SCELBI I/O Cable Fabrication

October 28th, 2015

Here is a picture of my first reproduction SCELBI I/O cable. I’ll need to build four of these before I can hook up the reproduction TTY and Cassette interface chassis that I plan on building.



For connections between the main and I/O chassis, SCELBI used a standard cable that featured 11 pin Amphenol connectors on each end. Male connectors on one end connected to the main chassis and female connectors connected to the I/O chassis. The connections are straight through, pin 1 to pin 1, pin 2 to pin 2, etc. They used 12 conductor cable with two wires connected to pin 11 (ground) on each end. The cables that I’ve been able to examine, averaged approximately 32″ long from connector face to connector face. The power supply cables and the cables that ran from the peripheral chassis to the peripherals themselves were different than these cables. Several different kinds of SCELBI cables can be seen on the CHM website.

12 conductor cable can be expensive. I ended up finding a roll of 50′ of Belden type 8747, which should be enough for my purposes. It cost me, $50, including shipping. The reel that the cable came on, has a UL stamp dated Nov 1, 1979. It’s been around almost as long as the original SCELBIs!

Original SCELBI cables had metal shrouds with strain relief covering the interface between the cable and the connector at each end. A while back, I bought a lot with two varieties of plastic shrouds on eBay. I’m using a different style of the plastic variety on each end of my reproduction cables. I’d use the smaller shroud on both ends, but I don’t have enough of those for all the cables that I need to build. At some point, I hope I’ll find a batch of the original metal type shrouds and be able to do a swap.

The Amphenol connectors and shrouds are expensive, but you can sometimes find deals on eBay or in surplus dealers inventory. If you just look at standard suppliers like Mouser or the like, connectors may cost up to $14, each, and the shrouds about half that. If you don’t shop around, you may end up paying close to $50 for the parts to build a single cable. I probably have a bit less than half of that invested in the pictured cable.

I once ran across an ad in a 70’s era magazine that listed the connectors for $.25 each. Those days are long gone.

SCELBI Backwards Capacitor Update

October 26th, 2015

Take a look at these two images of original SCELBI boards.

Backwards Capacitor

Backwards Capacitor

The second one has the plus sign manually removed and the capacitor reversed. I did a quick test in my “lab” and found that indeed the “negative” side is about 1 volt higher in value than the “plus” side. Those capactors definitely should be reversed. While you are at it, make it authentic and manually remove the “plus” sign.

SCELBI Cassette Write Board Build Instructions

October 25th, 2015

I’ve been working on writing detailed build instructions for the SCELBI Cassette Boards. I need to do this, because I haven’t found original build instructions. There have been a few challenges with the cassette boards, as the schematics haven’t always matched existing boards. Also, the initial version of schematics I had, were missing some labeling information on resistors and capacitors. I recently obtained images of some later schematics and got to examine a original board that help fill in some of those gaps.

My reproduction was built to match the earlier schematics and a couple of different boards that I had images of. As part of this documentation process I had a chance to compare an original board with my reproduction. Here is a side by side scan of my reproduction next to that original SCELBI write board.

Original and Reproduction Write Card Comparision

Original and Reproduction Write Card Comparision

You can ignore the circuit in the relay socket. That was put there for debugging purposes, way back in the 70’s. It was basically using LEDs to indicate when the RELAY circuit was activated. The different color substrate is due to evolution of technology. The old green substrate is no longer made, because it fluoresces which affects modern photo based PCB fabrication processes. Some reproduction guys are dying their modern boards to give their reproductions the old time look.

Going through the later schematics and this board revealed a few things.

  • one of the electrolytic capacitors is reversed. I need to take a closer look at that before something bad happens. This is a serious issue as inserting electrolytic capacitors backwards can cause them to catch on fire.
  • This card has plain 7493s installed and seem to work fine. Clearly, the problem I experienced with this part in location Z4 is a marginal issue, and in some cases 7493s may work there with no problems.
  • This board uses (and the schematics have at this location) a .33uF film capacitor, though putting a .47uF capacitor here will have no effect on performance or behavior.
  • The 74121 timing circuit uses a 47K resistor instead of 56K. This is reflected in the later version of the schematics that I have. I’ll have to try the 47K resistor in my circuit to make sure everything is OK with that setup.
  • Finally, I noticed that I accidentally put a 1K pull up resistor in a location that calls for a 10K resistor (R5) in my reproduction. This should have no affect on performance, but I plan on fixing this mistake.
  • Here is my current version of the cassette write board build instructions.

    8B- Cassette Write board – build instructions

    I tried to make them match the style of original SCELBI build documents. Note that I need to investigate a few more things including that reversed 10uF capacitor before finalizing this document.

    SCELBI Cassette Chassis Artwork and Front Panel Screen

    October 23rd, 2015

    Here is the artwork I’m preparing for the front of the reproduction SCELBI cassette chassis.

    Artwork Front Cassette Chassis

    Artwork Front Cassette Chassis

    Dimensions are 6″ x 3.5″. I’ll either directly silk screen the chassis or make decals. The crosses are drill points for binding posts and Amphenol connectors. I believe the holes for the binding posts will be 1/2″. The Amphenol connectors also get 1/2″ pilot holes, and then will be punched out with a Greenlee chassis punch.

