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lost all picture just says v loss can you help me

"V-loss" (which stands for "Video Loss" ) is typically something that your DVR will say when it is working normally but isn't detecting video from the cameras. It simply means that the DVR is not receiving a signal and is a normal message to see, so chances are the DVR itself is fine.

If all of your cameras have gone out at once, it's probably your best bet to check the power supply (since having multiple cameras fail simultaneously is very rare), however it might also be prudent to try running your cameras directly into a monitor or TV to see if you can get a signal off of them when bypassing the DVR. If you're experiencing an issue like this, though, nine out of ten times either the power supply or cameras are to blame.

I have got a camera system with 12 cameras that are connected to a dvr. 6 of them are not working. Is there a length limitation on the rg-59 coax? Also there are 2 cameras that if I disconnect either of the 2 the other camera shuts down.

If you're using RG59 coax, you typically want to keep your runs at 700 feet or less unless you've got amplifiers or repeaters on the run with your cameras. If you're using siamese cable (which has the power along with the coax), you also have to keep track of how long your power cable is rated for - in other words, if your siamese has RG59 coax that's rated for 700 feet but 22-gauge power cable that's only rated for 400 feet, your siamese cable will only be good for that first 400 feet.

Given what you're experiencing it sounds like there might be something funny going on in your power supply - typically if you have a total of 8 cameras malfunctioning there will be some common reason for the problems (usually the power supply, sometimes the cable), and since you've got cameras that apparently linked in their power, it sounds as if there might be a wiring error somewhere. I'd suggest double-checking your cabling and the internal wiring on your power supply to make sure everything is as it should be.