Here’s the known evidence:

1 – John Blankenbaker described selling out to CTI and shipping all remaining parts and computers to them in South Carolina in his 2007 Felsenstein interview.  He said “I think I probably delivered a few computers to them because I had sold about 44 out of that 50 and so I delivered some to them” (emphasis added.)  This seems unusually vague for a meticulous engineer.  But it does emphasize the original run of 50 computers which he had always been planned, and says he "sold about 44” before sending remaining parts and computers to CTI in South Carolina when Kenbak Corporation closed down.

2 – CTI had at least 8 computers, and perhaps no more than 8.  We know this because Robert Nielsen, owner of a a Nielsen Institute of Technology/Nielsen Electronics Institute in South Carolina, acquired all 8 of his computers directly from CTI.  Some were acquired while CTI was still in business, some at CTI’s bankruptcy auction.  He said he purchased every Kenbak-1 related item they had, including documentation and training videos.  Four of his computers had the original Kenbak-1 faceplates (some had the Kenbak-1 name covered with a foil “CTI” label) and four had newly fabricated CTI silk screened faceplates without the card-reader slot.

3 – Robert Nielsen was adamant that CTI never made their own Kenbak-1 computer from scratch, but just completed or rebranded computers that Blankenbaker had already partially assembled.  The IC date-codes on the CTI computers go to late 1973, suggesting CTI purchased their own components and soldered them to PC boards, but the Kenbak-1 name on all printed circuit boards suggest they didn't fabricate any new PC boards, just used the ones Blankenbaker supplied to them.   If they made their own PC boards from the large plastic sheets, it would be trivial to remove the Kenbak name, and add their own.  Nielsen also believed that CTI never sold any Kenbak-1 computers to anyone except to him, and he feels he purchased every one of the Kenbak-1 computers they ever had.  Nielsen's statements would be hard to believe, but considering Robert Nielsen actually worked very closely with CTI during this time, he may be right.  His school was a big CTI customer, and when CTI was initially considering buying the Kenbak-1 rights, they asked Nielsen to review the computer and tell what he thought.  Then he worked closely with the CTI sales team making up brochures and manuals, and even making video tapes to instruct the sales force on how to use and demonstrate the Kenbak-1.  Blankenbaker thought CTI may have made a sale to a school in Canada, but this is most likely based on an inquiry the school made to John, who referred them to CTI.    

4 – John Blankenbaker made his first prototype computer, using the “Rev_” (no character after revision label) printed circuit board.  This board required many modifications and jumpers to get it running.  Blankenbaker then designed and fabricated a second “Rev A” printed circuit board which was almost correct and just required a few changes and jumpers to work.  This was sort of a "revised prototype" or "beta prototype" and only two examples of this “Revision A” computer have been found, neither of which Blankenbaker ever seems to have sold - one apparently was shipped to CTI along with left over computers and parts (Nielsen3,) and one he mailed to Boston’s “The Computer Museum” in 1986 for their contest (Serial #216.)  All other computers (every single one he sold to customers) appear to have the error free “Rev B” circuit boards.

5 – John put serial numbers on some computers, but not all.  It seems likely that he put a serial number on all of the computers he sold or at least completed for sale, but he didn’t number the Prototype, or the two “Rev A” computers (see below about Serial #216.)  He usually placed a serial number label on the back of the case, and also stamped the serial number onto the PC board.  He explained that rather than starting at “1” or “100” he started at 167, the last 3 digits of his address.  The highest known serial number is 216, the 50th in that sequence, on the computer he donated in 1986 to The Computer Museum in Boston.  However, this 216 as a true consecutive number is doubtful.  While number suggests it was the very last one he made in his run of 50, its “Rev A” PC board and very early IC date codes proves it was actually one of the very first development/prototype computers he made.  Also, this computer had no serial number stamped onto the PC board.  I suspect Blankenbaker didn’t plan to put a serial number on this computer, which he never planned to sell.  But when he needed one to send one to Boston 13 years later, he wanted to put on a serial number so it looked professional and complete.  As for the 8 computers he shipped to CTI, at least one had no number on the PC board, but had a numbered label on back (Nielsen 4,) and one odd computer (Nielsen7) had the odd serial number, “501” stamped on its PC board.  Perhaps CTI was considering their own numbering system starting with 500, maybe with the PC boards they assembled themselves.  In conclusion, serial number evidence offers little evidence to determine how many computers were made.

Putting all of the above together, we can make reasonable guesses the number of computers made.

Estimate 1: Focusing on “sold about 44 of that 50” and delivered the rest and parts to CTI: If we take that as literal and accurate, we arrive at a very high number.  If he sold 44, and kept 3 computers for himself) that adds up to 47, and then if we add in the computer that he “gifted” to his employee of 4 months, we’re up to 48 computers.  Then we add in the Kenbak-1 and CTI machines that CTI received or assembled, we have a total of 56 computers.  This seems unlikely, as Blankenbaker would have had to purchase 53 of the production “Rev_B” circuit boards, a non-round number.  Perhaps this number is more useful as a "upper-limit" of number of computers made.

