Max,
I did some more poking around and ran across this old thread that you started and possibly you'd forgotten about it. Andy pretty much identified the problem with this post I've quoted below. Andy confirms the sync stability remark with another post at the very end. This Marconi set you're working with definitely fits the category of having a cheap sync design where the sync is taken from after the video amp. Another way of doing this would be grid-leak DC restoration which takes place in the grid circuit of the video amp. That may give you the needed isolation. I'll see what I can dig up on that method.
http://mail.videokarma.org/showthread.php?t=250452
Quote:
Originally Posted by andy
Most sets drive the cathode with the video, and it's coupled to the video output tube via a cap. The brightness control is coupled to the cathode with a resistor and controls the DC voltage on the cathode. All you really need to do is replace this resistor with a diode (cathode toward the brightness control). Adding a .1uF cap between the brightness control and ground will improve the DC restoration a little, but might cause sync, or blanking problems on some sets (some old/cheap sets take the sync off the video output tube, and don't have any blanking circuit). You will also need to add a resistor in series with the grounded side of the brightness control to keep the maximum brightness level reasonable.
As a quick test, you can simply clip the diode in parallel with the brightness control to cathode resistor, and turn down the brightness a little. If you connect the diode backwards, it won't do anything.
|