Re: Trying to fix an Apple II+ : blue & orange colors "sparkle" & cap with only one connection

[Linards Ticmanis <ticmanis@gmx.de>]

On 01/20/2013 08:23 PM, eccofonic wrote:

> There is a 10uf cap labeled "C17" in the "Apple II Reference manual
> addendum" schematic, but it is connected to pin 3.

Sorry, I can't help with that one.

> When the computer is turned on from a cold state... if it is booted
> immediately to a graphic containing blue & orange colors... these
> colors are purple and green instead.
> 
> Gradually, these areas start to "sparkle"... imagine hundreds of  dots
> randomly flickering brighter...
> 
> And the blue and the orange colors "fade" in.
> 
> But the "sparkles" remain, mostly in the orange areas.
> 
> I read that Woz devised a circuit to add more color to the original
> Apple II, so maybe this the culprit?

If you have a board without RAM size blocks, this circuit will be a
fully integrated part of the regular mainboard electronics.

Can you read circuit diagrams and twist your brain into a few knots? If
so, try to find a copy of "Understanding the Apple II" by Jim Sather.
It's long out of print, but there are scanned PDFs of it floating around
on the net (just google for it). This superb book describes the whole
Apple II/II+ circuitry in excruciating detail, but makes it as easy to
understand as possible. You will find the blue/orange circuit as part of
the video circuitry near the end of the book. I would think that with
that knowledge and your description of the fault, you should be able to
narrow down the culprit to one or a few of the 74LSxx logic chips, most
of which are luckily still fairly easy to get. My rough guess would be
that one of the chips at board positions A8 to A11 is faulty, but with a
bit more thinking (which I'm currently too lazy to do) you should be
able to narrow it down to one or two of them.

Regards,
-- 
Linards Ticmanis