    I tried doing a direct transfer from a laser printer printout, but I’m not satisfied with the result. This is essentially the same process as the laser printer PCB etch method. You print onto glossy paper then apply the image by melting the toner onto the target material. The problem is that with a PCB, you can roughen the surface with sand paper to get the toner to stick. I’m not interested in damaging the surface of the chassis and the surface of the chassis is too slick to make it stick consistantly.

    I’m doing a fourth round on the front panel silk screen. The third round failed when I messed up registration of the two layers of transparencies that I used as a mask. I use two layers to ensure a good image. This time, I didn’t have them lined up exactly and I ended up with a nice screen, with a double image towards one end. The good news is that I think I have solved the other problems that I have been dealing with.

    Bad Screen

    Bad Screen

    SCELBI 8B front panels status

    October 18th, 2015

    Now that I have basic tape support going on the OS/X emulator app, I’ll get back to work on screen printing the front panels. My first two attempts to make a screen failed. On the first one, I didn’t get the screen stretched tight enough. The second attempt failed when I couldn’t rinse out the emulsion – I had probably left it dry too long. Hopefully third time is a charm.

    Version 2.0 of OS/X SCELBI Emulator App Available

    October 18th, 2015

    I’m really excited to announce that this version contains an initial shot of cassette tape support. There are also some cosmetic UI improvements, a nifty application ICON and a memory configuration menu that includes built in MEA EPROM support.

    Release notes and a download link can be found on my emulator web page:

    Working KenbaK-1 Going Under the Hammer in November

    October 17th, 2015

    Auction Team Becker has it listed in the catalog for their November 7th, Science, Technology, Automata and Fine Toy auction. Their estimate is 20,000 to 40,000 Euros. One sold recently at a Bohams auction for $31,250, so I expect that is how the Becker’s came up with their estimate.

    The Kenbak-1 is often referred to as the first personal computer. It was advertised in the September, 1971 issue of Scientific American magazine for a price of $750. Inventor and producer of the Kenbak-1 was John Blankenbaker. He says about 50 of them were sold.

    It’s I/O was limited to front panel LEDs and switches and memory was only 256 bytes. I personally think it was so limited in functionality, that it’s just not that significant of a product. The main target market turned out to be the educational market.

    This machine is one reason why I alway include the word practical, when describing the SCELBI-8H as the first “practical computer marketed to hobbyists and individuals”.

    SCELBI OS/X Application Status

    October 16th, 2015

    My OS/X Macintosh emulator now supports reading and writing SCELBI compatible AIFF audio files, but I have to do some major work on the associated TTY terminal emulator window, so that I can successfully run the MEA editor in emulation.

    A while back, I had introduced 8008 cycle counting and timing on the OS/X version of the emulator, so that I could support SCELBI front panel operations and run it at prototypical speed. This made implementation of the tape read fairly straight forward in that environment. I recently added a configurable memory menu option to switch between an 8H and an 8B configuration and have a built in MEA PROM image that can be enabled, as well.

    SCELBI OS/X Emulator Reading a "tape"

    SCELBI OS/X Emulator Reading a “tape”

    Once I clean up the emulated tape interface a little, I’ll make a new version available for download. The editor part of MEA will still have problems until I fix the TTY terminal behavior, but there are a lot of other improvements in this version that may interest people.

    Once I get the TTY emulation fixed, the OS/X emulator will have practically all the functionality of a real SCELBI 8B or 8H with TTY and audio tape interfaces.

    I’ll look into what it would take to add it to Apple’s app store. I also might work on converting it into a IOS app for iPhone or iPad at some point. I did some experiments with IOS a while back and had basic 8008 instruction execution working, so I know a little bit about what it would take to do that port.

    TTY Chassis Views

    October 14th, 2015
    TTY Chassis Front

    TTY Chassis Front

    SCELBI TTY Chassis Back

    SCELBI TTY Chassis Back

    TTY Chassis

    TTY Chassis

    Inside TTY Chassis

    Inside TTY Chassis

    Here are a couple of pictures of a rare original SCELBI factory TTY chassis. Can anyone find what’s wrong? Hint – the schematics can be found at

    SCELBI Intel Hex to AIFF Sound File Converter Utility

    October 12th, 2015

    This program turns SCELBI 8008 code and data that is in Intel hex format into AIFF format sound files that is readable by the SCELBI cassette interface when played out of a computer or digital music player earphone or headphone out port. Note that the SCELBI only will read continuous data using the cassette read command. At this time, the program does not fill in gaps, so the Intel hex format file must also be continuous.

    The program is a work in progress, and there is some ugliness in the commenting, coding style, etc, but it works well enough, that I figured I’d share it, rather than wait to find the time to perfect it. I put the source in the retro-restore newsgroup files section under 8008 Systems/SCELBI. Be aware that you need to download an AIFF “C” language library from source forge to build it. I had to make a change to the library source to make it work for this application. In function AIFF_ReadOpen(const char *file, int flags) add this code to the end of the function to initialize buffer2 pointer and length.
    r->buffer2 = NULL;
    r->buflen2 = 0;

    Since it doesn’t divide perfectly into normal audio file sample rates, like the Apple II sound format does, the SCELBI’s FSK encoding took considerably more thought to reproduce, which made this an interesting little project.

    It should be possible to use the program as a starting point for other audio format conversions that use FSK or similar encoding methods. See my previous program that converts from Audio to Intel hex format.