Estimate 2:  Perhaps 44 were made/completed, not actually sold:  As a builder, Blankenbaker may have focused more on how many he completed and kept that number 44 in his head, but 35 years later during the Felsenstein interview, he was asked how many were sold and how many were sent to CTI.  This may have caught him off guard, so he said “about 44” rather than get into a lot of detail about the three he kept, and the one he gifted to his employee of 4 months.  He didn’t mean to deceive, so said “about” to be keep his uncertain numbers honest.  If he made/completed 44 machines, that might include the 3 he kept for himself (prototype, and Serial 183 and Serial 216) and the one he gifted.  If we add the 8 computers we knew CTI had, we get 44 + 8 or 52 computers all together.  

Estimate 3: Focusing on Blankenbaker’s initial planned run of 50 machines:  John had always planned to produce an initial run of 50 production machines, as in his quote.  This probably does not include the prototypes ("Rev_" or "Rev_A") which he never planned to sell and made before ordering the final PC boards.  Adding the run of 50 production machines, to the 3 prior prototype/developmental machines, would make 50+3 or 53 total computers made.

Estimate 4: Counting PC boards:  The costliest and most unique computer part, the PC board, would probably have to be purchased as a large order (like all 50) to get the best price.  He probably ordered just one of the initial prototype ("Rev_") boards to test it out the design.  Then a small number of the improved/corrected boards ("Rev A") until he found the last of the board errors.  But then when the circuit was finalized, it would make sense that he would want to order the full run of 50 of the production circuit boards ("Rev B") at once to get the best price.   If he bought all 50 of the production “Rev B” boards at once, and we add in the prototype board, and the two known "Rev A" boards, he would have a total of 53 total printed circuit boards to turn into computers.  Lastly, we know he kept one bare PC board which never had any components soldered to it.  It was photographed many times during interviews after 2005, loaned to Grant Stockley for his Series 2 reproduction, and even loaned to Achim Baque to copy.  So with one unused PC board, it seems logical that 52 PC boards made into computers, and one other board is still owned by Blankenbaker which has no components.

Caveat....

There are many of assumptions and opinions in the above.  But it does give a discussion why the frequently stated "44" or "50" probably isn't correct.  This page has been revised many times, with input from others.  Let us know if you have an opinion.  See our “Contact” page.

There are several reasons for variation.  First, different instructions take different amounts of time to execute.  Most instructions are two bytes, and some instructions with indirect addressing modes require many steps and memory accesses to execute.  Since memory is implemented serially, when reading a byte from memory, there may be a shorter or longer delay until the required byte circulates to the output of the 1024-bit shift register to be read.  In fact, the vast majority of execution time on a Kenbak-1 is spent waiting for a required memory byte to be read from the serial memory.  Secondly, each Kenbak-1 has a different clock frequency.  Quartz oscillators were not used.  Simple RC circuits , with imprecise capacitors were used, and those components may change as they age.  Last, it seems that Blankenbaker experimented around with different capacitors in the clock circuit.  His first 3 computers all had different capacitors installed, changing the clock frequencies.

The best way to know, execution time is empiric measurement of a typical program on a real machine.  When running the simple "counting" program (on the first page of the kenbak.com web site) on an original computer (Nielsen #3 or "Gray Case") the high-order bit flashes on and off almost exactly once per second.  This corresponds to 768  instructions per second.  Unfortunately, after 50+ years this Kenbak-1 doesn't have a clock speed of 1 MHz -  it is now around 1.6 MHz.  This may be due to aging of components, but it's most likely that this very early version were smaller (it was a "Rev A" early beta-prototype.)  If the clock speed was decreased to 1 MHz (by adjusting resistors or capacitors on the multivibrator clock) it follows that the instruction rate would fall to around 480 instructions per second.

A second Kenbak-1 can also be similarly measured.  A YouTube video shows John Blankenbaker running a similar counting program on the Prototype Kenbak-1 in 2018.  (https://www.youtube.com/watch?v=JNU6PFJj4tg)   John entered the counting program by memory, so it's probably the same counting program except the "A" register was incremented rather than "B" register.  This program is published in his "Laboratory Exercises" manual, as the very first program a student enters and runs.  The YouTube video shows the high-order bit turning on and off 7 times in about 11 seconds.  That would correspond to about 488 instructions per second.  That suggests it's clock rate is quite close to the  nominal 1 MHz.

One more computer was then measured:  a "Kenbak-1 Series 2" reproduction computer made in 2007 (see kenbakkit.com)I.  While this is a modern reproduction, it uses the same circuitry and capacitors as the production "Rev B" computers, but newer transistors, and it seems to be operating at 428 instructions per second.  I have not yet measured it's clock rate, but suspect it's around 0.9 MHz based on ratios of speed.

