75-300 ohm Transformer Connection Question - AVS Forum
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post #1 of 4 Old 11-02-2013, 01:50 PM - Thread Starter
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Could someone tell me the difference between a 75 to 300 ohm transformer that attaches to four screws vs. the one that attaches to two screws? Is there a difference? And should the one that attaches to two screws be attached to the UHF or VHF screws, does it matter?

I'm curious because I have an old TV from the '70s and I have it connected to the cable box via coaxial connections using the transformer. I recently got a new cable box (Comcast Cisco RNG100) and it's producing a very annoying buzz and static noise on all the channels, especially on the guide and menu. I didn't have a problem with the old box (SA Explorer 2200). I'm trying to figure out what the problem is. I thought maybe it's the transformer and how it's connected.

I had the cable box temporarily connected to a CRT TV from 2000 and I didn't notice the buzzing or humming. It was connected using the coaxial connections. So that's telling me something, but I'm not sure what exactly.
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post #2 of 4 Old 11-03-2013, 07:38 AM
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Since you have to tune the old TV to channel 3 or 4 for that kind of hook-up everything is VHF (i.e. CH 3 and 4 are VHF frequencies).

I'm guessing that new cable box is radiating noise that the old TV is picking up as those old TVs weren't shielded for today's modern digital circuitry noise 'generators.' Try moving the cable box further away from the TV via a long coax. I had a similar problem with an old TV and a DTV converter box and that did the trick. I also had some luck separating them with aluminum foil and placing the box away from the tuner section (TV tuner knobs) but it wasn't 100%.

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post #3 of 4 Old 11-04-2013, 10:04 AM - Thread Starter
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On the back of the TV, there are two screws for UHF. Then below that there are two screws for VHF. But, in between those two VHF screws is a 25 awg wire sticking out with a U-shaped metal thing on it that looks like it would attach to a screw. What is that wire for? I don't recall ever seeing a wire like that on one of these TVs.
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post #4 of 4 Old 11-04-2013, 11:32 AM
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I dunno, maybe an optional ground wire. Hook everything up VHF and check to make sure it tunes in&out as you change from channel 3 (or 4, whichever you have chosen from the cable box). Might try both 3 and 4 to see if one works better than the other.

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