An Unusual Kenbak-1 (Rev A Board)

(and a Secret Error in Serial #183?)

A Mysterious Kenbak-1:

The Grey Bottom (Nielsen3) Kenbak-1 has always seemed odd. Initially it seemed to be a "Frankenstein" using an earlier unpainted case, yet had a later unusual CTI serial number slapped on the back. I contacted John Blankenbaker about this in 2013, to ask if he recalled anything about a Kenbak-1 with a grey bottom.

He couldn't recall this, but did say "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."

This computer was never actually sold by John. It was apparently included in the big load of computers and parts that was shipped to CTI when Kenbak Corporation closed in 1973.

Besides the grey case, someone at CTI stuck an unusual "SERIAL NO 001" sticker on the back. No other machines, Kenbak-1 or CTI branded, had a CTI serial number sticker. Maybe CTI was just experimenting with how to re-label the Kenbak-1 to re-brand it for mass production. This serial number sticker also calls it "Model 5005" which seems to be a mistake, this should be "Model 5050."

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 attached to the front panel, that's practically permanent. Removing the front panel from the PC board would require extensive desoldering and resoldering in a delicate small area. And that shouldn't be needed, as any malfunction of front panel switches or lamps would be easy to replace just that part. So the next logical question, was this "marriage" between the grey case and the computer done by Blankenbaker, or someone at CTI. A hint lies in some "blue-paint" on the front panels.

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 case. It's unknown if this is because it was attached to a freshly painted blue-case or if the glue from the switches ran down and made the panel stick to the paint. Regardless, this proves that this front panel had previously been installed in a fully blue case. And since removing the front panel from the circuit board is difficult and with little reason to do so, it seems clear that this computer, the PC board and front panel, was once in a totally blue case, and at some point was transplanted into the grey bottom.

Inside the Case gets More Interesting:

The Insides get more interesting. Everything (except the grey case) looks at first like a normal production machine. But the printed circuit board is marked "Kenbak 10000 Rev A." No other Revision A board had ever been described (until 2/2023 when the computer in Computer History Museum, serial 216, was noted to have a Rev A board.) It was assumed that Blankenbaker went directly from the prototype to the Revision B boards. In fact, some called the prototype board a "Rev A" board, just because it came before "B" but it was not marked that way. Is this newly discovered "Rev A" circuit board an intermediate between the prototype and the production models? Sort of a "Second Prototype" or "Try Number 2?"

John's Prototype Computer was labled "Kenbak 10000 Rev " without any letter. This is un-deniably the first Kenbak-1 printed circuit board John Blankenbaker made (photo courtesy Achim Baqué of Western Germany.)

This unusual computer is labeled "Kenbak 10000 Rev A"

Most other Computers are labeled "Kenbak 10000 Rev B"

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

The "Rev A" circuit board was noted 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 did he make more than 1 computer with this newly found "Rev A" board? Did he order a single board, and when he found it still had mistakes, did he moved directly on to the Revision B boards. Unfortunately, he couldn't recall these details. Too much time had passed. These details, indeed, are apparently lost to history.

The "Prototype" printed circuit board, with extensive correction and modifications. Right Click for a magnified image of all these corrections. (photo courtesy of Achim Baqué of Western Germany)

So we sought to answer these questions ourselves. Is this "Rev A" really an intermediary link between the prototype and the protection models?

To answer that question, a comparison was done between the prototype, this new "Rev A" board, and the "Rev B" boards. The prototype PC board had a whole lot of modifications. It did not have the RC circuitry to initialize the bit counter clock, so that was added. Numerous orange jumpers and cut traces especially around IC 61, 62, and 63. Above the memory clock drivers is a real "rats nest" of extra components, and even an extra transistor soldered in. The prototype PC board required many changes.

Contrast that with the "Rev A" board on the right. At this magnification, you can't even see any fixes. It looks perfect. It looks just like a "Rev B board, just missing one disc capacitor on the left.

However, if you look closely, several error corrections are apparent. Below are several places where IC leads were pulled out of the holes, and soldered to a neighboring pin.