Regarding capacitors used, we can examine early computers. The first prototype computer used two different capacitors, a big brown mica capacitor, and a large ceramic disc capacitor.  The Nielsen3 Revision A computer seems to use two big brown mica capacitors (which made the clock a bit too fast.)  The Revision A John2 computer in the Computer History Museum uses two large ceramic disc capacitors.  But then all other production Revision B computers seem to show very small ceramic disc capacitors.  John was definitely trying out different components in the clock circuit, probably aiming for a round 1 MHz, as he wasn't too interested in optimizing speed.

My opinion differs from those of John Blankenbaker.

I think schools were resistant to purchase the Kenbak-1 because machine code programming is not as good for introductory programming as higher-level languages that could be obtained with a time sharing system.  John thought the problem was the slow and complex school budgeting process, and perhaps that's the excuse the schools gave him.  John did a great job of marketing to schools.  He traveled to educator conferences to demonstrate his computer.  He put advertisements in popular education magazines and journals.  John correctly recognized that timesharing systems were his main competition in schools.  Timesharing systems allowed schools to lease terminals and phone lines to connect to a large central computer which served many schools and businesses.  He acknowledged this competition in an advertisement in November 1971  Nations Schools which compared the cost of buying a Kenbak-1 to the higher cost of renting time sharing terminals.  But the problem wasn't the cost, it was the capability.  Timesharing systems offered high level languages, such as Dartmouth BASIC, or COBOL which are much better for teaching high school students than machine code.  A student can make much more useful programs with BASIC than they could with machine code.  Schools usually teach machine or assembly code programming much later in computer science or engineering curriculum.  So while John recognized his competition, I don't think he understood why schools were choosing his competition.  

Why the individual customer (the hobby, or amateur customer) didn't buy is more complex.  I suspect the level of detail in advertisements or sales brochures was probably too limited for individuals to shell out such a huge amount of money (equivalent to $5,500 in 2025.)  John thought he didn't focus on this hobby market enough, but he did reasonably well.  He got an early article written in the Amateur Computer Society M John's advertisements focused on how "fun and educational" the Kenbak-1 was, but lacked enough details for customers feel confident they knew what to expect.  I suspect customers were weary of these mail order advertisements.  It's not at all surprising that only about 6 individuals bought his computer, and that only included about 3 engineers or programmers (numbers from the Amateur Computer Society newsletter.)

Had John sold 100 or 200 computers that first year, history might have been much different.  He could have hired more help to assemble the computers and handle administrative details.  Then John could have done what he did best: focus on designing and improvements.  Maybe he could have even made a "Kenbak-2" when microprocessors and cost effective RAM chips came out.

For the 1986 Boston Computer Museum contest the industry expert judges realized their first job was to decide what qualifies as a "Personal Computer."  Even defining what a "computer" is was not obvious.  They had to decide if they should include programmable calculators, analog computers, and even the cardboard or plastic "toy computers" like the "Geniac" which were commonly sold from the 1950's through the early 70's.  The judges decided not to include programable calculators, and the mechanical, or analog computers, focusing only on digital computers with a stored program.  They even made the arbitrary decision to disqualify any "kit" computers, and focus on items which were commercially available.  Many people have offered opinions of what a "personal computer" should be.  The Deutsches Museum in Munich Germany saw the ambiguity in definition, so placed a note on their Kenbak-1 saying it is the "first personal computer for educational purposes."  But that does little to clarify the ambiguity. 

In November 1999, an early and thoughtful treatise tried to name the "First Personal Computer."  The "Blinkinglights.org" site (permanently archived here) asked "what was the first personal computer?"  They do an excellent job of explaining the difficulty in defining a PC, and listed several contenders, usually ruling them out.  Unfortunately, they go down the rabbit hole of including the cardboard and wire "toy computers" like the Geniac, the paper-clip computer, and others, before finally settling on the 1950 "Simon" which most people would not consider was a true computer, and definitely not a stored program computer that people think of today.  Instead of convincing readers that "Simon" was the first personal computer, many readers were left feeling the Kenbak-1 matched their own notion of a first personal computer.    

While only around 52 were made, almost all Kenbak-1s were sold to high schools, colleges, and technical schools.  Hundreds of students from 1971 to 1976 and beyond used these to get their first exposure to computer programming.  This was a pivotal time when every high school wanted to start their own computer classes.  But even people who never saw a Kenbak-1 in person were likely inspired by the idea of a $750 computer.  Thousands of people and educators saw the many advertisements and it got them imaging a world where they could have their own computer.  The Kenbak-1 left a legacy of interest and imagination which eclipsed it's very dismal sales numbers.  Now 50+ years later, hundreds of enthusiasts still buy and make Kenbak-1 reproductions and learn to program them, share programs, and share the joy of experiencing this ancestor of modern computers.

John and his wife aren't as active as they used to be.  They recently moved to a senior community, so he doesn't have to do all the hard home maintenance.  But he remains excited to do new experiences, and traveled to see his first solar eclipse in the spring of 2024!