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Re: Trying to fix an Apple II...
:: On Thu, 15 Apr 1999 18:50:48 -0500, Rubywand <rubywand@swbell.net> said:
> Jeronimo Pellegrini writes ...
Hi.
Sorry for the delay. Our newsserver has a problematic disk, so
sometimes we stay sometimes without news.
> Could be. You could remove the pot and check resistances to
> determine the value needed for a replacement. Even if one of the
> terminals is blown, you can use the remaining two to get the max
> value. The pot swapped into an Apple 'Disk ][' drive (to have an
> external adjustement) is 5k; but, there is no guarantee that a
> different drive would use the same value.
It goes from 0 to 50k in the board (I know this may change if I remove
it, but at least I know that adjusting it should make some difference,
since the resistence changes....) I could try removing it, but I don't
trust my ability to solder it back :-( - It's a very small thing,
and there's almost no space between it and the board.
A friend told me there's a layer of something over the disk, and if it
is "drilled", some material from inside could leak over the disk
surface. And indeed, that's why some particular disks are not working
(could be by some other reason, but I'm sure these are "dirty" disks:
they have som eoil-like thing over their surfaces). I pu one of these
disks inside the drive, and as usual, the drive stop working. Then, I
took a look at the head... It was visibly dirty. I celaned it, and the
drive works again! Fine.
But still I can't see why adjusting the pot makes no difference in the
drive speed reported by software... Anyway, I could make almost all
disks work now.
> My experience with A2 clones is just about zero. Perhaps
> someone here knows more about a clone which displays "COMPUTER" upon
> start up.
We were not allowed to import computers for a long time in
Brazil. This has changed. So all we had were clones in that time (I
also have a ZX81 clone). This one is probably asiatic.
> If it lets you enter upper and lower case, it is likely to be
> at least an early IIe clone. If it lets you run "128k" software and
> double-hires games (like King's Quest) in 16 colors, it is an
> enhanced IIe clone.
I believe it's an early IIe clone, although I can run 128k software (I
can backup one whole disk without having to swap disks several
times! And I can use a RAMdisk when I run CP/M) - I have a 128k
expansion card... And there's no cable coming from the card and going
into the main board (as I've seen in 16k cards).
But I think I'm not familiar with this "double hi-res" term - and that
means I've probably never used this mode.
There are 3 potentiometers in the mother board, all related to the
video: one for brightness and two for color adjustment. And there's
a place for another potentiometer, which was never put there.
> Good idea to experiment! I rechecked the old II+ schematic and,
> as you suggested, it does use a 555 to cause blinking (flashing)
> characters to blink. The condition is that character bit 7 be 0 and
> bit 6 be 1-- a blinking space cursor would just need to be a
> $60. Sorry about the mistake. So, you may be looking for a 555, 556
> (dual 555), or 558 (quad 555) which needs to be replaced or, maybe,
> just pulled and re-socketed to make a good contact.
I see... And when I go back in the text I've written (to put the
cursos over it), sometimes it shows another character (this happens
with symbols and lower case letters). I think I'll buy two 555s and
replace them. I have to go buying some ICs anyway (one of my Z80s
stopped working, so I have only one Z80 for both the ZX81 clone and
the Apple Z80 softcard... It's a pain in the neck to have to pull it
from one board and socket it in the other so often)
Thanks!
J.
--
Jeronimo Pellegrini
Institute of Computing - Unicamp - Brazil
mailto:jeronimo@dcc.unicamp.br mailto:pellegrini@iname.com
http://www.dcc.unicamp.br/~jeronimo