Archive for January, 2013

SCELBI Chassis Prototyping

Monday, January 28th, 2013

Prototype Chassis with backplane installed – top view

SCELBI chassis-top view

SCELBI chassis-top view

This is the result of some hacking of a backplane into a BUD AC413 chassis. The AC413 is the same width (12″) and depth (10″) as an original SCELBI chassis. However the BUD AC413 is 3 inches tall, while the original SCELBI chassis 3.5″ tall. During this prototyping/hacking effort, I found out a few important things.

  • Cutting aluminum is not hard, but it’s best to drill holes with a jig or at minimum use a punch to mark the center of the holes in the aluminum. I eventually used one of the scrap backplanes from the first delivery to create a jig for drilling holes.
  • On a “real” SCELBI chassis, the cutout for the edge connectors fits very tightly around the perimeter of the edge connectors. Making the cutout any larger than necessary, makes it very difficult to locate the support screws on the sides of the backplane. The edge connectors extend 8 7/8″ left to right and 5 5/8″ front to back. The backplane is 9.5″ wide and if the hole is made 9″ wide, that leaves only 1/4″ overlap per side to drill a 9/64″ hole for the #6 mounting screw. If possible, it would be best to cut the hole for an exact 8 7/8″ side to side fit. Though it doesn’t look like it, my prototype left too much space between the edge connectors and the chassis, making fitting the side mount screws a challenge.
  • The cutout starts 1/2″ from the front of the chassis and is centered between the edges.
  • SCELBI Chassis Bottom View

    SCELBI Chassis Bottom View

    A view from the bottom.

  • Screws holding the edge connectors/card guides have both a washer and star washer, but no lock tight on the original SCELBIs. I don’t have card guides and have used 1/2″ #6 machine screws.
  • Screws holding the backplane to the chassis have neither a washer or a star washer, but a held in place with red Locktight on original SCELBIs. I haven’t used Locktight, because I want to find shorter screws and I have more work to do on the chassis with the toggle switches and I/O connectors. The 3/8″ screws are a bit too long on my reproduction. A 1/4″ screw might be a better match to the original.
  • The screws at the front of the backplane do no go through holes in the backplane, but the edge of the nuts hold the backplane in place.
  • When soldering the edge connectors, only the pins with connections and pads are soldered.

    One more thing – I positioned one of the holes for the edge connector mounting screws 1/16″ too far back. This hole will need to be drilled out to properly fit a #6 screw. However the error is so small that the enlarged hole cannot be seen under the nut of the screw or the ear of the connector.

    An insulated stiffener running down the center of the board between the connectors would have been a good addition to this design.

    Stacks of SCELBI PCBs

    Monday, January 28th, 2013

    Figured I’d share a picture while I still had a pile of them

    Stacks of SCELBI Boards

    Stacks of SCELBI Boards

    SCELBI CPU clocks

    Saturday, January 26th, 2013

    SCELBI clocks

    SCELBI clocks

    update: this clock setup is incorrect!, the second clock starts too late and last too long. It must finish before the first clock rises.

    The clocks are adjusted to run at 500 KHZ. Note that the build manual mentions that clocks are supposed to be self starting, but may not start with some user power supplies. Sure enough, I’m seeing this issue with my bench supply. It is easy to kick start by shorting both ends of electrolytic cap C9 (located near trim pots). Note that adjusting these clocks takes a bit of twiddling.

    SCELBI CPU card under construction

    Saturday, January 26th, 2013

    Here it is, without the CPU socket, which will be made up of old fashioned Molex strips of pins. 7400 series parts are soldered directly to the PCB, without sockets. I will be socketing SRAM parts on the SRAM board, when I get to that.

    SCELBI CPU under Construction

    SCELBI CPU under Construction

    I’ve been following the old SCELBI documentation which can be found here:

    The documentation is actually quite good, which saves me a ton of effort in writing my own. However, I’ll share a few additional tips here.

  • The CPU card contains both 7404 and 74L04 parts. Don’t mix them up like I did. 🙂
  • There are 8 1K resistors
  • Actual boards that I have seen, use 3K instead of 3.3K resistors
  • The 5K trimming resistors have the pin that is offset from the other 2, offset by 10 mills. Bourns offers two spacings, either 20 mills or 10 mills. Somehow, though I knew the difference, I ended up with the wrong version. I was able to bend the pin under to fit, for now, until I am able to replace them with the correct part.
  • The are 8 1N914 diodes and 2 zener diodes. The document only mentions 9 diodes.
  • I’m using modern 6.2 volt and 12 volt, 5 watt zener diodes for the over voltage protection circuit. Actually, I don’t have the 6.2 volt ones on hand, at the moment, so they will be installed later. The leads on the 12 volt zeners are too big for the holes, so I drilled out the hole with a 3/64″ bit mounted in a pin vise. I drilled in a bit in from each side, rather than straight through from one side, so I didn’t risk delaminating the pads. This effectively removes the plating, so I soldered the end that had a trace on the top side on both sides of the PCB. This isn’t noticeable.
  • After soldering, I clean the flux from the board with isopropyl alcohol.
  • I obtained parts from, and It is possible that Unicorn may be able to supply complete parts kits in the future, as I am going to provide them a complete BOM, once I get the prototype working.

    batch of custom chassis

    Friday, January 25th, 2013

    Cory is working on getting a batch of custom Bud chassis made for the SCELBI and needs a head count.

    Here is the information that Cory has on this custom build

    I have gotten a quote from Bud on a custom chassis to match the measurements we have of the “factory” one.

