Comparison of Original Kenbak-1 Computers

Much can be learned by examining the internals of the original Kenbak-1 computers. Several long held beliefs become unlikely. Initially, it was accepted that John Blankenbaker made one prototype computer, then 50 production computers. He then shipped unsold and unfinished computers to CTI Educational Products in South Carolina when Kenbak Corporation went out of business. One contemporary (Robert Nielsen) felt certain that CTI never made any computers from "scratch" but just re-branded the machines Blankenbaker didn't sell, and completed some incomplete computers. Much of this appears incorrect. What's more, we get clues on the order the computers were made, better than serial number evidence, which seems inaccurate.

Comparison of Printed Circuit Boards of Extant Original Kenbak-1 Computers.

Above is a changing gallery of different original Kenbak-1 PC boards, from the first prototype, to some of the later made CTI machines. Click on the arrows to stop transition.

Evolution of Printed Circuit Board Revisions:

The prototype had a very different board design, and even the power supply board was different. It not only required a lot of rewiring (jumpers and trace cuts), but several components (a transistor, a capacitor, and a couple diodes) had to be added on top of the PC board to get it operating. The prototype's power supply was also different, it outputted three voltages, +5 volts, -12 volts, and -5 volts. Later revisions did not output the -5 volts, and put the -5 volt Zener diode and stepdown resistor onto the logic board.

But after this first prototype was working, before moving directly to the production boards, we see that Blankenbaker made a revised PC board which was labeled "Rev A" which had most of the errors and problems corrected. It was greatly improved, but still not quite perfect - it required a couple jumpers and modifications to work. Only two of these "Rev A" computers have so far been found, and neither one of them was sold by John Blankenbaker, who probably only wanted to sell the "perfect" final "Rev B" boards. One of these "Rev A" computers was apparently shipped to CTI Educational Products in South Carolina, as part of the large hoard of old parts and partially completed or unsold computers, and the other computer was the one Blankenbaker kept in his attic until 1986, when he needed to to send a working machine to Boston's The Computer Museum for the early PC contest. So it is likely Blankenbaker didn't think the "almost perfect" "Rev A" board was good enough to sell to customers, even though it functioned well. It seems to be a "second-try" or "Beta Prototype."

Evolution of Component Styles and Dates:

While interesting changes of components are visible in different computers, the date codes on the TTL chips give objective evidence of age. The latest date code of any IC gives a earliest date the board could have been made (unless a failed IC was replaced, which happened in some of the Nielson computers, which were used for many years in a technical/electronics school.) Examining these date codes, along with PC board revisions, we deduce a few facts. First, it seems that John Blankenbaker didn't just buy all the IC parts as a lot of 50 all at once. There does seem to be earlier IC's clustered in the earlier computers, and older IC's in presumably later made computers. Second it's clear that the CTI computers tend to have IC's made as late as August 1973, after Blankenbaker sold out to CTI. This suggests CTI purchased their own IC's, and assembled some computers themselves, after they took over the product. And while they wanted to rebrand the computers from the "Kenbak-1" name to the "CTI-5050" name. It's unlikely that they made any of their own PC boards, however, since it would have been very easy to scratch off the "Kenbak" name on the mylar sheets, and write "CTI" but all the PC boards, but they didn't do that. I suspect they didn't produce any additional PC boards, and just finished up the PC boards Blankenbaker supplied to them.

The CTI machines also used different electrolytic capacitors, black ones, rather than the orange used by Blankenbaker.

Table of Differences and Hypothesized Order of Production:

Taking into account the known information about PCB revision and IC date codes, and other differences, we can piece together a suspected order of production. Unfortunately, most of the extant computers lack adequate internal photos to complete this table. It seems likely that the serial number sequence does represent order of manufacture for almost all computers, except serial number 216, because this computer was definitely one of the earliest prototypes, the PC board was not stamped with the same serial number, and it's most likely John Blankenbaker just attacked the last serial number (50th in his sequence) to the case just prior to sending it to Boston's The Computer Museum in 1986.

Below are the Original Kenbak-1 computers which have No Clear Basis for ordering them. Mostly, these computers are lacking enough information to place them in the above list. While it may seem obvious that any CTI labeled machine must be later, as they were made later in 1973, that's not always the case. We known that Nielsen8 was clearly labeled CTI on the front panel, but the circuit board clearly is an earlier Kenbak-1 production computer, with the orange electrolytic capacitors used on the main board, and the PC board was stamped with the serial number 203.

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