Solved! re: hum on the output of the vcr/dvd sys

[reble]

The vcr/dvd recorder combo is a Funai model ZV427FX4. There is hum (not very loud) and the volume is lower in the tv as appose to when the tv is connected to the cable tv DVR via the HDMI cable. And there is no hum via the HDMI cable. I have a portable dvd player with it's own screen. When I play DVD's that I recorded off the DVR, there is no hum in the recording. But when I play the same DVD in the VCR/DVD sys the hum is there. The hum is there when I play commercially made dvd's or vhs tapes. I have checked all of my cables with a VOM. All the cables are fine. I have a drawing of my sys layout. Here is the Dropbox link https/www.dropbox.com/s/xurhv2i3mapyani/tv sys layout.jpg?dl=0
PS: I have the 3 lead video/audio switch on the VCR/DVD output for testing other dvd players and other things. I am currently working on a Commodore Amiga A500 sys. The A500 only has video and stereo out. No RF out period.

 

[americanaudiophile]

The portable DVD player may not go low enough in frequency to reproduce the hum which would likely be at 50 or 60 hz depending on the country.
If the VCR/DVD has a three prong AC cord you can use 3-2 AC adapter to lift its' ground.
You could try to isolate the AV output of the VCR/DVD player with an audio transformer. You might need a composite video isolation transformer too.
Eliminate the AV switcher as well just to rule that out.
If you can connect another DVD player to the TV that will help determine if the VCR/DVD has a problem or if the system has a ground loop issue

 

[reble]

When I play the same DVD in my laptop, there is no hum there.

 

[americanaudiophile]

The hum isn't recorded onto the DVD. Its caused by either a defect in the DVD/ VCR or a ground loop.
http://web.mit.edu/~jhawk/tmp/p/EST016_Ground_Loops_handout.pdf