Osbourne OCC1 problem

Ade Vickers wrote: Something in the video generator circuit on the digital side by the looks of things. Probably pre-character-generator -- those characters look fine and they're in the right place, but they're the wrong characters. So that's anything pre-character-generator basically -- buffers, latches, RAM, not necessarily in that order. Point me to a PDF of the schematic and I'll see if my overworked brain can come up with any other ideas :) Have a read through the Tektronix "ABCs of Oscilloscopes" guide - that's on <http://www.tek.com/> somewhere (their search engine usually works fairly well, or try Google). That should be enough of a reminder of the terminology, and how to make the scope work. Now the things I'd try in order: Set the scope to 0.5V/div, AC coupled and connect it between +5V and ground. See if there's any significant noise on there. If there's around 0.1V or less of noise / pulsing on the power line, you should be fine. Any more than that and you want to check the power filtering. Check using various ground and +5V points -- this might expose a bad track or via. Does the keyboard work, or is the machine totally crashed after booting (assuming you leave the terminator where it is)? Does a CAT (or DIR, or whatever the relevant CP/M command is) show something similar to a directory list? Check the address lines for the display RAM. If it's DRAM you're pretty much screwed, but with SRAM you can see if the count is incrementing normally or if there's an output line stuck low. If the RAMs are socketed (and the same type), try swapping them around. If you know the main RAM is good, swap each VRAM chip with a main-RAM chip until either the main RAM test fails or the video stops glitching. This, of course, only works if the main and video RAMs are the same type. A screen full of one character may well be a power-on test or something similar... If it's happening after power up then something is very badly wrong. -- Phil. classiccmp at philpem.me.uk http://www.philpem.me.uk/