I'm having a video hum problem. (Probably audio hum as well, though the video hum is unmistakable.)
I have a new Panansonic HTIB (
http://reviews.cnet.com/Panasonic_SC-HT920/4505-6740_7-30895706.html) that I just hooked up to my system, which includes TV, VCR, and DirecTV satellite receiver. The HTIB main unit (DVD player) is connected to a powered subwoofer (active subwoofer). The video hum is only present when watching DVD's, not when watching the satellite.
The satelitte box has a 3-prong power cable, while the powered subwoofer has a 2-prong power cable. (The TV and VCR also have 2-prong power cables; the DVD player is powered by the subwoofer.) What I've found is that unplugging the satellite box eliminates the video hum.
I've tried plugging the satellite box into another power recepticle, including in another room, but the hum is still present in varying degrees. (I should mention at this point that my house is 80 years old, for what it's worth.) As a test, and using an extension cord with a ground recepticle, I plugged the satellite power cord ground (third) prong ONLY into the extension cord's ground recepticle, leaving the other two prongs unplugged. The hum came back. This leads me to believe that it is indeed a ground loop issue.
After reading many articles and posting on the site about eliminating ground loops and interference, I decided to try using an isolation transformer. I purchased this one:
...since I figured the Jensen was a bit expensive and I didn't even know if that would solve my problem. It did not solve my problem. I'm not even sure if it helped at all as I didn't see any noticeable reduction in hum. I tried connecting it at the wall outlet for the satellite cable, at the satellite receiver, at the point where the satellite receiver connects with the VCR... pretty much everywhere I could. No luck. Then I read somewhere--maybe on the Jensen site--that satellite receivers may not be affected by an isolation transformer. I'm certainly not sure.
Any guidance would be much appreciated. I'm guessing that something is not grounded properly but I really have no idea how to fix it.
An 80 year old house can be expected to have electrical service that may be fine for lights and motors, but resistance in the neutral path will cause all kinds of issues, like yours. Also, if the stereo is on one circuit and the display is on another that uses the other Hot cable, while it's supposed to be sharing the neutral, it won't be the ideal situation, as far as the power supplies are concerned. Try to connect everything to the same circuit and see if some, if not all, of the noises go away.
You may have a ground loop on the audio cables, too. If you're using a digital coax, the ground is still connecting the two pieces and if your hum goes away when you unplug the coax, try using an optical cable.
If you can, run a new power feed to the equipment. The 80 year old wiring undoubtedly has more than a few connecting points and they won't necessarily make sense. Half of your house may have the neutrals combined in a box for a ceiling light, like mine did. Of course, I didn't know this when half of my recepticals went dead after I removed a ceiling fixture but I found it while searching for whatever could be disconnected.
Sat systems don't always have the same causes of hums and noise bars, but some are more than similar and coincidental. One cause is that the cable may not be very good or the terminations may be less than correctly installed. When the feed runs parallel to power wires, it's easy to pick up EMI. If they must be in the same place, cross them at a right angle and separate them by at least 12", more is better.
Cable, phone and satellite feeds are required to be grounded at the point of demarcation for the building, by the National Electrical Code. If they aren't grounded outside of the building, they are required to be grounded as close to that point inside of the building to the electrical panel or a cold water pipe that has no breaks between that point and the ground outside. The problem with this is that most of the cable and satellite installers either don't know this, or don't care. They're paid on a per-job basis and if they have to spend more time at one job, one or more jobs won't be done and they lose money. That's too bad. I want my house to be wired by the provider as well as I do for my customers and when they don't ground it, I call them back whether it's for my place or for a customer. I don't want to have a call-back, because some schmuck didn't think this was necessary.
Cable has a different cause, usually. They try to squeeze as many customers onto one amplifier and at some point, their ground's integrity is compromised. This causes voltage to be present on the shield and the resulting noise. A cheap RF isolator can work well but in some cases, the Jensen is needed. I was working on a job a couple of weeks ago and when we fired up the Cable TV, it hummed like crazy. We estimated that it had close to 60VAC on the cable, so we used a cheap isolator, since we weren't expecting any noise. Worked like a charm and the video quality was great, most likely because of the short distance. My cable had over 15VAC and I measured it after getting a good jolt when I connected it with one hand, behind the equipment. I saw a spark, too. I had to do the same for a customer last year when the new system hummed and that was all the time, not jsut when cable was on.
The way way to check for this it by making a short coax jumper that has no connection to the shield. If the noise is gone when this cable is used, isolation is needed.
DO NOT TOUCH THE SHIELD FROM THE CABLE BOX WITH ONE HAND AND GROUND YOUR OTHER HAND.
