Idiosyncratic Apple 1 video thoughts

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Idiosyncratic Apple 1 video thoughts

It's been a while since I posted an update about my experiments in trying to make the Apple 1 video work on modern monitors and I wanted to post an update. I had been wondering the reason the video was shifted to the right and after answering Uncle Bernie’s question (https://www.applefritter.com/content/hsync-apple-i-intentionally-10-us-wide) about the horizontal blanking period, noticed it was way off. The sync tip should be 4.7us and it was 9.7us and the "front and back porch" was off by 5us as well. After a little digging to understand the video generation on the A1, I found the horizontal sync signal on the A1 schematic at the 7400 at C15 and fortunately that chip also had two unused gates. Using one gate to invert the signal I fed it into a 556 dual timer to move the sync tip and then shorten it. Then after re-inverting the signal I returned it to the A1, without any modifications to the A1 board. So, I breadboarded it and got the timing right (10 turn pots really help) and it worked and was stable on all of the composite monitors I've tried it on so far. Then I built up a smaller perf board plug in version. When I get a chance (read as “stop being lazy”), I’ll lay out a miniature board with surface mount parts that shouldn’t be much bigger than the original chip it replaces.

 

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Joined: Apr 1 2020 - 16:46
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There was a one transistor fix to this problem 2 years ago . . .

My own solution to this Apple-1 quirk was published 2 years ago here:

 

https://www.applefritter.com/content/how-fix-nonstandard-apple-1-video

 

. . . but alternative solutions are always welcome.

 

I'm still hunting down and killing quirks in the Apple-1.

 

The  most recent "discovery" was that that the Apple-1 crystal oscillator is rotten, see here:

 

https://www.applefritter.com/content/wanted-apple-1-owners-frequency-counter

 

And here is the remedy I've worked out:

 

https://www.applefritter.com/content/how-get-apple-1-crystal-oscillator-target-frequency

 

I'm deeply indebted to Woz that he planted enough  bugs/quirks in his - otherwise great, for the time - Apple-1 to give me a fertile field to work on. Otherwise my life after retirement would be just too boring . . . .

 

- Uncle Bernie

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