View Full Version : Ground Loop / Hum
ihmeyers 11-09-2008, 10:19 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 11-09-2008, 12:33 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 11-09-2008, 06:46 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 11-10-2008, 07:30 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 11-10-2008, 10:55 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 11-10-2008, 12:03 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.
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 11-10-2008, 12:31 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.
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 11-10-2008, 01:15 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 11-10-2008, 01:48 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.
:thmbsp:
ihmeyers 11-10-2008, 01:49 PM I don't have a SAMS. I should get one. I am cautiously optimistic that the Phillips modulator will be better.
wa2ise 11-10-2008, 11:58 PM 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 11-11-2008, 07:46 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 11-11-2008, 11:58 PM Rig up this: http://www.audiokarma.org/forums/attachment.php?attachmentid=117380&stc=1&d=1226469480 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 11-12-2008, 05:19 PM Rig up this: http://www.audiokarma.org/forums/attachment.php?attachmentid=117380&stc=1&d=1226469480 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 11-12-2008, 06:52 PM 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.
Dave A 11-12-2008, 07:14 PM Is the power supply on-board or a wall-wart? Either way, it could be faulty and leaking AC to the modulator. What happens if you move the modulator to another set?
ihmeyers 11-12-2008, 07:59 PM On board power supply. Haven't tried the modulator on another set but did try a different modulator on this one. It was worse.
I think it's got to have something to do with overloading the set because it screws ups some of the colors as well. Not very noticeable on programming but the DVD welcome screen is purple at the top of the screen when it should be all dark blue (I tried the DVD player on a new set). No amount of AGC adjustment will fix it. Bob suggestion about adding caps helped somewhat but didn't fix it. I'd switch DVD players but the one I use doesn't have macrovision so I don't have any problems with protected DVDs.
zenithfan1 11-12-2008, 08:12 PM Sounds like a purity issue, it shouldn't be purple at the top no matter what, unless there is something purple there of course. Is there a stray magnetic field around? Maybe the power supply on the DVD player?
ihmeyers 11-12-2008, 10:19 PM Hmm. The power cord for the DVD player runs right next to the back plastic casing for the picture tube. If I adjust the fine tuning on the set the purple will become, red, green, orange. When finetuning and AGC is optimally adjusted the top of the screen is purple. Maybe I need to re-route. I've also observed that there's an rf choke on the DVD power cord. I think I'll try removing that as well....
Nope. That didn't help. Moved away both the power cord from the modulator and the DVD player. Still have that purple along the top. Also a dark shadow along the right hand side of the screen. Almost like a one sided cataract. It's not there on program content and isn't a problem on broadcast channels. Also if I adjust the fine tuning on DVD program content I can make those rainbow colors appear at the top of the screen.
freakaftr8 11-13-2008, 10:25 AM Dont mean to sound dumb, but are you sure your modulator and your tv are set top the same channel. Just for s&g's, try tuning it to channel 3 and fine tune your tv in on the oppisite channel. It's always possible that the modulator could me misprinted...
ihmeyers 11-13-2008, 04:32 PM Dont mean to sound dumb, but are you sure your modulator and your tv are set top the same channel. Just for s&g's, try tuning it to channel 3 and fine tune your tv in on the oppisite channel. It's always possible that the modulator could me misprinted...
Tried that too. The modulator appears to be labelled correctly. Does it sound like an overload on the set from the modulator? The Phillips modulator I got earlier this week was actually worse. Problem exists even if I go through a wireless modulator like a rabbit. Maybe the output is too high on the DVD player, but its really the only option I have since it doesn't have Macrovision and nearly all others do. Really at my wits end with this...:sigh:
freakaftr8 11-13-2008, 07:38 PM Wow! Wild! Dude try another DVD player... Do you have a video game console you can run through the rf modulator, also disconnect one lead from your tv input, see if the symptom changes.. Im assuming that the blue background is normal standby for the DVD player orr not? The purple blob looks like a putity glitch. One more thing, try tweaking the horiz hold back and fourth very slightly..
ihmeyers 11-14-2008, 08:24 AM Wow! Wild! Dude try another DVD player....
The firmware on this DVD player has been changed to remove Macrovision. With an off the shelf SONY DVD player I was getting that copy protection pulsing on about half my DVDs. I don't think there are any other DVD players without Macrovision and I preferred not to buy another one anyway. Maybe I'll have to...
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