Z80 cursor but no text

[albertusj]

Yes, monochrome.

[1024MAK]

You want something like this...

Mark

[RetroTechie]

To protect my ZX80 I'm thinking of introducing an optocoupler into the video signal path.

Most important for protection against ESD damage or such, is to have series resistance in the signal path. Something that limits a current spike to values that eg. a protection diode in a 74xx IC's output won't mind. If a € 0.10 transistor behind that 'eats it', who cares. Some thoughts:

• Forget optocoupler. It might be very difficult to find one fast enough to pass a 6.5 MHz signal without too much distortion.
• ZX80 schematic show some series resistors in the signal path. So you could limit yourself to adding a buffer transistor (emitter-follower).
• To have buffer transistor output reasonable voltage levels, you may have to fiddle with those series resistors' values. And maybe add a pull-up (hey! marked R33 on schematic. There's already a position for that on the board?). If you need help with that I could do some calculations. Been there, done that on multiple occasions.
• Filtering the 5V supply shouldn't be needed if 5V comes from a linear regulator like 7805, and IC's have proper decoupling. If 5V comes from a switching 7805 replacement, then filtering may help. But even then not needed.
• Series capacitor: 100..220 uF. Even 47 uF works, much bigger than 220 uF is overkill here.
• Putting 100 nF ceramic in parallel with that is pointless imho. You don't even WANT signal edges to be as-steep-as-possible. Just short enough such that pixel time >> transition time. So rise/fall times in the order of say, 20 ns (or even a bit more) is not just acceptable, but preferred. If it was beneficial, you'd find such 100 nF's in video output circuitry everywhere. Not the case. So use a low-ESR cap if you want but leave it at that.

[1024MAK]

Another member has reported a similar problem here.

Mark