Some of the minor corrections of Rev A board, where pins of IC's 54, 65, and 123 were pulled out of the hole, and soldered to neighbor.

Several jumpers on the "Revision A" PC board back.

More corrections are visible on the bottom of the Revision A PC board. Several PC board traces were cut, and several jumpers were placed. Keep in mind that the Revision B PC boards had zero corrections. They were perfect, only requiring a single jumper on the back because there just wasn't enough room on the board for the copper trace.

It is clear in comparing these PC boards, that the Revision A was definitely an intermediary step, between the prototype, and the Revision B boards. But did John sell any computers produced with this Revision A board? Or was this a unique "second try" or "semi-prototype" that wasn't good enough to sell?

Were Several "Rev A" Boards Made, or is this a Unique Example?

The final question to ask, is if Blankenbaker produced just one of these "Rev A" boards, found it's mistakes, and immediately fixed the mylar sheets to order "Rev B" boards. Or did he perhaps order several "Rev A" boards to save time and cost, found and fixed mistakes, and not wanting to waste money, produced some Kenbak-1's with the hand repaired "Rev A" boards? This was unknown until February 2023, when the Computer History Museum in California took some high resolution photos of their Serial Number 216 computer, and they also had a "Rev A" circuit board.

But there is something interesting about this particular Rev A board in the "Grey Bottom Case" computer. It shows something that any electronic engineer would recognize as a "undone correction." In the photo on the right we can see that the PC board has a copper trace which was thought to be incorrect, and cut in two places (the copper metal was scraped off, opening the circuit.) But then there's a jumper negating these cuts. Cutting a trace then reconnecting it is not what happens when you're just fixing known errors in a printed circuit board. It's what happens when you're trying to locate an unknown error the first time, trying to get a circuit working. It's a sign of "debugging" not just "fixing." This and other changes, suggests John was trying to fix something without precisely knowing where that problem was. This "undone correction" is good evidence this board went through some "debugging" of the actual printed circuit traces. It's good evidence that this PC board was the first Rev A board that John Blankenbaker assembled, debugged, and got running.

A tell-tale "Undone-Correction" which shows a debugging process.

Putting it all Together: This Computer might be the Second Kenbak-1 Computer that John produced.

This Revision A computer was definitely made after the Prototype Kenbak-1's board, and before the Revision B production models. And until comparisons can be made with the other Rev A board at Computer History Museum in California, seems possibly the first Rev A board assembled. It is hoped in the future that the CHM will allow the bottom of their PC board to be examined, to see how many jumpers and trace cuttings are visible. That would tell us more about the development and debugging process.

It may never be known for certain how many Rev A and Rev B boards exist. John said in his Felsenstein interview that the board cost "wasn't very much." I suspect the corrections needed on the Rev A boards might have prevented him from ever selling any of these Rev A boards. John was a bit of a perfectionist, and may have wanted to only sell the perfect Rev B boards, rather than sell an "imperfect" board to a customer. It's possible he never sold a Rev A computer to any customer. One got shipped to CTI when Kenbak corporation folded, and one got donated to The Computer Museum when they needed a working computer for their contest.

Epilogue: Were the "Revision B" PC Boards Really Perfect (Serial 183's Secret)?

In the above discussion, the Prototype "Rev (blank)" circuit board is described as requiring a lot of wiring corrections, and the Revision A circuit board still had several errors requiring corrections. Then the final Revision B boards are described as being "perfect boards" requiring only a single jumper on the back side, due to the lack of room for a single connection.

In September 26, 2022, John Blankenbaker told me a surprising statement about a mistake in the production machine's printed circuit boards: "All of the machines except the very last one (Achim owns it now) had an undetected error in the memory drive circuits. The machines still worked with this error though when it was corrected the waveforms were much better. One of the drive transistors was wired incorrectly in the etch." John is essentially saying that all of the production Kenbak-1 computers have a wiring error in the memory clock signals, which still allowed the machines to function, but was none-the-less wrong. But he couldn't be talking about Achim's machine (the prototype, the first machine) which had a completely different circuit board. That had a completely different clock/driver circuit wiring, so would have little influence on the completely redesigned production machines. He was definitely talking about the serial number 183 computer, which he repaired in 2010, and which carried the same circuit board as the other production machines.

