My Mark-8 Minicomputer

By Bob Bell

The year was 1974. Someone showed me an article in Radio-Electronics magazine about building a computer – a Mark-8 personal minicomputer. That's nice, I remember thinking, but what would I use a computer for? I was in 9th grade and had my goals were to be an electronics technician and a ham radio operator. In fact, I had just been accepted by the county vo-tech (vocational-technical) for the 10th grade to study just that. I went to vo-tech for three years when something started to change. Yes, I still wanted to be an electronics technician and a ham radio operator, but I was discovering there was more out there than amplifiers and transmitters. When we started studying digital electronics, I knew I had found what I had been looking for. I saw immediately how digital electronics could be applied to create the building blocks of a computer. I still wasn't sure what I could use a computer for, but I soon started wanting one.

Radio-Electronics Magazine In my senior year, I participated in the co-op program, where I got to spend a few hours everyday working as an apprentice in a local electronics business. My first position was in a television repair shop, where I did well, but the owner had to lay me off after just a few months due to economic conditions. Another opportunity came my way at a company called Syntonic Technology. I started working there part time in the shipping department and part time testing PCBs for an electronic terminal called Wiltek. The work on Wiltek soon grew to where I was doing chip-level troubleshooting on all parts of the system and no longer doing any of the shipping.

I graduated from high school, turned down an almost certain appointment to the Naval Academy and instead went to Penn State for Electrical Engineering. I continued working at Syntonic and this is where I ran into a real crazy guy who went by the name Joe Pietz. It was Joe who inspired me to explore advancing my digital electronics career into computers. Joe actually bought one of the first Cromemco microcomputers in 1977 and kept it at Syntonic. Cromemco was one of the companies trying to capitalize on the enormous success of the Altair, the first microcomputer available in kit form. At night, Joe wrote Z80 assembly language that actually ran with little or no modifications the next day. He let me use the computer, and while I didn't know assembly language, I could write programs in Fortran. By now, I had a lot of ideas for what I could use a computer for and I was really aching to have one of my own. Unfortunately, I didn't have $5,000 to pony up for one like crazy Joe did.

Then along came SD Sales with their Z80 starter kit. This was a single board Z80-based microcomputer that had everything anyone would need to get their own computer running. All parts were included for $275! This was something I could afford, so I wrote the check and in a few days I had my very own computer! Parts, that is. I had a lot of experience with electronics, Heathkits, and the like, so this was no problem. I assembled the kit and had the computer running in a couple of days. This was so cool at the time, that I didn't even mind learning Z80 machine language to program it. Over the course of several years, this computer was modified several times and appears as you see it now in the photos below. You can still see the basic Z80 starter kit board buried in the enclosure.

Z80 starter Kit
Inside the Z80
S-100

It still works, and I use it occasionally to program EPROMs. Even though I had my own computer, I still wanted the capabilities of saving programs on disk and interacting with the computer through a keyboard and video monitor, like Joe Pietz's Cromemco.

In 1978 I started collecting the pieces of an S-100 computer system in the hopes of someday getting them all to work together. I found a Z80 CPU board kit, which I assembled, and a used Wangco model 82 5 1/4" floppy drive. I bought a 64KB memory card with only 32KB installed, and an Artec 12 slot motherboard and card cage. Along with a homebuilt power supply, and other pieces scrounged from who knows where anymore, I managed to get the system running sometime in 1979. Over the next 10 years the system morphed from a mess of wires and boards kludged together to the nice system you see here (it's been pulled apart to diagnose and correct an intermittent boot problem).

This computer is still running remarkably well, although I have started to experience degradation of the floppy disks. To combat that effect, I run disk tests every year or two on all the floppies, and I replace any that report errors even after reformatting. Of course, all the files are indexed and I maintain at least three copies of every file on a combination of both 5 1/4" and 8" disks. I hope to write more about this system in the future. Anyway, back to the Mark-8.

For quite a few years now, I have been trying to find an Altair 8800 or 8800b, or an Imsai 8080. I have seen these occasionally on eBay, and they usually go for more than $1,000, but I keep looking. One day I saw an ad for the PCBs and rare ICs for the Mark-8. I instantly recalled my high school days and the article from Radio-Electronics. Was it still possible to build one of these? From the looks of the eBay ad, it seemed possible. No, it wasn't an Altair, but I learned that the original Mark-8 is in the Smithsonian and there are perhaps only five or six Mark-8s now in operation in the world. The challenge was made. I decided I would set out to build one and I purchased the bare-bones kit of parts from the eBay auction.

Read Bob's Mark-8 story in its entirety.


Bob Bell and his wife, Barbara, are empty nesters in Harrisburg, Pennsylvania, where Bob grew up. He became interested in electronics at age 10 and received his BS from Penn State in Electrical Engineering. Bob has worked in computer design, systems administration and management, and currently works for a Harrisburg Academy as the director of technology. Bob also runs a small business, BCSTech. In addition to his interests in old computers, Bob also plays the pipe organ in his local church.

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