I was playing with my Apple 1 yesterday when the reset began to fail. It has worked intermittently but now does not work at all. It displays the / but never does the carriage return. I installed my address and data bus display onto the expansion bus, ran in single step and found that the monitor gets as far as FFF7 and then instead of performing the RTS back to the keyboard monitor loop at FF1F, it continues to FFF8 and eventually loops around at FFFF. I have so far replaced the 6502, the 6820, the 8T97's and the ROMs to no avail. Do any of you have suggestions for fixing this? Kinda weird that so much code is correctly run before it fails.
Have you checked the RAM?
Yes, I have now installed known good ram. The results are the same.
I also have a working replica to help with trouble shooting. Don't know where to go from here.
Can you remove the 74123, and measure the capacitance at the 47pf cap?
Also does the keyboard work ok with the replica? Sometimes it's the obvious thing and the keyboard is the issue if you have separate key switches, the key switches can develop some corrosion and then when you use them it gets knocked off, but if you don't flush out the switch they can intermittently cause a problem. mind you this on on switches that event been used for a while and/or are kept in a humid environment. If it's a datanetics switch, I usually remove the screw at the bottom (not sure if it really helps) and using an eye dropper flood it with Isopropyl Alcohol. Press the key a lot to loosen anything and flood again, then let dry and reinstall the screw.
If it's not a keyboard issue, and you know your ram is good, let's start with the cap near the 74123.
I fixed it. Thanks to Corey and his recommendation on checking the keyboard. I found that I must have bumped the keyboard connector while installing, bending a pin to short to an adjacent pin. Not sure what signal got messed up, but straightening the pin fixed the problem. BTW I have a George Risk Industries model 756 keyboard.
I did check the capacitor as Corey recommended but found it to be 39pf and the resistor measured 29.5k.