Nielsen3

Nielsen3 / Gray Case

Provenance: This is one of the eight computers which Robert Nielsen acquired in the 1970's. Nielsen learned computer programming in the Navy, then opened up technical schools teaching programming, electronics, and even truck driving in Charleston, SC. His technical school utilized electronic training kits from CTI Educational Products, so they approached him in 1973 to ask him to review the Kenbak-1 computer. They were considering buying the Kenbak-1 production rights to modernize their educational products. Nielsen absolutely loved the small computer. He wanted to incorporate it into his computer curriculum. He also worked with CTI to develop sales and training materials for the computers. When CTI went bankrupt, he purchased all the Kenbak-1 computers and materials they had at auction. All of Nielsen's 8 computers had ventilation holes drilled in the top of the case, as they would overheat and lock up otherwise. Then when his technical school closed, he stored all eight of the computers and documents in large storage bins in his garage. In the early 2000's, he contacted many museums in hopes of selling his computers, but they only wanted free donations. Finally, in 2003 he contacted Herbert Eisengruber who ran the Nova Scotia Computer Museum, who bought seven of his 8. Eisengruber kept the seven computers on public display for several years, until the museum closed, and he sold the computers individually.

This particular computer, Nielsen3, was purchased from Herbert Eisengruber of the Nova Scotia museum in 2009. Internal examination shows it to be one of only two known early "beta-prototype" computers using the "Rev A" circuit board. The other "Rev A" computer (John1) resides at the Computer History Museum, in Mountain View, California. Neither of these two "Rev A" machines were ever sold by John Blankenbaker. This particular computer was apparently shipped by Blankenbaker to CTI in South Carolina, when he shipped a huge load of left over parts and computers after they purchased the Kenbak-1 rights. This computer seems like sort of a "Frankenstein" pieced together from discordant parts -- while the internals show it was one of the first computers completed, it uses in an unpainted gray bottom case, and has a unique "CTI Serial Number" sticker is on the back. It appears that CTI was experimenting with methods to re-brand the machine, and no other CTI Serial Number sticker has been found on any other computer. When Robert Nielsen was asked why the gray bottom case was used, he said "they ran out of the blue cases" so used this unpainted early one. When Blankenbaker was asked about the case, he didn't recall the details, but said that gray was the standard "off the shelf" color of the cases, and for the first few computers he painted them blue himself. For later computers, Bud Industries supplied the cases painted blue, and with the proper holes drilled in the bottom and back for the PC board standoffs. There are a number of intriguing revelations on close inspection of this machine, described below.

Front/Top

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Back/Top

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Front Top

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Gray Bottom Case

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Bare Top Case

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Bare Bottom Case

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Component Side of PC Board

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Bottom side of PC Board

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Backlit bottom of PC Board.

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Closeup Matrix Photo of Top of Printed Circuit Board

This will only probably be visible on a wide screen monitor, not a phone screen. Not sure why left-most photos are yellow.

Top/Front Running

Computer with case removed, opened up, running a program. Few original Kenbak-1's are still running. I think only 4 are still working.

Closeups of interesting or unique parts.

Blowup of Clock Circuit

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The two dark brown mylar capacitors on the left (next to the orange electrolytic capacitor sticking off the board) set the clock rate in the multivibrator circuit. This computer has a clock speed near 1.6 MHz, probably due to the choice of capacitors. The first couple computers Blankenbaker made (prototype, and the two Rev-A computers) used different capacitor combinations, but the "Rev B" production computers all used small orange ceramic disc capacitors, producing a standard 1 MHz clock rate.

The Mylar Clock Capacitors

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This may be why this particular Kenbak-1 has a faster than usual clock rate. These brown Mylar capacitors are different. There was one in the first prototype, two in this computer, and all other capacitors in clock circuits were ceramic disc capacitors. Production computers used 0.0015 microfarad capacitors. The code "471" would signify 0.00047 microfarads, making the clock faster. It's not quite proportional to value, as the transistor switch times affect this. The follow number "103" would signify 0.01 microfarads which would make the clock slower, so I don't think that's the value code. The resistors and transistors are the same, so breadboarding the components to check clock may give confirmation.

