Ground Loop / Hum - Videokarma.org TV - Video

ihmeyers · #1 11-09-2008, 11:19 AM

I've been trying for the past month to stop the hum when using my RF Modulator with the Admiral. It's definitely video not audio induced as it appears even when no audio cables are connected. I'm using a high-end Monster S video cable from the DVD player to the modulator and a decent coax from the modulator to the set. I am almost at the point where I want to scrap the modulator and try something different. On some programs the level of hum is pretty intrusive. It's not present on any off the air channels so I'm pretty sure it's not a tube or something on the board. Any ideas?

Thanks!

freakaftr8 · #2 11-09-2008, 01:33 PM

I have had the same issue with an RF modulator.. Why dont you try a VCR first and see if it cures the problem. When you get a hum/buzz is it with a screen thats bright, or does it not matter whats on the screen, as it would do it no matter what.. Your set might need a slight touch up in the AGC pot (automatic gain control) this pot is on the back of the set somewhere near the RGB screen pots.

wa2ise · #3 11-09-2008, 07:46 PM

The amplitude of the input video may be too high. This can cause the sound carrier to cut in and out at the vertical rate (making a 60ish Hz buzz) and the horizontal rate (usually not noticeable). White areas in the video makes it louder.

An easy to reduce the video amplitude is to add an additional 75 ohm termination to the video line. Some consumer equipment has the amplitude set too high, by about 20%. Makes a consumer think that the equipment makes a brighter and higher contrast image on the screen.

ihmeyers · #4 11-10-2008, 08:30 AM

Thanks for the suggestion. How/Where would I do this?

I tried adjusting the AGC yesterday with no success. I also ordered a different modulator. I'm currently using the Radio Shack model and I ordered a Phillips as I hear that's about the best consumer one available. Actually cheaper than the RS too. We'll see if that's any better.

jpdylon · #5 11-10-2008, 11:55 AM

The RS model should have an adjustable input impedance. Make sure its set to 75 ohm and not 1k or you'll overload the box with the av from the DVD player.

freakaftr8 · #6 11-10-2008, 01:03 PM

YES!
This could be an issue with the radio shack model. I have one and if set to 1k the screen will we very washed out as well and cause one heck of a buzz through overload.

Quote:

Originally Posted by jpdylon

The RS model should have an adjustable input impedance. Make sure its set to 75 ohm and not 1k or you'll overload the box with the av from the DVD player.

ihmeyers · #7 11-10-2008, 01:31 PM

Quote:

Originally Posted by freakaftr8

YES!
This could be an issue with the radio shack model. I have one and if set to 1k the screen will we very washed out as well and cause one heck of a buzz through overload.

Where do I make the adjustment? There isn't anything on the back other than a Ch3/Ch4 switch and I didn't see anything yesterday when I opened it up?

freakaftr8 · #8 11-10-2008, 02:15 PM

I found on my set (magnavox) that a touch up on the quadrature coil in the sound demod circuit cleaned up alot of the buzzing I was getting. Sometimes as components age they drift. When they drift coils that control the frequency of these circuits need a touch up. Do you have a sams on this set?

krazyknuc · #9 11-10-2008, 02:48 PM

Hello
Yes, cable tv is probably the culprit.
I have been told it can be caused by having more than one 3 prong (grounded) device plugged in too. Some people use a 3 to 2 prong converter to fix this but this is illegal in Canada and I'm guessing there is a good reason......like fire hazard or something.
I ordered a "ground loop eliminator" from parts express a couple of years ago.
It was expensive but it did what it claimed it would do.
Maybe one of the electronics guys on this site know how to make a "DIY" version for cheap.
If not, I would go for the parts express version.
The only thing that works better is unscrewing the cable connection.

ihmeyers · #10 11-10-2008, 02:49 PM

I don't have a SAMS. I should get one. I am cautiously optimistic that the Phillips modulator will be better.

wa2ise · #11 11-11-2008, 12:58 AM

Quote:

Originally Posted by krazyknuc

Maybe one of the electronics guys on this site know how to make a "DIY" version for cheap.

Some 75 to 300 balums are isolated (no DC connection between the 75 ohm F connector, and the 300 ohm twin-lead. Use a pair of these with the 300 ohm twin lead terminals back to back.

ihmeyers · #12 11-11-2008, 08:46 PM

Wa2ise

At the risk of sounding stupid (which I'm quite good at), I don't follow how to construct this. Would you mind explaining it further to me?

Thanks!

Ian

wa2ise · #13 11-12-2008, 12:58 AM

Rig up this:

A pair of 75 to 300 ohm balum matching transformers back to back, with 0.01 caps between. You'd need to solder the caps to the spade lugs.

ihmeyers · #14 11-12-2008, 06:19 PM

Quote:

Originally Posted by wa2ise

Rig up this:

A pair of 75 to 300 ohm balum matching transformers back to back, with 0.01 caps between. You'd need to solder the caps to the spade lugs.

Thanks for the info Bob. Definitely an improvement. Not perfect, but better. Do you think it would help if I played around with the value for the caps?

wa2ise · #15 11-12-2008, 07:52 PM

Quote:

Originally Posted by ihmeyers

Thanks for the info Bob. Definitely an improvement. Not perfect, but better. Do you think it would help if I played around with the value for the caps?

Yes, you probably could make the caps smaller, like 1000pF. But be sure to use the same value for both, so the system stays balanced (else the TV set and such would tend to leak the CATV signals into the "air").

Another possible solution would be to run a heavy braid wire from the CATV shield (outside of the F connector) to the TV or the audio amp's chassis. This would divert hum currents from impressing themselves on an audio cable connection.