Operating an Original SCELBI 8B

I recently spent a few hours with Mark Arnold and his original SCELBI 8B. This is the very machine that Mark used to develop SCELBAL with, way back, in 1975. Towards the end of our debug session, we had the machine in a largely working order!

Mark Arnold with his SCELBI-8B

Mark Arnold with his SCELBI-8B

We spent a lot of time, troubleshooting what appeared to be an intermittent EPROM issue. Finally we discovered that that the edge connector on the EPROM board wasn’t making reliable contact with the connector. After examining the board in detail, we noticed tarnish on the edge connector contacts. We used a soft pencil eraser to clean the tarnish off of the contacts and the intermittent operation completely disappeared. Fixing a computer with a pencil eraser was a first for me, though I expect others have done the same. Once the repair was done, Mark was able to reliably run a variety of MEA operations without any problems.

With only 4K of SRAM installed, we were unable to load the full floating point SCELBAL, but we were able to load tiny SCELBAL using a simple bit-banged RS232 port. Once loaded, Mark successfully entered and ran some small BASIC programs. Weeks later, I understand that Mark still has the system powered on and it responds to MEA commands, without any need to reset or reboot.

In order to declare that the machine is in 100% working order, a few things still need to be checked out. We had only 4K of SRAM installed, the other 8K should be added back into the system. We used a modified version of MEA with the page 76 EPROM changed. I created a version of page 76 EPROM with a 2400 baud driver, but we were having some issue with corrupted serial output characters. We don’t know if that was a ground loop problem or a baud rate problem with the modified page 76 driver. Serial input at 2400 is working perfectly, but the timing for output is done differently, so it could be either.

Finally, the cassette interface needs to be connected and tried out. I don’t really expect any issues with that, as I have used that particular cassette interface with a reproduction SCELBI 8B, back when I was first troubleshooting my reproduction cassette interface.

5 Responses to “Operating an Original SCELBI 8B”

  1. Jack Rubin says:

    Congratulations on hooking up with Mark. It would be great if you could do an extended interview with him!

  2. Mike says:

    Mark has some interesting stories to tell, and I hoping that he will be able to find to time to share them with us.

  3. Len Bayles says:

    Mike,

    That is awesome! I just went out with Jon this week. It’s really fun going down memory, and future lane. Can’t wait to get my Scelbi built and load Scelbal. (Hint: I only need a front panel, of graphics to do my own. 🙂

    Years ago Mark emailed me after I had posted a copy of Scelbal on my site. He was very nice and only asked me to post proper citations along with the file. That was very generous of him to allow the 8008 community to enjoy a piece of important history. Along with your work many more people are experiencing time travel back to the 70’s.

    Keep hacking away! You are awesome!

    len

  4. Dave Williams says:

    Fantastic! Great to see a SCELBI still with the original owner and with such history. Am I right in thinking this would be the only original 8B capable of running MEA / SCELBAL?

    Dave

  5. Mike says:

    I heard about one other original that was powered up a few years ago, what I heard is that it had some SRAM issues, but otherwise mostly worked

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