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?