Adam Thornton wrote on Fri, 5 Jul 2019 11:38:56 -0700 > I have an Apple /// that I've had for many years; it's never worked. > > When you power it up, you get a checkerboard screen, where half the squares > are solid white, and the other half have a little mosaic pattern in them. > > Looks like this: > https://share.icloud.com/photos/0NHNkEG9ssPsi65ojivBteKaQ > > Does this failure mode ring any bells? Obviously the video signal is being > generated well enough to sync a composite output. Any idea whether I > should start by replacing the CPU or the ROMs?
You have 40 by 24 very crisp characters there, so I would guess everything about the video is ok. DRAMs are supposed to have random data in them when power is first applied but that is not normally the case. Instead half of the bits tend to always come up as 0 and the other half as 1. These are normally in alternating rows inside the DRAM (all 1s in one row and all 0s in the next row) which often leads to a checkerboard pattern when used as a frame buffer. Normally the various DRAMs in a byte don't have the exact same pattern so you might get different colors or characters than 00h and FFh. This indicates that the processor never wrote anything to the framebuffer, so a bad CPU or ROM is likely though other chips could also cause the software to not execute properly. Given the Apple /// design it is very likely that all the clocks are good. Another defect that leads to a similar visual effect is if the character ROM is bad, but then you tend to have a uniform screen and not the checkerboard. The Apple /// is infamous for having chips work their way out of their sockets due to heat, so you might start off checking for that. -- Jecel
