Altair 8800c Build Troubleshooting Request

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

Re: Altair 8800c Build Troubleshooting Request

Postby tep392 » April 19th, 2024, 9:39 am

Your getting close! Lift J pin 3 and verify that it is following pin 2. If it isn't, then it's not getting enabled by J pin 1 going low. The enable signal is from M pin 6 and goes through jumper J3. Make sure J3 is in the correct holes.
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Re: Altair 8800c Build Troubleshooting Request

Postby mafisher » April 20th, 2024, 6:27 pm

I am happy to say that I found the problem and I can now RUN, STOP, DEPOSIT and EXAMINE. Thank you to everyone who helped me get to this point. I am going to continue to post here my fumbling along towards a fully working system in case it is useful to anyone in the future.

What was the issue you ask? It was indeed the pSYNC line [79] being pulled low. After lifting any IC pin "listening" on the front panel card and CPU card I still had the same issue. I then used a meter to see if I could find something else pulling the line and narrowed it down to the CPU card where I traced the signal path and discovered that I had mistakenly put one end of a bypass cap into a via instead of the correct hole beside it. Lesson learned, use stronger reading glasses when assembling boards.

I currently have the 8080 CPU card, font panel card and an FDC+ loaded in the machine. I am able to load and read back the "Kill the Bit" program but it does not function which I suspect might have something to do with the settings on the FDC+. I will continue to play around with settings on the card to see if I can get that program running or at least find a simple program that demonstrates a working system without a serial card and I will also now build my 88-2SIOJP card.

UPDATE: I am able to successfully load and run the simple three byte program from deramp's "Altair 8800 - Video #2".
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Re: Altair 8800c Build Troubleshooting Request

Postby tep392 » April 20th, 2024, 9:21 pm

Congratulations! I'm glad you found the problem!
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Re: Altair 8800c Build Troubleshooting Request

Postby AltairClone » April 21st, 2024, 7:08 am

I bet this progress feels great!
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Re: Altair 8800c Build Troubleshooting Request

Postby Wayne Parham » April 21st, 2024, 9:40 am

Dude, that's awesome! Congrats!
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