G'day all.

I notice that a lot of older video equipment particularly cameras from the 70s seem to output video with a certain level of DC voltage. As a result I have zapped two of my AV inputs of my Panasonic DVD recorder some months ago. Now I have been trawling the web for a simple passive video DC blocker circuit schematic but haven't found any, though have found a couple of video DC blockers for sale:
http://www.extronelectronics.com/product/product.aspx?id=vidcbl&subtype=61&s=4
http://www.cs1.net/products/audio_authority/1182.htm

As I couldn't be bothered trying to order these internationally I have decided to try and make my own passive DC blocker circuit even though I don't know exactly how to make one. I put a 0.1uf capacitor in series and 75 ohm resistor in parallel and tested it out. It successfully removed the DC offset from the video signal, however the picture is not completely stable and it warps and bends as the picture gets brighter.

Anyways I was wondering if anyone here can provide me with a schematic of a passive video DC blocker that can produce a nice stable picture? Any help will be much appreciated.

You just need a much bigger capacitor. It should pass at least 30 Hz into 75 ohms, so
1/(2XpiX30xC) = 75 gives C= 70 microfarads. Use a 50 or 100 uFarad lytic, and put 100k resistor to ground to make sure it stays biased in the right direction. Reverse the lytic if the output of your device has negative DC (I'd guess not likely).

Edit: you didn't say if you removed the 75 ohm before attaching to your destination device. The destination should have 75 ohm input, so only use a 75 ohm resistor (across the 100 k ohm shown) for testing with a scope while not connected to a destination device.

Thanks for the circuit. I gave it a go but got the same results, a distorted picture.

But it's all good now as I've just found another circuit that will limit DC down to 0.2 to 0.6 volts which I guess is pretty acceptable for DVD recorders, I put it together and it works very well. The circuit can be found here http://www.telestream.net/pdfs/app-notes/app_CM_DC_Offset.pdf . I had to add a 75 ohm resistor in parallel from video signal to ground to make the circuit work effectively to limit DC.

I am using this for the viewfinder output from my JVC GC-3300E colour camera http://www.troysvintagevideo.741.com/gc3300.html as the viewfinder is not working and I want to use a little Casio handheld TV in place as the viewfinder. That circuit has effectively limited the DC from over 3 volts to about 0.6 volts which I guess should be okay for the AV in of a small handheld TV.

hi guys,,,,
That gives about 11v DC out of 9v AC which then is input on the 7805. ... (way above the ones existing on the video signal) and after your comment, ... Instead of your circuit, I would remove the resistor and inductor, ...:banana: