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Old 12-09-2007, 10:54 PM
Cappellanus Cappellanus is offline
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Join Date: Jul 2007
Posts: 24
Default Another Finished Kenbak-1, how many are functioning now?


A week ago, I finished my Kenbak-1 Series 2, and am quite happy with it! I haven't had a chance to exhaustively check all instructions, like "set bit" and "bit test and jump" but the jump and mark instructions, rotates, conditionals, and other things work, so I suspect 96% of the circuitry has been tested out. Interesting: with microprocessors, if one instruction works, we suspect the whole computer will probably work. But with small scale integration processor design, a minor bug can hide in a little used instruction.

It "almost" worked when first turned on, but I noticed when I entered data to memory with the front switches, two bits would make neighboring bits also turn on before storage in memory. Review of the documents and a little reasoning said this should be near the "I" register, the only place where the suspect bits sit next to each other (parallel). Sure enough, I switched IC-6 and IC-20, and different bits became "sticky" suggesting it was not a soldering error. I replaced IC-6, and problem fixed.

A few comments:

1) Spray painting the front panel with clear enamel or laquer isn't really an optional step. The deep texture of the brushed metal will suck a finger print off the cleanest hands.

2) Wow! there were a lot of extra capacitors when I was done. I placed one everywhere I could, and still had lots left over. I guess having more than needed is better than too few. It's interesting that Erik's machine used *zero* decoupling capacitors, just two electrolytics on the board for +5V and one for +12V. I suspect John designed the board with lots of decoupling caps, but realized it ran fine without them. Standard design rules of the time, however, suggested lots of decoupling caps were needed.

3) The board scans and documentation never show c16 and c17, and how exactly they should be positioned. But a search of the internet showed documents at Erik Klein's web site and "compusaur" seem to show it. I notice you suggest c16 and c17 aren't necessary. Since Blankenbaker installed them on the back of the circuit board, I suspect they were a later modification to get rid of a little glitch which only rarely cause problems, maybe due to borderline spec IC's.

4) Interesting about the second ground connection from the power supply to the ground of IC-114 and IC-115. I think I know why Blankenbaker did that. The ground current from these two IC's could potentially be pretty high. You may notice that the open collector lamp driver IC's can source up to 15 volts, so higher voltage lamps (12-volt) could easilly have been used without modification. But also, each output can drain to ground up to 40 milliamps, or 240 milliamps/chip. I think he was worried that's a lot of current to go along a thin PC-board trace, and if not connected directly to power supply, a significant voltage drop may exist. I'm not sure of the state of PC board quality in 1971. He often used several plated through holes to carry power lines from one side of the board to another. Interestingly, my second ground inadvertently popped off my power supply, and the LED light brightness was just about the same. I haven't investigated why, but I can only guess the ground current dissipated through the the inputs of these two chips to the output of their drivers, but standard TTL can't sink much. I may investigate this more when I get time.

5) Blankenbaker had an amazing circuit, but has anyone noticed how he never tied an unused input directly to VCC? I can't understand why he always tied the /SET and /RESET inputs of the 7474's to a pull-up resistor, instead of directly to VCC? That's why we have several 1K resistors spread around the board. I wondered if he wanted the pull-ups for debugging so he could set or reset the values from time to time, but the pc-board was all-the-way production with no other debugging facilities. But he even tied the input of an XOR chip to a pull-up. The only answer I can figure, is he didn't know that TTL circuitry allows tying directly to VCC to keep an input high. Is that possible? Of course, John was a Physics and Mathematics major in college, who by chance got caught up with computers after his Junior year of college, and didn't have *formal* electrical engineering education. Besides, TTL chips were fairly new at the time, and many people with experience in other logic circuits may have become accustomed to tying inputs to high or low through a resistor. Don't yell at me, yes, I'm suggesting John's design was a bit naive in one technical aspect, but that's quite understandable to someone who's roughly "self-taught" at a time when TTL chips were fairly new, and few people even in digital electronics had used the 7400 series TTL. Does anyone else have another theory on why he used all the pull-ups?

6) I'm not sure if I'm supposed to use the pan-head screws that came with the case, or the smaller ones in the bag. Do you feel the smaller screws are more authentic? They don't look all that different to me.

7) A little piece of metal to cover up the "expansion" slot in the front panel wouldn't be a bad idea. I'll probably make one up. The only Kenbak-1 I've seen missing this is Erik's machine. I just noticed for the first time: The CTI labeled machines left off this slot completely.

8) I used automotive spade-connectors to attach to the power switch on the front panel. That allows completely removing the front panel.

9) In retrospect, I'd like to raise the power supply a bit. It's just a bit above the PC board, and hides too much of the Kenbak-1 pc board. If it was just 1/2 or 3/4 inch higher, you could see a bit more of the "guts". As it is designed, you can't even see the "kenbak" logo on the pc-board.

10 It's interesting that the transistors were opposite pin-out (ebc instead of cbe) which makes the transistor orientation look wrong, when compared to Erik Klein's machines. Did you use the same transistors as Erik's?

This was a lot of fun. I'm surprised with how complete the kit was. I figured I would have to supply a lot of miscellaneous wire and hardware, but didn't. Well done!!!

Last edited by Grant Stockly; 09-10-2012 at 09:14 PM.
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