The possibility of a long un-noticed error intrigued me, as I and others had examined every portion of that circuit. I even recreated the circuit board with a schematic generated netlist, while looking at the original board to make sure traces were roughly in the same place. I found it puzzling that I, and others who fixed and debugged these computers could have missed such an error. I spent a great deal of time looking over the clock waveforms, and carefully tracing out the circuit connections. Yet everything appeared correct. Why couldn't I find the error he was talking of?

Pictured is the schematic of the CMOS shift register clock drivers. There are two of these, and each has 3 transistors. So I searched over the circuit boards to try to find out what's wrong. I could find nothing. Then I looked at the oscilloscope tracings for the clock signals (psi1 and psi2 in the schematic. ) It's tracing is below.

While the two phases are a bit bumpy/rough, this is because there's no filter capacitor anywhere near this circuit, and the power supply is showing the current surges. But both of the two clock signals are the same shape, which doesn't support anything like the reported error.

Unable to find anything wrong, I asked John again about the error for more details, and if he could recall which transistor he had to fix.

Unfortunately, John didn't recall details. He wrote:

"The circuit [he means the schematic] was correct. One of the transistors was wired incorrectly. I don't remember which one. I found the problem when I compared the the phase 1 circuit to the phase 2 circuit. In particular I believe it was the transistor that was the farthest (of the six) forward toward the front panel. To correct it, I cut the etch to one the three leads and installed a jump with the correct signal. That wasn't hard to do. My memory has gaps in it."

Now armed with a bit more definitive information that I was on the right track, I again searched over the circuit boards, yet could not figure out what he was talking about. My oscilloscope tracings of phi-1 and phi-2 match.

I put this problem on the "back burner" until several months later, when the current owner of Serial #183 was finally able to send me some photos of the "corrected" circuit board, and it in fact did show modifications of a transistor right where I thought it would be. These photos aren't great quality, but indeed shows there was some recent (dark flux) changes around the Q5 transistor.

top of board of Serial 183 machine

bottom of board of Serial 183 machine

The above red circles show the top and bottom of the circuit boards where the Q5 transistor is. Even with the low resolution and blurry photos, relatively fresh (dark) flux shows more recent soldering. And the Q5 transistor is not oriented like any other Kenbak-1. The flat side is pointing to the lower right. Compare this to Serial 197's orientation, where the flat side points to the upper left. This is definitely where he made his change, but it appears to me the circuit tracings were wired correctly in the first place, and the oscilloscope tracings seem to show the two phases match each other.

So what is going on here?

"normal" Serial 197 for comparison

I'm beginning to suspect the problem may not be a true error. It's possible John replaced a faulty Q5 with a substitute transistor, then noticed it wasn't looking right on the oscilloscope. The 2N5449 transistors originally used aren't produced anymore, and substitute transistors often have different pinouts.

Here are several possible pinouts for a bipolar transistor in a TO-92 package.

If looking at a transistor with the flat side towards you, and the leads pointing down, the emitter, base, and collectors could be in any of the following orders:

EBC: Most common American pinout, as in picture.
ECB: Most common Japanese pinout.
CEB: uncommon, but known.
CBE: uncommon, mostly seen in European transistors (But THIS matches the Texas Instruments 2N5449 Pinout!!!

It does appear a substitute pinout could have flipped leads, and caused faulty clock signals in this level shifter circuit.

EBC:most common American pinout

Until we get more information, I wonder if John replaced Q5 with a substitute transistor which had a different pinout, maybe reversing the collector and emitter. Most designers know the experience of getting "lost" when trying to follow printed circuit board traces, having to flip from front to back of a board. Like getting lost in a forest. Maybe he didn't notice his new transistor pinout was different, so he thought the circuit board was wrong. But he managed to fix it regardless. Until clearer photos, or the part number of the replacement transistor is available, this remains a mystery.

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