Hi all!
Last week I asked some questions about a broken MSX I had bought second
hand. I had troubles with the screen, with basic and with the diskdrive.
Thanks to some people the problem for the screen and basic has been found.
Yesterday I did some testings with the diskdrive and I think to have found
out where to search for the problem.
Because I have not answered to everybody who send me a message and because
some other people could be interested I send this text to the
mailinglist...
Screen messed up and basic not working
The first reactions I received told me one of the vram chips had to be
damaged. But I didn't really beleive it because I couldn't understand why
basic and the diskdrive where not working either. But when I heard the vram
chips were 4 bits wide and a byte is stored in two chips things became
clear, especially when I heard that basic reads lines from the video
memory.
Each byte stored in vram lost its highest 4 bits. So the patterns of the
characters in vram had a blank space of 4 pixels wide. Resulting in only
two pixel wide characters in screen 0. Unfortunately I didn't realize this
at first, because the monitor I used was not that sharp and with white
characters on a black background the two pixel characters were looking
wider than the 4 pixel blank space! But now I know it I can see it very
clearly.
This problem also means, that only the characters 0-15 are placed on the
screen. If characters with higher numbers where placed, the highest 4 bits
were zeroed again. So there was no difference between normal letters and
capitals for example. Even the characters 'A' and 'Q' etc were the same.
Furthermore, when basic reads these characters it won't interprete them as
(unknown) basic and there won't be an error message. Only the prompt 'Ok'
is given! So the broken vram chip also affects basic from running
correctly.
I don't think this is going to be a very hard operation of repairing it. I
have opened the 8245 again and I have been searching for the vram chips. My
guess is that the IC's u16-u19 are used as video memory. So my last
questions on this problem are:
- Are the IC's u16-u19 used as video memory?
- If so, which one is causing the problems? Or in other words, which one is
used to store the highest 4 bits in screen 0?
- What are the costs for replacing a vram chip? First for doing it myself
(material costs) and second for bringing it to somebody with some
experience in it. If these costs don't vary too much it seems logical to
let somebody else do the job...
I would like to thank everybody who has helped me with this problem. I've
had quitte some discussions with some people. And I didn't always give the
correct information but I've already explained this was caused by the
monitor I used. I'm sorry for that, but at least I know a little bit more
about a MSX is working...
Diskdrive problem
As I said in the beginning of this message, I did some more testing with
the diskdrive. I have asked if I could connect it in my 8235 and when I
heard this was possible I tried this out. At first I thought it was
working, but when I looked better yesterday I could see it was working.
Only somehow it seemed to be making more noise. But for 100% sure I can say
the following things:
- The 8245-drive is not running on my 8235
- The 8235-drive is running on the 8245
- The 8245-drive is running when connected to an external power supply.
So this means somehow the power to the drive-motor is blocked somewhere
inside the drive. I don't know if it is due to a broken diode or transistor
or something like that, but I think that it can be repaired by replacing
the print on the downside of the drive with one of a drive with a broken
read/write head. So from two not working drives I want to make one that is
working. My questions on this problem are:
- Is it possible to make the drive working using the method described
above?
- If it is possible, who has a broken drive for me?
- If it isn't possible, who can help me with a drive that is working? On
last year's Zandvoort fair I saw two 8245-drives being sold for NLG 10 a
piece. They were said to be working, but because I didn't know if they
would work in my 8235 I didn't buy them. Later I decided to take the
risk,
but of course both drives were sold already. And now I'm feeling quitte
stupid, because I don't think I can find a working drive again for that
little money...
The end...
Again I want to thank all the people who have helped me and I hope I have
supplied enough information for answers on my last questions. If not,
please mail me!
Greetings,
Jasper.
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email: [EMAIL PROTECTED]
www : http://huizen.dds.nl/~the-laut
