I’ve got the modified CRT working pretty well as an X/Y monitor, but one thing I’d really like to do is to be able to display the composite input signal unchanged and then blend in additional tones to interfere with it. As it is now, I can toggle the X and Y deflection signals between external input and the internal signal, but using external signals to try to recreate the image on the composite input is, no surprise, effectively impossible. I can get the vertical deflection pretty close with an external 60hz sawtooth (and the results can be pretty fun), but the horizontal is way too sensitive to get anywhere near working that way.
The two approaches I can think of would be either
-tap the H and V deflection signals from the main board, attenuate them down to audio levels, blend them with the external audio inputs and rout that all through the amplifier I’m drivign the CRT with
-get some potentiometers/rheostats that could safely handle the output of the external amplifier and the internal signals, and wire them up as simple passive blend controls
The second one would be a lot simpler but I’m hesitant to try it because it seems like there might be some potential to damage things (back-current from the amp to the main board or vice versa). Any advice?
Also, the way it works now I have to use the composite input to actually have the beam appear, which I expected but a side effect is that the beam dims during the vertical blanking period, so there are dim sections or breaks in the lines that get drawn. I’ve done quite a bit of Googling but I can’t find any information about adding a direct brightness control of the beam to a modded CRT, but that seems like it would be the way to get rid of the blanking period issue (and of course the logical next step would be to build a simple VCA to modulate it and have a Z input). If anyone can point me in the right direction for that, please do! Would it be as simple as identifying the cathode pins, measuring the voltage that’s being supplied to them with a white screen being displayed at full brightness, and then building/buying a DC source that would supply continuous voltage to them and swapping that in for the composite?