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Old 10-18-2007, 04:06 PM
Cappellanus Cappellanus is offline
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Default IC-99 Mystery (modification)

I see talk about IC-99 (the un-used chip) getting hot. There is a bit of a mystery regarding this, but I really think the original machines *DID NOT* have a chip in this place.

I think the original Kenbak-1's, produced both by John Blankenbaker and CTI just left this extra spot empty, no chip at all. My evidence? Look at the below board (online) at the Nova-Scotia computer museum. See
http://www.computermuseum.20m.com/im...0open2%20b.jpg
This is actually a CTI labeled machine (so one of the later produced ones) and it clearly shows an empty spot where IC-99 goes.

But Eric Klein's machine at
http://www.vintage-computer.com/imag...kBoardBack.jpg
(which I think was where much of Grant's information came from) has an IC placed in this spot. Now look closely: On the back of Eric's machine, there are some cut traces, and jumper wires placed between the extra IC and the rest of the circuit. Notice that these jumper wires are slightly different looking than the original power and switch/light wires, suggesting later modification.

I tried to follow this, but it appears that Eric's machine had some sort of modification. Has anyone tried to schematic this out, to see what his modification does? It appears to me that some extra function is made to start or stop the machine, as this modifies the start/stop flip-flip state circuitry.

Anyone have any ideas what they were trying to do here? You can see the modified machine wiring really well on Eric Klein's web site. I think it would be quite reasonable to just leave this chip out of the kit.
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File Type: jpg temp5.jpg (43.2 KB, 322 views)

Last edited by Grant Stockly; 09-10-2012 at 09:17 PM.
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Old 10-18-2007, 05:25 PM
Geoff Harrison Geoff Harrison is offline
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Good find.

It's certainly safe to leave the chip out since it's connected to nothing other than Vcc & gnd. I took mine out.

Looking at that close-up on Erik's site, it looks like the trace from 116.8 to 107.13 (the X4 signal) has been cut and redirected to pin 1 on IC99. That's one of the inputs of a nand gate, the other input is connected to a wire that wraps around to the top of the board that has been cut. The output of the nand seems to be routed to both inputs of another nand to invert the signal again and routed back to 107.13.

When X4 is true and location 377 (input) is being read, the K register will start shifting data in. I wonder if the wire that was cut allowed another input channel that bypassed the buttons.
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Old 10-18-2007, 07:40 PM
Grant Stockly Grant Stockly is offline
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I didn't notice any of those wires and I spent months looking at the board! Ok, I was looking at the top and not the bottom...

Should the switches otherwise act normally? Erik does report that his Kenbak does not work like he thinks it should. Memory does not seem to be written. If the lights are displaying the wrong thing this might be why.
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Old 10-18-2007, 08:04 PM
Geoff Harrison Geoff Harrison is offline
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Quote:
Originally Posted by Grant Stockly View Post
Should the switches otherwise act normally?
I doubt it. It's hard to be sure just from the pictures, but it looks to me like he'd need to take that cut wire that goes from IC 99 around the side of the board to the top and tie it high. Either that or just restore the cut trace between 116.8 and 107.13
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Old 10-18-2007, 08:29 PM
Geoff Harrison Geoff Harrison is offline
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Looking at those pictures of Erik's, it seems that machine has had several mods made at some time in its history, all of them were backed out except the one involving IC99. Someone has cut (and restored) traces and connected wires to IC pins that involve states that seek instructions and update the P register, plus some others that involve the K (data lamp) register. It's all quite tantalizing.

Fortunately, unless there are some other cut traces that I missed, all those mods have been restored.
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Old 10-19-2007, 03:03 AM
Cappellanus Cappellanus is offline
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That modification on Eric Klein's machine is interesting because it is the first (and only?) modification known on a Kenbak-1. Common sense suggests that this should be an input or output, the only thing that a Kenbak-1 lacks to do "real work." We've probably all wondered how to make an input or output (of course, Grant's investigations show the light lines can be used, maybe with a low-pass filter.)

I seem to recall that Herbert Eisengruber (Nova Scotia) had some type of machine interfaced up to a Kenbak-1, but maybe I just dreamed this up. At one VCF meeting, in the pictures, I thought he had some later hardware that was hooked up to a kenbak-1, but now I can't find a source, and the pictures don't make that clear.

Look at this picture....
http://home.comcast.net/~msmith6020/P7160223a_copy.jpg
towards the bottom right of the picture is a photo showing a kenbak-1 with a later box on top of it. Was this a input/output interface? If anyone knows Herbert, maybe they could ask him. Herbert's machines came right out of a technical school, so I wouldn't expect them to have the modifications such as a privately owned machine could have attained.

In any case, maybe we'll never know details of why Erik's machine was modified, as a whole-big circuit board with lots of logic could have been interfaced into those few signals, and all that is long lost.

Oh, and Grant: I didn't get to the bank before they closed, so no money order. I'll definitely get it mailed by Saturday, priority mail. Don't give my kit away to anyone else!!!
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Last edited by Grant Stockly; 09-10-2012 at 09:18 PM.
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Old 10-19-2007, 04:56 AM
Architect Architect is offline
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Default Son of John

Perhaps Eric's machine is the one that was modified by John's son. See:

http://www.bitsavers.org/pdf/kenbak/...baker_1986.pdf

At page 8 (last page) he refers to three machines: the prototype, the one that is now in the Computer History Museum and one modified by his son.

Just speculating.
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