Hi folks, I’ve been troubleshooting another one of “uncle” Clive Sinclair’s offspring (though most folk don’t know his involvement), the Grundy Newbrain. It’s a typical early 80s micro with Z80 and 32K RAM made up of 16x 4116 DRAM.
What’s uncommon with this particular one is that it works….mostly. The annoying thing with the fault with this one is that I watched it happen - the whole screen is inverted so instead of white-on-black I now get black-on-white with garbled characters. Initially the display was fine but with the rightmost line of pixels missing from each character so I suspected the character ROM (2332 mask PROM) or 74LS166 bit shifter that supports it, but then as I watched the whole screen inverted and stayed that way, see pics. I’ve attached the schematic of the board, but for now my general question is what could cause the whole video stream to be inverted? ISTR that with the ZX80 you could invert the screen with a jumper that switched output from a 74LS165 bit shifter from the Q to /Q pins but the 74LS166 doesn’t have an inverted output, unless I’m reading the datasheet wrongly of course :) http://www.binarydinosaurs.co.uk/newbrain-05.jpg <http://www.binarydinosaurs.co.uk/newbrain-05.jpg> http://www.binarydinosaurs.co.uk/newbrain-14.jpg <http://www.binarydinosaurs.co.uk/newbrain-14.jpg> http://www.binarydinosaurs.co.uk/newbrain.pdf <http://www.binarydinosaurs.co.uk/newbrain.pdf> Cheers! — Adrian/Witchy Binary Dinosaurs - Celebrating Computing History from 1972 onwards