Shift Registers

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Power Supply

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Power Supply

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Power Supply without Capacitor

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The white rectangles were high power low resistance current shunt resistors to measure current.

Checking large Capacitor

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Measuring the 50 year old capacitor. While most electrolytic capacitors aren't expected to last very long this one is holding up well.

Power Supply Transistors

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generous amounts of heat sink compound

Washers used as standoff between heat sink and pc board

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a bit of a "hack" to get proper spacing of the PC board from the heat sink.

The shift register memory chips

Without the Intel 1404 shift registers which becoming available just before the Kenbak-1, it would have been very hard to make 2048 bits of serial memory.

IC55 leg tie

1769 x 887

Several IC legs were pulled out of the socket, and tied to the leg next to it. I suspect this was an input which was not connected in the PC board. Early TTL IC's would oscillate and draw heavy power if an input was left floating.

IC64 leg tie

1370 x 775

IC123 leg tie

990 x 603

Some Curious Details about this Unusual computer.

The Gray Case Bottom?

The bottom half of the case is gray, not the usual blue. I asked John Blankenbaker in 2013 if he remembered a gray bottom case, and he didn't recall it, but said "I don't remember about the gray bottom on the case. That was the natural color of the case as purchased on the shelf from Bud. The first two computers that I shipped to customers were hand crafted. I modified the case myself. Later, the units shipped by Bud were painted with the necessary holes in them." The first owner, Robert Nielsen, says he was told they "ran out of blue cases."

CTI Serial Number 001?

While this is a Kenbak-1 labeled machine, somone at CTIput on a strange CTI serial number sticker saying "Serial No. 001". No other machines, Kenbak-1 or CTI branded, had a CTI serial number sticker. This serial number sticker also calls it "Model 5005" which is incorrect, and should say "Model 5050." Maybe this sticker was just an experiment by the CTI employees trying to decide how to rebrand the computers to their own product, but then decided against using these stickers.

Inside the Case, things get More Interesting:

Looking inside, it was noticed that the printed circuit board is marked "Kenbak 10000 Rev A." This is unusual. At this time this was discovered, no other Revision A board had ever been noted before this. The prototype computer was printed with "Rev " without any letter after it, and all the other production machines said "Rev B." Most people thought Blankenbaker went directly from his Prototype to the production machines, and even called the prototype a "Revision A" board, even though it didn't include any letter, and assumed since it was before B, the prototype should be called "Revision A." But this computer shows that Blankenbaker actually created a middle version, sort of a "Beta-prototype" after the first prototype was made, and before the production machines (Revision B) were made.

After this computer was noted to have a "Rev A" circuit board, it was discovered that the computer in the Computer History Museum in Mountain View California, Serial 216, also has a "Rev A" circuit board. These have been only two computers with this early "Rev A" board found, and interestingly, neither one of them was ever sold by John Blankenbaker. The Serial #216 was kept by John Blankenbaker in his attic until 1986, when he needed to send a working computer to Boston's The Computer Museum, and this Nielsen3 was never sold, it was shipped off with all the parts and partially completed computers to CTI, when Kenbak Corporation folded.

"Rev " prototype

"Rev A" beta-prototypes?

"Rev B" production computers

Can we Verify when these "Revision A" boards were Made?

Luckily, John Blankenbaker didn't buy all of his integrated circuits in one large purchase. He made one purchase before making the prototype, then purchased some more before the Revision A computers, then purchased more before the Production Revision B computers. Looking at date codes, we can see evidence of that. Through high resolution photos of several computers we can read the date codes on TTL integrated circuits. This is detailed in the Comparison of Original Kenbak-1 Computers page, but here's a bit of that information:

Prototype computer: Latest Date Code 7103 (mid-January, 1971)
Nielsen3 Revision A: Latest Date Code 7129 (mid-July, 1971)
Serial #216 Revision A: Latest Date Code 7129 (mid-July, 1971)
Serial #183 Revision B: Latest Date Code 7143 (late-September, 1971)
Nielsen1 and 7 Revision B: Latest Date Code 7337 (mid-September, 1973, but some may be 1974 dated. assembled after Kenbak Corp folded)

This confirms the two "Revision A" machines were after the first prototype, but before the production machines.

