Reply 20 of 54, by mkarcher
Bernkastel7734 wrote on 2022-10-25, 19:49:I do not have much experience with that, but maybe it's RAM related issue. In Olivetti M20 hardware manual I have found such Video logic diagram.
The Video BW pin should always be driven either high or low by the "video control" circuit (assuming it's a TTL level signal, as in MDA). Even if the video RAM failed, you should be seeing clear edges and defined levels. Looking at the DC offset in the MDA image: Do you connect scope ground anywhere? The MDA signal shouldn't be going below zero if DC coupled (the screenshot says it is DC coupled), and not connecting ground on a handheld battery-operated scope can result in "false AC coupling", as well as degrade the signal overall.
I recommend to check whether the BW video pin is connected to anything by trying to inject negative voltage into it and check whether it's clamped. With most IC technologies, injecting negative voltage at an output pin of an unpowered chip is clamped to -500mV. Use a meter in diode check mode, red wire to GND, black wire to BW Video. If the meter indicates "open circuit", the video output trace is likely interrupted somewhere.