prior to Preamp.
If you are a bit over, it isn't the end of the world, other than losing weak signal sensitivity.
If you a lot over, locating the Preamp or Distribution Amplifier at the END of the Coax run
could help prevent Overload, due to the extra loss (every 1 dB of Loss reduces IMD by 3 dB).
You might need to add a SMALL amount of additional Loss to maximize the Dynamic Range.
2011-09-17, 06:33 PM
well i figured out what was wrong with my preamp that i was inquiring about in a previous post 2743... well the reason the preamp wasn't amplifying my vhf signal was that there was no vhf signal to amplify. When i put up the new antenna i made a new rg6 cable... well and i messed it up when i did, i was rechecking everything in the setup when i had a vison, i cut all the braded wire off the rg6 so there was no outer conection thus no signal! Oh well got it figured out now and didn't cost me anything, just alot of time and headaches...
2011-09-17, 08:03 PM
Signal travels down the centre conductor, not the braid. The braid acts as a "faraday cage", which captures external RF and grounds it out to prevent interference with the centre conductor.
Of course if your braid isn't grounded out, this could result in interference with the conductor, which could result in poor performance.
2011-09-17, 08:28 PM
Even thought the signal is on the core the braid is part of the circuit loop so cutting it back breaks the connection. No?
2011-09-17, 08:55 PM
No, the braid doesn't handle desired signal whatsoever, and it's not part of a circuit that accomplishes signal distribution. At no point should braid and conductor form an electronic loop.
None of your tuners or devices attempt to capture signal from the braid. It's merely part of an electronic cage and system that protects the conductor in conjunction with the foil and dielectric. The protection works both ways: It prevents RF ingress and egress.
2011-09-17, 10:01 PM
unless of course if the cable that was broken was the downlead and had DC riding on it to power up the pre-amp, then it wouldn't work at all with a break in the Braid.
2011-09-18, 12:27 AM
The braid connection must be intact to conduct the DC for the preamp and the signal. They both share the same piece of coax, but do not interfere with each other because of a capacitor and choke network at each end. A capacitor will pass RF, but it blocks DC. A choke (inductor) passes DC, but it blocks RF.
Because of the "skin effect," the signal travels on the outside of the center conductor and the inside of the braid. At the instant that the RF is going down the center conductor, it is going up the braid, because they are 180 degrees out of phase. You must have continuity for the signal to have a return path, just as it is needed for DC. The DC current uses the full cross-section of both conductors. Yes, the braid does act as a shield, but it is also a conductor.
When you walk into a dark room and turn on the light switch, you are making a complete circuit that provides a return path for the current to flow. Just as it is needed for AC, it is also needed for DC and RF. There is even a path for the return RF current of a dipole antenna, which is through the air from one end of the antenna to the other.
The proof, of course, is that his problem is now solved.
There is another proof, if you have cable. If there is a slight break in the shield, the low analog channels will be snowy, but the high channels will still be OK. This is because the high channels can jump the small gap that has enough capacitance to provide a low reactance for the higher frequencies.
I drew a diagram of the power supply when I was doing a thread for troubleshooting the CM7777 preamp on the other forum. C2 and L form the network of the power injector.
Good job of troubleshooting, Bill. Thanks for letting us know.
2011-09-18, 12:49 AM
Cutting back the braid causes a discontinuity in the impedance of the coax
transmission line, which in turn causes the signal to be reflected back up
the coax (huge SWR), rather than being delivered to the tuner.
2011-09-18, 01:02 AM
majortom, I was speaking of coax in the general sense. But you're correct: In the context of using a CM7777 preamp, there must be continuity with the braid to power the amp. And as this amp doesn't have a fail-over or pass-through feature (like some of the newer CM amps), no signal (or very poor signal) would result.
Either way, Brantford Bill, I'm glad that you found the problem and were able to resolve it! ;-)
2011-09-18, 01:57 AM
Thanks for the update on the HDP-269 input.
There certainly is a need for a preamp that can tolerate strong local signals without harming weak signals. YU1AW has a DIY preamp with interesting specs:
Do you have any input specs yet on the AD CPA-19? All I can find is "High overload resistance for more consistent reception in urban and suburban areas."
The specs for the CM3410 say: Amplifier Gain 15 dB and Output Level 25 dBmV, which seems to imply max input of +10 dBmV (-38.8 dBm).
