Apple-1 Replica video/character/Sync issues forum

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mrtitanic777's picture
Last seen: 12 hours 47 min ago
Joined: Jul 23 2020 - 04:40
Posts: 71
Apple-1 Replica video/character/Sync issues forum
Hello everyone! I'm Logan I'm from the Apple-1 enthusiasts group on facebook, and I just wanted to create this forum for people who are having issues with their Apple-1 replicas such as Video sync, character display and terminal issues.

To start off this forum I thought I'd tell everyone my problems with my Apple-1 Replica, so I recently just finished my Apple-1 replica I believe in November 2020, and all I had left to add was PROMs and a 2519.  

In late December 2020, I got my PROMs and a 2519 and so upon initial boot up I wasn't getting 40x24 character display on the terminal display. I get the flashing "-@" symbols and such like normal. But only half of that screen is being displayed, the rest the screen displays flashing "@" symbols from top to bottom but not on the thin left and right side borders of the screen. It's kinda hard to explain.  

I'm gonna try to add some photos, I have some MINIMAL spare parts:

-5300uF 15V Cap

-3 1404As

-A heatsink

I also just ordered these, they should be here soon.

-1 New DS0025

-5 New 74161s not (As)

-2 New 74157s

-10 New 1404As


Hopefully this helps! Any questions, ask away?!




P.S. I slowed down this video so everyone could see it.



Last seen: 9 hours 3 min ago
Joined: Apr 1 2020 - 16:46
Posts: 800
Logan's above problem is fixed ...

... I was able to help him to debug his Apple-1 over the phone, and all is good now. Normally my service / coaching is only available to Apple-1 builders who bought one of my tested and burned-in IC sets on Ebay, but I made this one exception because Logan had bought some other parts from me --- and I wanted to collect evidence to make my point that any Apple-1 builder is well advised to use a known good IC set to avoid lots of frustration and wasted time and money. You don't need to buy one of my IC sets though - hint, hint ! - as they are a lot of work and making them steals a lot of my time - but you can also transplant a known good IC set that works to a new build using the procedure I have outlined here:

This transplant procedure using a "donor" automatically creates a second known good IC set on the donor, and any bad ICs out of the tube can be spotted quickly. Some may die during the first hours of operation, though. It's called the "bathtub curve". Or "infant mortality". With 40+ years old NOS ICs, it happens more often than with recently made ones.

Anyone not following my advice better be aware to possibly face a world of hurt. Debugging an Apple-1 can be daunting.

This is how I guided Logan though the debugging process:

First, I told him to remove the 555 timer and short pins 1 and 3 on the 555 socket. This got rid of the blinking. I wanted to know if the garbled trash on the screen came from the cursor blinking signal path or from some problem with the screen buffer. We were lucky and once the 555 timer was neutralized and there was no blinking anymore, the trash was gone and the screen output was OK except it still did roll due to the monitor not acquiring VHOLD. The bad luck was that his monitor / TV would need a remote to adjust the VHOLD, which he did not have. But even with a rolling picture, stuff can be read from the screen.

After this first step, I knew the problem is in the signal path which modifies the MSB of the 6 bit character code circling in the 2519 shift register. The MSB is pulsed  to make the cursor blink. Actually, it will make any character it thinks to be a cursor blink.

As a second step, I told Logan to put the 555 back to restore the blinking (and the original misbehaviour) and to  replace the 7402 at C10 as it is in that signal path.

No success. The symptoms still were there.

As a third step, I told Logan to replace the 7408 at C12 which contains the AND gate in this signal path and this IC turned out to be the culprit. The problem was gone.


The whole debug session took about 30 minutes on the phone. It could have taken less time without the complication to use 74LS ICs as the replacement parts because Logan had no suitable 74XX at hand. So I had to go to my own lab and fetch 74LSXX to try out first that they do work in my own Apple-1. In hindsight this was a waste of time as they did work, but as a general rule, you want to know if a not-so-perfect replacement could do its job in the given target socket or not, or if it introduces a problem of its own.


I think that Corey takes $750 per hour to fix Apple-1, so I should have billed Logan $375 for the half hour of my expert advice.

Greedy lawyers bill about $600/hr and produce no tangible goods, so this is not asking for the moon as a good Apple-1 was born that evening.

But I'm not that greedy. Logan has learned his lesson and I have another data point for my chamber of Apple-1 horrors.


And another case so I can prove that using known good IC sets is a must for any Apple-1 builder who wants instant success, and not weeks and months of agony and frustration. And a waste of money, too. Just look at the spare parts list in post #1 which Logan had bought in a futile attempt to fix the problem by himself.


One final observation / hint: Logan was confused by the weird stuff that shows up on the screen after the Apple-1 is turned on. Same thing happened with some of the other builders I am coaching. This is a misconception and may lead to unfounded suspicions that the Apple-1 is faulty. A golden rule is: whatever you do, YOU MUST CLEAR THE SCREEN after every power up of an Apple-1. Otherwise the 2504/1404 shift register sitting below the DS0025/DS0026 may contain many fake "cursors" which completely screw up any legit screen output from the Apple-1 firmware. There must only be ONE AND ONLY ONE cursor pulse in the whole 1K bit cycle.  Which you can accomplish with the CLR SCREEN signal given from the keyboard, or a gimmick switch installed in the keyboard socket.



P.S.: I'm not so sure what post #2 means. Looks bogus to me. Don't send your Apple-1 to strangers !








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