Re: Trying to fix an Apple II...

[Rubywand <rubywand@swbell.net>]

Jeronimo Pellegrini writes ...
> 
> 
> :: On Thu, 15 Apr 1999 18:50:48 -0500, Rubywand <rubywand@swbell.net>
> said:
> 
> > Jeronimo Pellegrini writes ...
>
 ....

> I could recover 3 disks already.
>
> But even with a clean head, the reported speed doesn't change when I
> adjust the minipot. The drive's behaviour changes, but not the speed
> reporteed by software. Strange.
  ....

     Yes; it sounds like your drive is a bit off-speed (since more diskettes
do not work).  It could be that your drive speed checking software is messing
up. If you have 60-cycle AC power, you could shine a fluorescent light on the
strobe pattern on the drive's pulley or flywheel (assuming it has one) and use
this to detect speed changes and to get a good adjustment (by adjusting for a
stable pattern).

     Possibly the strobe pattern has a ring for 50Hz AC, too. 


 ....

> > 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 replaced both, but the problem persists.
> I wish I had the schemes... I could check the places where things
> could be wrong.
> 
> Not only the cursos doesn't blink (it remains invisible), but also
> flash mode doesn't work. And inverse mode works, except for lower case
> letters - then it shows other ASCII characters:
> 
 ....

     Your 555's could be okay. You probably have a bad or loose logic chip--
one which gates in the Inverse signals (from Text chars and/or from the 555
which handles blinking).  Try pulling and reinserting 74LS02's and 74LS08's
and 74LS86's-- particularly the ones close to the Char Gen ROM. (The IC's may
not be LS series; for example, they may be 7402's, etc. or 74S86's, ... .)  If
these all seem to be okay, try replacing the shift register IC (74166 or
similar) near the Char Gen ROM.



Rubywand