I built an Apple-1 replica. The machine is working, but there is a problem. When I power on, the screen is ok, the cursor characters are blinking, and the _ characters are showing. But, when I push the clr button, the screen will be blank. The cursor isn't show. If I power off, and then quickli power on, the cursor is showing, and blinking. There are some other character, but if I push the reset button in the keyboard, I get the prompt, and the machine is working. I can run the Basic, and everything working. But if I push the clr, the screen will be blank, and the cursor is missing. What can i do? I tried replace the 7450, 74174, 75157, but nothing change. Perhaps is 6820 bad?
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Could be a bad 6820, I’d also check that your reset switch and wiring are solid. You could also have a bad/intermittent solder joint. I’d also try touching up the solder on the board after checking the reset wiring. Also reseat the chips on the board.
When that is done, if it still doesn’t work, let us know and we can start deep troubleshooting.
I checked the reset line, and everything ok. I think, the computer section is ok. I replaced all logical ic -s in the terminal section, i tried 74LS and simple 74 series too except the 74LS10, because i haven't. I swap the two 74LS10 IC-s. I replace the 2504 shift registers too. But did not solve it. I checked the cls circuit, including the 7450,74174, and the wire too.I measured the cursir signal, in the C11, C12 and C13 IC-s with oscilloscope. The cursor signal is missing, when push the clr. If I pull it down to GND the 74174, ICC7 pin 1, the screen will be full blinking cursor character, and the machine is working, just the screen isn't blank. I will make a video.
I think I'm following what you did in the video.
What happens if you clear the screen first then press and hold reset for about 1 second and then release?
The same thing happens. I've already tried every combination. It's like if the terminal session is blocked.
What happens if you simplify things? Pull the ACI, pull RAM block W and see if you get a clear reset. Do you have a spare 6820 or 6821? Does everything seem to work consistently when you only have the terminal section populated. I actually think it is a CPU section issue at this point.
Ok. I tried. I pulled the ACI and the W RAM IC-s. The situation same as before. Although sometimes begin write _ characters continuously. But the CLR issue same. Unfortunately I have no replacement 6820, but I ordered one.
Do you ever get the underscore issue if you only populate the terminal section?
The underline signal problem was because I did not connect the keypad and it was probably STROBE signal interference.
I removed all the ICs from the computer section. No changes. I think the computer section is fine. I examined the PCB by microscope. The soldering is fine. After CLR, the CURSOR pulse does not appear on the WC1 line or on the WC2. If I drag WC1 or WC2 to GND for a moment, the screen will be full of @. Receives characters from the computer.
Your right something does seem weird. When you cycle the power you should still get some characters popping up. I don’t think I have ever seen a completly clear screen.
If I'm fast enough when switch off then on, the screen will be blank just the cursor blinking. I ordered some logical ICs from another manufacturers, and I will check the all wire between logial IC-s in the CLR and cursor circuit.
Hurray! I found the answer to my problem. I replaced the 6820 and two 78LS10, but the problem left. Then I replaced again the 78161 ICs, about for the fourth product. I thought now I would use russian 74161, This type KR1533IE10. I saw in a post that someone made Russian parts to Apple-1, this is where the idea came from.And it working perfect. :) Probably that the wrong "Last" and "LastH" signals were causing the error.