John Blankenbaker probably never wanted to sell these "imperfect" "Revision A" machines. He only sold the perfect "Revision B" boards as he was a bit of a perfectionist. An early development revision of a product, after the first prototype, but not ready for production, is often called a "Beta Prototype" and that's a fitting name for the "Revision A" computers.

Time to Ask John Blankenbaker about this "Rev A" PC Board:

The "Rev A" circuit board was first noticed around 2013, but the mystery didn't really start to "ache" until 2022, when I finally decided it's time to ask John. It's been a long time, 51 years. Would he remember how many different "tries" it took to get the PC board right? And how many of these "Rev A" beta-prototype circuit boards did he make? Did he order just two of the second-try circuit boards, or more? Unfortunately, he couldn't recall these details. Too much time had passed. These details, indeed, are apparently lost to history.

Was this early "Beta Prototype" always in the Gray Case?

It is easy to switch cases on a Kenbak-1. Just a few screws, and the PC board and front panel lifts out. The power supply can easily be swapped out - it only requires a few wires to be desoldered. But once the PC board is soldered to the front panel, it's very difficult to remove. Removing it would require extensive desoldering and resoldering of many wires in a delicate small area. And removing the front panel shouldn't ever be necessary, since any malfunction of front panel switches or lamps would be easy to replace just that part. So was this "marriage" between the gray case and the electronic circuitry made by Blankenbaker from the start, or later. A hint lies in some "blue-paint" on the front panels.

Blue Paint suggests Case was Changed

Here is a close-up of the back side of the front panel. There is clearly blue paint on the front panel, right where it would touch a bottom case. It's unknown if this is because it was attached to a freshly painted blue-case (John painted the first few cases blue himself) or more likely because glue from the switches ran down the panel, sticking the painted panel to the bottom case. Since the paint only seems to be on the lower side of the panels (and other Kenbak-1 computers have traces of paint in the same area) I suspect the glue from the switches dripped down, and made the front panel stick to the cases. Regardless, it's a safe bet that this computer once sat in a fully blue case. I suspect Blankenbaker needed all the blue cases for the important "Production Computers" to sell, but since Blankenbaker had no plans to sell this early beta-prototype computer to a customer, he put this into the unpainted gray case.

One Last Interesting Finding:

In examination of the Nielsen3 computer, one other interesting thing was noted. There is a curious cut of a printed circuit tracing, which was then reconnected by soldering in a jumper. This is something any electronic designer will recognize as an attempt at debugging a non-working printed circuit board, where the connections are in doubt, but the location of the error is not known. This is not something done when a previously working circuit stops working, due to a component burning out or a connection going bad. It's only done the first time, when trying to figure out why a new printed circuit board is not working. This suggests John Blankenbaker was debugging the printed circuit design, trying to fix something, when he wasn't quite sure where the problem is.

In this photo, underneath the horizontal white wire, is a printed circuit trace, which is cut in two places. The cuts are highlighted with the red arrows. White scratches are visible on the PC board substrate.

But after cutting this, the maker realized that trace was not the problem, so a short piece of wire was soldered to the two sides, fixing the signal line to how it previously connected.

This is not a procedure which would be done in just fixing a circuit which previously worked, and stopped working. This is a procedure that is done after a new printed circuit board is fabricated but doesn't work. And once one printed circuit board is made working, the correcting changes can be made directly to subsequent boards, without having to make a change, then undoing that change.

So in summary, while at least two "Revision A" boards were made, it seems likely that this board was the first one John Blankenbaker got working, as it shows signs of back and forth debugging changes. Once this circuit board was working, later boards could be fixed with simple cut traces in some spots, and jumpers in other spots, but would not have a trace which is both cut, and then jumped back to it's original state.

But this can't be known with certainty until the other known "Revision A" computer (currently at the Computer History Museum) has the bottom of the PC board examined. High resolution photos of the component side of the board exists, but no photos show the bottom of their circuit board.

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