I hope this exact problem was not discussed before. I tried the search function, but I am not able to limit the results to the Apple II forum.
I have a relatively good working Apple II: boots, loads software and games. However the video signal is bad, there is a lot of noise in the signal.
I am able to get picture on my monitor, but it is noisy (obviously) and has a bad contrast ratio between black an white.
I tried to diagnose the problem and attached an oscilloscope to the internal aux video connector:
On the left the sync level is visible, followed by a lot of black and the APPLE II text.
As you can see, the noise level is significant high. And there is a short glitch down to almost sync level between real sync pluse and the text.
I recorded the video-signal and the sync-signal as well (the color burst signal is inactive and almost noise free, therefore I skipped it).
I think most of the noise is introduced by this signal. The high level seems a bit low (3.6v).
The sync signal shows less noise and has a better high level of 4.1v. However the sync glitch is clearly visible.
Does somebody have an idea what might cause the noisy video signal. I traced the sync glitch a few years ago and I temporarily fixed it by adding a delay. However, this did not improve the picture quality.
During my invesitgation I probed the ground level at different places of the board (top-bottom, near to far away):
The more distance between the probe ground and the probe, the more noise is visible.
I thought that it might be a problem of measuring high frequency signals over long distances.
However, the noise pattern resembles the pattern in the other signals and the video output.
Does somebody have an idea how I should proceed from here? Has somebody reference mesurements to compare to?