$c050: graphic mode $c051: text mode $c052: no graphic/text mix $c053: mixed text/graphic page $c054: select text/graphic page 1 $c055: select text/graphic page 2 $c056: select low-res graphic mode $c057: select high-res graphic modeI would suspect blown chip or bad chip/socket contact in the address decoder circuit. The 74LS259 at F14 board location does all the video mode decoding: That chip contains 8 addressable latches where A1 to A3 selects the latch and A0 connects to the data input. The chip is enabled by a memory acces in the $C05X address range. If your Apple still beeps on power-up then the $c05X decoder still works 'cuz it ticks the speaker. (Whew! I haven't opened my Apple ][+ books for almost a decade :). If that chip works then you'll have to trace the problem into the image generator circuit. Another option is to swap the board completely but that's a less instructive option :)
You might find Apple 2 books in public libraries or colleges. I still have my copy of "Understanding the apple 2". That's where I got the info.
I hope you have the tools of the trade: logic probe and (better yet) a scope. Don't forget antistatic protections. Good luck,
Yves Bryan Parkoff wrote:
GR & HGR Do Not Work
It does not work on Apple II+. I tried to type GR or HGR, but "@" was
filled from top upper-lef to the bottom down-right. I mean that it will
show only text, but no graphics. Do you have ideas that it may have
something wrong with the type of chip in the motherboard. Where can I find
Apple II hardware (Not Apple II Reference) book and/or Understanding Apple
II book?
I should be able to repair motherboard myself without sending it to the
repair deptartment. It is very hard to find where repair department
locatio. Thanks a lot!!!
Bryan Parkoff
BParkoff@satx.rr.com