The two diodes, D1 and D2 and the PCB "bar" track were apparently provided to connect to a shield to protect the circuitry on the board, or maybe to protect the user...MaFli wrote: ↑Tue May 16, 2017 6:52 amI've also traced down the components that are not populated (R36, R37, C3, TR1, D1, D2). Looks like they tried to fix the back porch of the video signal. Does anyone have any idea? Even more weird is the D1, D2 divider. Were these intended to be zeners or other clamping devices in case something went wrong with the power rails? And why is the center point between the two connected to a dummy piece of copper in the middle of the top layer? That is the long rectangular bar in the middle. Clue anyone....????
There are/were a number of television video systems around the world, some use positive sync signals and some use negative sync signals. Same as in some television video systems, the video signal is a positive going voltage for peak white, while in others, positive is total black. So R36, R37, C3 and TR1 may have been provided to allow for one of these other television video systems.
I don't think it's anything to do with providing a back porch, as at the time ZX80's were expected to be used with a black and white / monochrome TV. So the missing back porch did not matter. Further, I don't think that circuitry would have the intended effect.
Mark