Tested another U950, from a working scope of a friend, both U950 work on his scope, both doesm't work on mine. It seems like grid bias is not clamping the ac waveform at the correct amplitude. If I ground the inverting input of U1890 lowering its output near 0 volts everything starts to work perfectly (intensity pot can completly dim out the traces and I got no artifacts or spots on display), when the RC delay starts to increase again the output voltage of the op amp, however the same problems reappear. I have already tried to swap U1980 with a good one but nothing changes, same problem, (its behauviour was indeed correct, it follows the rc delay as it should, so that was expected).
