How Does The Startup Screen Get Populated?

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How Does The Startup Screen Get Populated?

I'm wondering whether anyone can tell me how the Apple 1 startup screen gets populated with alternating @ and _ characters? I just finished a second board and instead of alternating characters I'm getting an entire screen of flashing @ symbols, and the board doesn't run reliably. This board is populated with a lot of period correct Fairchild and Signetics ICs from the mid-late 70s, so suspecting that one of these was the culprit, I swapped all the ICs from my other 100% working Apple 1 board, but the problem remains. I'm now suspecting one of the passive components - perhaps a vintage Philips 22uF cap, but knowing how the startup screen gets populated with alternating characters would help narrow it down.

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Bad 22uf Electrolytics

Turns out the problem was indeed bad vintage Philips 22uF electrolytic caps. I guess this messed up the timing of the terminal section. I did notice that the flashing was quite a bit faster than normal, and one of these caps is involved with setting the timing of the 555, so that makes sense.

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Hi,You searched well. The

Hi,

You searched well. The "startup board" is generated using the 555 timer, corresponding to, inter alia, for the blinking of the cursor. The signal along with the pulses from the multiplexer output 74157 (C14) goes to the input 6 shift register 2519. Before the screen is cleared, inputs I1-I5 of chip 2519 are low, because  - input 15 (E - CLR) of 74157 chip (C4) activates them when it is in the low state. In the high state, input 15 causes the outputs 4,7,9,12 (Za-Zd) to be low.

Input I6 changes its state to the clock cycle of the chip 555. Now look at the contents of the chip 2513 (character generator). The address 000000 corresponds to the "@" character and 111110 to the "_" character.

 

Hope I was right. If I was wrong, correct it.

 

Regards,

Mateusz / SQ9PXB

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Thanks for the explanation,

Thanks for the explanation, that's super helpful! I couldn't find this information anywhere and I really enjoy learning more about how the circuit works.

 

I'm surprised that the faster timing of the 555 caused only @ symbols though, and that they all flashed. But I estimate that it was running around double the normal rate, so could it just miss the _ characters in this case?

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