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  #16  
Old 02-21-2015, 12:56 AM
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I can't see a problem in the video....

If the over modulation lights stay lit with the controls all the way down, then that is a problem with hum (probably 60Hz) getting into the signal. It could be a result of your AV source having a different ground voltage than the modulator, or mod output being connected to something with a different ground potential. Using a power line isolation transformer to power things independently may work to fix it. You can trace the source by unplugging things till the lights go out.

My agile mod likes to randomly introduce hum in it's output, and interfere with my fixed modulators....Some BT modulators are old enough that the lytics have gone bad, and I suspect mine is one....Yours could also be one.
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  #17  
Old 02-21-2015, 01:00 AM
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I fixed the over modulation lights as mentioned above.. The problem is there.. You don't see the noisy video? actually the camera picks it up more that what I actually see, that's not a pixely youtube video, that's actual noisy video from the TV screen, it's very noticeable...
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Last edited by tvcollector; 02-21-2015 at 01:06 AM.
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  #18  
Old 02-21-2015, 01:11 AM
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Sorry bout that. You posted while I was writing my previous post.
I could not see video noise...Granted digital video systems try to suppress noise to get better compression and fine noise is harder to record.

If you have been using a cheaper modulator before this how much more noise can you see with the BT modulator? It is hard to get noise free RF video, so there should be a threshold where (say at normal viewing distance) it is very difficult to notice which should count as good enough.
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  #19  
Old 02-21-2015, 01:17 AM
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I was using a regular cheap RF modulator to run from the digital cable box RCA outputs to the modulator and output coax to the TVs, seeing how the cable box doesn't have a coax output, and there is no noise with the cheap RF modulator..

Here is another youtube video with someone outputting directly to TV with their Blonder Tongue and the video is perfect..
https://www.youtube.com/watch?v=KQ_83G8UEgs
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  #20  
Old 02-21-2015, 08:03 AM
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Quote:
Originally Posted by Electronic M View Post
I'd go with a UHF bowtie dipole.

I'll also the praises for good old oven-rack in fancy-dress with bowtie(s) style of antenna. Damned good at what they do.


The discussion on baluns in this thread so far is spot on. That is to say, your bog-standard 75 to 300 ohm TV balun is absolute crap if you're trying to transmit power through it. (TBFH, the average TV balun is just crap, full stop.)


http://www.kyes.com/antenna/balun.html has a good writeup/table on the kind of losses you'll be facing. -3dB means you're losing HALF your power, -6 means you're losing THREE-QUARTERS. So for VHF use, you'll be lucky to see 40% of what you put into it come out the other end. For UHF it's a roulette wheel style thing where on a given channel you might only lose 30% of your power, but if you slightly alter the length of the cable at some point suddenly you're losing 90% but on a previously unusable channel you're getting great throughput.

The page I linked also includes information on rolling your own 4:1 balun, but this link will make things a bit easier. http://n-lemma.com/calcs/dipole/balun.htm The important thing here is that you do need to know with some level of confidence what the velocity factor of the cable you're using actually is. Dig through your coax stash until you find something that has a series number on it that you can turn into a spec-sheet with google. (If you see Belden, you've got it made.) Once you plug the known velocity factor into the calc, next you'll plug in the video carrier center frequency(Not the channel center frequency.) for your channel of choice, and the calculator will return a length value with a hilarious number of digits east of the decimal point. For your purposes here you can round to the nearest 0.1" and be OK. Good cable-prep and soldering skills will help, but if this is your first time making RF signal transmission line connections, spending half an hour practicing on a few feet of junk coax is a good idea, so that you get a feel for where and how to begin the cutting/stripping process so that your finished length is as close as possible to the ideal desired length.


I'm also in agreement that you probably want to attenuate the output of the modulator if you're direct wiring it to a set.
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  #21  
Old 02-21-2015, 10:29 AM
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Thanks for all the info.. I suspect that will come in handy when I get a unit that's functioning properly..
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  #22  
Old 02-24-2015, 02:23 AM
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Made an Antenna.. Haven't cut the lamp cord yet.. Waiting on another unit..



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  #23  
Old 02-24-2015, 07:18 AM
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I wonder what kind of antenna would be needed with multiple modulators and an active combiner.
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  #24  
Old 02-24-2015, 03:42 PM
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I'd cut all but the coax connector side off of the box and make it so that the closest the wires come to each other is the point at which they are soldered to the connector....
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  #25  
Old 02-24-2015, 07:47 PM
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I'm able to keep this unit and get a full refund.. Right now I'm testing all the electrolytic caps on power supply board with my ohm meter and eico cap checker.. So far all the bigger caps are either testing out of range, not giving a reading at all, and the Eico leakage test eye opens slightly instead of all the way at their 35v ratings.. And small cap is not even making contact with one lead (pictured)

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Last edited by tvcollector; 02-24-2015 at 07:53 PM.
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  #26  
Old 02-24-2015, 07:52 PM
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For shits and giggles I'd order up all the standard electrolytics from capacitorworld and see what happens.
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  #27  
Old 02-24-2015, 07:55 PM
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I'm going to, then I'll have 2 units working hopefully.. Look at the pic, that's the way that cap was when I opened it, not even making contact.. I suppose I should replace those rectifier diodes too..
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  #28  
Old 02-24-2015, 08:05 PM
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The glass is -3dB.
 
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Quote:
Originally Posted by tvcollector View Post
I'm going to, then I'll have 2 units working hopefully.. Look at the pic, that's the way that cap was when I opened it, not even making contact.. I suppose I should replace those rectifier diodes too..
They seem to have gotten a bit toasty... Granted they probably have eleventy-million hours in a hot and cramped headend rack on them. Looks like there's room to stand the replacement rectifiers up off the board when the lid is on, plus an extra 5 cents per diode x4 for the good ones isn't a big deal when the shipping charge is considered.
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  #29  
Old 02-28-2015, 02:10 PM
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I just got my second BT modulator model "A" model, and it's preset for UHF 23, which I'm going to leave it, and it's working great, and I'm just using a bowtie dipole antenna that I don't have extended yet.. I'm able to pick up signal at the end of the block.. I tried the GE set, and the UHF actually comes in on that set.. No pic though, but sound comes through.. The caps arrived today for the problem modulator, I'll recap that and use it for a VHF channel.. Probably 7 or 8..

What would you recommend broadcasting with UHF? just a standard botie Antenna or shango066's method?
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Last edited by tvcollector; 02-28-2015 at 03:41 PM.
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  #30  
Old 02-28-2015, 05:30 PM
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Quote:
Originally Posted by tvcollector View Post
I just got my second BT modulator model "A" model, and it's preset for UHF 23, which I'm going to leave it, and it's working great, and I'm just using a bowtie dipole antenna that I don't have extended yet.. I'm able to pick up signal at the end of the block.. I tried the GE set, and the UHF actually comes in on that set.. No pic though, but sound comes through.. The caps arrived today for the problem modulator, I'll recap that and use it for a VHF channel.. Probably 7 or 8..

What would you recommend broadcasting with UHF? just a standard botie Antenna or shango066's method?
Glad you got a working mod. I'd probably go with a UHF bowtie. Odds are at UHF, without a SWR meeter like Shango has, it will be hard to cut the length close enough to optimum frequency/ impedance match for there to be much benefit....But you could try if you are curious, and I could be wrong.
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