2011-09-18, 03:35 AM
I don't recall ever seeing Max Output/X-Mod specs for any of the A-D Preamps.
Perhaps you can query ADTech.
Although I have seen the CM3410 "Max Output 25 dBmV" spec on various RETAIL websites,
incl. www.channelmasterstore.com, they are NOT found in either the Product Sheet
or Manual on www.channelmaster.com or in the 2011 C-M Catalog.
"Output Level 25 dBmV" is insufficient information....unknown HOW MANY strong signals and
the resultant X-Mod (Crossmod, or Intermodulation distortion) level. C-M and W-G usually
use -46 dB Crossmod spec (Distortion Noise floor is 46 dB BELOW individual signal levels).
A variety of 2, 5 or 8 (strong) signals are sometimes included, using "standard" deratings.
Note that 25 dBmV is NOT EVEN CLOSE to typical OTA Preamp "Max Output"
specs (~50-60 dBmV) with 2 or 5 strong signals:
See "DTV Preamp Signal Overload Calculator - Rev L".
I have a digital copy of C-M's 2007-2008 Terrestrial Catalog: NO Dist Amps of any kind.
I also have C-M's 2003 DTVS Catalog Ed7, apparently before CM3410 came out.
Fol. is one their "Indoor" OTA and CATV (Return Path) Dist.Amp. Series:
Model 3042 3043 3044 3045
Number of Outputs 1 2 4 8
Amplifier Gain 13 dB 22 dB 22 dB 22 dB
Gain Each Output 13 dB 10 dB 8 dB 4 dB
Noise Figure 3.5 dB 3.5 dB 3.5 dB 4.0 dB
Max.Output Per Channel (@ 36 channels)
30 dBmV 25 dBmV 23 dBmV 19 dBmV
They also had MATV models:
Model 7720 7721 7722
Gain 13 dB 21 dB 32 dB
Noise Figure 3.5 dB 3.5 dB 3.5 dB
2 channels (-46 dB X mod):
56 dBmV 54 dBmV 57 dBmV
36 channels (-54 dB CTB):
37 dBmV 35 dBmV 38 dBmV
Note that the 2 channel (strong signal) spec indicates both an Output Level and
the resultant CTB and/or Inter-Modulation (Crossmod) Noise floor. This important
piece of information is MISSING from the CM3410 specs.
The 36 channel (CATV) spec for the second group indicates both an Output level
and -54 dB CTB....but the latter spec number is MISSING from the first group.
More info on CTB (Composite Triple Beat Distortion) and other typical CATV
measurements can be found here:
Here are some typical CTB and other spec numbers for CATV Drop Amps:
Given how close CM3410 Max Output number comes to 36 channel spec numbers,
perhaps we can ASS-U-ME that C-M engineers conducted a CTB test???
If so, I'm not so sure we can accurately extrapolate a 36-channel test to derive
performance with only a few strong signals....linearity is NOT guaranteed....
2011-09-18, 03:41 PM
well i sure felt silly once i realized what the problem was, and i wasn't going to post what it was just to save some face :confused:, but i thought if i could do this than certanly someone else could too and if my little brain fart helps someone else try to trouble shoot than i thought i better share what had happened. By the way the lead that was the trouble was the one connecting the wire from the antenna to the preamp. Fixing the braid instantly fixed the signal.
2011-09-18, 03:53 PM
We all do things like that, Bill. That's how we learn.
My most recent "learning experience" was when I tried to measure the voltage of my 12 volt jumper pack with my DMM. I couldn't understand why the test leads were getting VERY warm until I noticed that they were connected to measure 10A. That meant the internal low-value current shunt resistor of the meter was connected across a 17 Ahr AGM battery! :eek:
Thanks for sharing; it will help others with a similar problem.
You buy an antenna amplifier, you follow the instructions and install it.
Now, the fun begins;
You turn it on, there is no signal!
You turn it off, the signal comes back!
You turn it on and play with strength knob (or screw) there is still nothing.
You get another amplifier, another brand, from another store the same shing!
After the forth time, I decided ask experts where to buy a saddle.
NOT a single amplifier that I have tried have let the juice pass trough (when turned on) let alone amplify it.
What the hell am I missing.
I am not an engineer, but I have done my house electricity myself and have build circuit boards.
2011-09-25, 06:52 PM
Well, Xiar, that usually means you're using an amp where you shouldn't and it's overloading.