On 2017-Sep-25, at 12:42 AM, Adrian Graham via cctalk wrote: > 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>
Howzabout: On schematic pdf.pg4, what looks to be the video pixel stream SOVSR (from IC436 on pdf.pg3) feeds an OR gate of IC433, thence inverter in IC450, XOR in IC434, combined with some other stuff in OR gate IC464, resistive mixing with the composite sync CMSNC, to produce the composite video CMPVD. The XOR of IC434 is the point of interest, the state of the other input of that XOR gate will invert-or-not the pixel stream. It is labeled as being fed from RVF (ReVerse Field or Frame?). I haven't looked at the schematic long enough to find the source of RVF. Hardware configuration? Bit of an IO port from software configuration? But I don't know, going from your pic isn't their a lot more screwed up than just reversed video?
