5 Jul
2019
5 Jul
'19
5:21 p.m.
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