    The chassis will still require drilling for holes, but will be 3.5″ and will have the cutout for the backplane. I didn’t want to have the holes cut because I’m not sure exactly how that will turn out. But holes should be easy with the correct bit and hole saw (cheap and easy enough).

    The cost is around $43 for each plus shipping for a quality of 10. Since I have to order them together figure with shipping of the 10 to my home and then shipping it back out, the final cost should be around $60 each. I’m in for 2 plus 1 for the MARCH museum at InfoAge in NJ. So that is 3 out of the 10. If we order 20, the cost is about 4 dollars less per unit, so I’m not too concerned about making 20. But 10 would be the minimum order.

    Who else is in and how many do you want? I’d like to place the order early next week. Also for the front panel I just need some final double checking of measurements so I can double check the front panel, they will be around $100 plus shipping in anodized blue with engraving/white paint for the lettering. I will also be placing orders for some without engraving so that we can use a home silk screen kit to make the lettering. When I’m ready to order just let me know which type you want.

    send an email to to get on the list.

    SCELBI PCB beta/introductory special sale

    Thursday, January 24th, 2013

    Beta sales of SCELBI boards sets is open from now through February 1st. The special beta/introductory price for a board set during this beta period is $225.

    The board set includes:
    1 Front Panel PCB
    1 CPU PCB
    1 DBB PCB
    1 SRAM PCB
    1 Backplane PCB

    You can optionally add an untested 8008D microprocessor for an extra $25.

    Shipping within lower 48 US states is free – shipping elsewhere is $35. Shipment will occur around February 2nd.

    Only 1 SRAM board allowed per set in this initial sale. Once system operation is confirmed and I get an idea of amount of interest, I’ll get another batch of SRAM boards made and will offer them for sale.

    Like the original SCELBI, all boards are two sided with plated holes and a 2 OZ copper layer. There is no solder mask or silk screen.

    Chassis mounting holes for the backplane PCB were purposely left off, in order for the builder to precisely drill holes to match holes in the builders particular chassis.

    At this time, I only know of one minor issue with the board set. That is that the holes for the over voltage protection zerner diodes were not made big enough for some higher wattage zerner diodes that have thicker leads. The easy fix for this, is to drill out the holes and, where necessary, solder from both sides of the PCB to ensure connectivity between front and back sides.

    During this special pricing period, PCBs and the 8008 should be considered untested prototypes that might possibly require some rework to be made to work correctly.

    Find more about the SCELBI, bill of materials, reference information, progress in bringing it to life, by following my 8008 blog category at

    Please send an email to for payment information.

    jOBs Movie Stills Posted

    Thursday, January 24th, 2013

    Mimeo owners might recognize what Kutcher/Jobs is holding in the first photo.

    SCELBI edge connectors

    Thursday, January 24th, 2013

    Here is an close up image of an original SCELBI backplane. Note that though wire wrap wire was used to wire the backplane, that the wires were soldered to the edge connector tails.

    back backplane - original sockets

    back backplane – original sockets

    Here is an image of one of my (faulty) backplanes, with a current production edge connector.

    back backplane - my sockets

    back backplane – my sockets

    So far, this is about the best match that I’ve been able to find for the original edge connector pins. I couldn’t find any exact matches at any of the surplus vendors. The part number I’m using is EDAC 305-044-520-202. these are in green and I found then in stock at As far as I know, all production SCELBIs used black connectors, but I couldn’t find the black version of the EDAC in stock, anywhere. They would have to be special ordered- part number for the black version of the EDAC connector is 355-044-520-202. In case you are wondering, pin spacing is .156″, row spacing is .140″. The holes in the backplane are .054″.

    Be aware that there many .156″ pin spaced edge connectors found these days have a row spacing of .2″. These will not fit well into a SCELBI backplane, as it was designed for .140″ row spacing.

    SCELBI front panel turn on – and some parts suggestions

    Tuesday, January 22nd, 2013


    It looks real pretty lit up like this. I still need to straighten out the LEDs and cut off the leads from the back side. I simply connected a +5 volt supply to +5 and ground connections on the edge connector.

    The three lights that are not on are -9 volts, run and stop. Run and stop are not driven by on board drivers. The other lights are driven by on board buffers. The inputs float high, so the LEDs come on, even with nothing attached.

    In this configuration, the board is drawing about 220 milliamps.

    The 6.3 volt zener which is used for overvoltage protection, is not installed.

    Based on the description in the assembly guide and the early flyer picture, the LEDs originally used in the very first units were mounted in metal case. Here is a link to an image of this type of LED.

    And here is a link to the flyer with a SCELBI with this type of LED.

    Later SCELBI’s had the same type of T1 3/4 package that we commonly see today. They are red in color and have clear lenses. All the surviving SCELBIs that we know about use this common LED. The LEDs I’m using are part number L05RWC from and seem like a good match for the originals.

    The fuse holders are Littelfuse 102074 which can be found at and elsewhere.

    SCELBI almost, but not quite…

    Monday, January 21st, 2013

    Almost there, but not quite – the backplane was printed reversed on the bottom side, so I need to get it redone. I accidentially sent the PCB vendor a reversed image for backplane bottom, and they blindly made it backwards. I need to get a new batch of backplane boards made before I can take money or ship. The PCB vendor have generously offered to redo and expedite the replacement backplane PCBs.

    The good news is that everything lines up nicely, with no issues. The other cards pass the visual inspection and mechanical fit tests.

    reproduction SCELBI mock up

    reproduction SCELBI mock up

    we are getting close, stay tuned for updates …

    – Mike W.