I have an apple 1 that I am trying to get operational. I'm hoping that someone on the list can give me a few pointers and help.
I have a user manual so I have all the technical information. So far I have the following troubleshooting information.
On power up and reset I have a blank screen. Sometimes on power up I get random characters on the screen which can be removed with the Clear Screen button.
1. The power supply voltages are correct and well filtered. +/-5V and +/-12V.
2. I know that all the chips are good as they work in my replica board.
3. I have pulled all the chips connected to the data and address lines and checked for continuity and line to line shorts. All measured OK.
4. All the data and address lines are toggling with the correct voltage logic levels. Not sure of the logic signal quality however seems similar to the replica.
5. Pulling the 2519 located at C3 gives a screen full of characters.
6. Lifting pin 12 on the 74157 at C4 fills the screen with left parenthesis.
7. The keyboard provides proper logic levels and works with my replica board.
8. The output of the 6820 does not change when pressing keys however does on the replica board. Pin 40 on the 6820 receives the key strobe.
I am suspecting that the processor for whatever reason is not properly setting up the 6820 on reset.
I have a scope and a DVM for trouble shooting. Wish I had a logic analyzer.
What else can I do to figure this out?
Have any of you used the single step circuit shown in the operation manual? Does it work?