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

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

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 onlinecomponents.com. 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.

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.
http://www.wired.com/gadgetlab/2012/10/the-history-of-led/

And here is a link to the flyer with a SCELBI with this type of LED.
http://www.scelbi.com/files/docs/advertisements/Flyer.pdf

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 led-switch.com and seem like a good match for the originals.

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

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

we are getting close, stay tuned for updates …

– Mike W.

No work today due to MLK holiday, I’m like a kid waiting for Santa…will send update to blog with early assessment and details on early adopter program later on…

As I layed out the PCBs and researched things SCELBI, I discovered several things that other SCELBI builders should be aware of. I fully expect that other things will crop up, like they did during my Apple 1 and Apple II builds, but here are a few things that I know about already.

+5 volt wire on DBB board

From the SCELBI CPU assembly manual….

Molex pins to make socket?

Anyone heard of this technique?

Please send an email to mike@willegal.net with a description of the exact molex parts to use, if you have.

Boards are expected to arrive next Monday. I’m still weighing details of the early adopters plan, but I’m leaning along these lines.

Assuming that board pass the eye test and some basic mechanical checks, I’ll probably accept payment for a shipment on the next Saturday or Monday after board arrival for those that want the best price, and are willing to risk untested HW.

Then I’ll ship a second wave after two more weeks, still with reduced pricing, but not rock bottom pricing.

After that, I’ll go to a standard price that I can live with going forward.

Just to give you an idea of pricing range I’m thinking about is in the range of $200 to $300 for a six board set, with no components.

SCELBI computer sold kits, as well as build up computers. Be aware that building a chassis for the SCELBI 8H can get surprisingly expensive, depending upon how close to a production system that you want to make it. For a bare bones setup kit type setup, you’ll need 10, 44 pin edge card connectors, and +5v and -9v power supplies, plus some switches and a chassis of some sort. You will need wire to connect the power supply and front panel switches.

For a more authentic “production” system, SCELBI used 14, 78S11 relay sockets for I/O and a total of 16 edge card slots. Also, each slot had card guides in production systems. The power socket is a 78S4 and the plug is a 86CP4.

The chassis is similar to a BUD AC-413, only 3.5″ high, instead of 3″. Cory is looking into the cost of getting a custom chassis made that is a close reproduction of the SCELBI original, along with an anodized blue front panel.

I’ve ordered a set of edge card connectors – once I confirm that they will work, I’ll post the part number. Also, I believe I have identified the correct card guide, but need to confirm that before I publish the number.

I think that all existing SCELBI 8H documentation, including assembly manuals and schematics can be found online at one of the following sites.

http://www.scelbi.com
http://bitsavers.trailing-edge.com/pdf/scelbi/
http://www.olson-ndt.com/Scelbi/

regards,
Mike W.

Experience has shown me that while recreating or restoring vintage computers you frequently encounter unexpected “vintage” problems. Some of these problems were pre-existing and have been forgotten long ago, by the original designers and owners. Other are related to using very old equipment that may have degraded or failed, while sitting on the shelf.

A few months ago, Cameron Cooper was discussing a problem of this type with me. It was related to a SRAM failure on an original SCELBI 8B that he was trying to get back into operating condition. What was most difficult about this debug effort was that the SRAM card could not be debugged with a scope very well, because it was buried between other boards. What he needed was a extender card. I knew I would also need one at some point in the future, so I designed one and sent Cameron the Gerber files, who had several made. Laying out this card only took a couple of hours. The hardest part was finding the correct edge connector, which Cameron managed to do.

Here is the result:

SCELBI card extender

There are other options, that could have been pursued. The SCELBI was originally built on pairs of 4.5″x6.5″ vector cards, mounted side by side. These cards are still available and Vector even sells an extender card.

http://www.vectorelect.com/Product/Extenders/3690.htm

A couple of these cards mounted side by side should work as a SCELBI extender. The main downside of this approach, is that these sell for around $45 each at Digikey, making the cost about $90. This is why we ended up making our own. With the connectors, our cost was about $60 each. The interesting part of this approach is that if we ever need a 4.5×6.5 vector board extender, we only need to cut the SCELBI extender in half.

There is a third option. That is connecting extension wires to the signals of interest on the board under test that extend to the outside of the chassis where you can pick up the signals with your test equipment.

There was one other advantage to making this extender card with Cameron. He was able to check dimensions of the finished board against his original SCELBI, confirming that I had it right.

Attached is a scan of the original SCELBI bill of materials for the boards and a consolidated version in excell format. I forget where I got the scan, and apoligize in advance for not giving credit to the person that did this work. I haven’t checked for accuracy, but it “feels” right.

The hardest part to find is the 8008, though there always seem to be a few on ebay (at crazy prices). I plan on offering a bundled 8008D as an extra cost option with PCB sets. The D part is a package type, not a speed option. Most original SCELBIs had C package type, but they are harder to find (and more expensive). American Microsemiconductor has thousands of 1101s in stock at good prices (under $4). I have some extra 1101s, and may offer a bundle of 8 as an extra cost option. 74XX parts are available from unicorn electronics in PA.

Each memory board supports up to 4 banks of 8 chips each bank, for a maximum total of 32 chips per board (1K of memory). You can add up to 4 memory boards into a 8H for a maximum of 128 chips and 4K of memory. The minimum is 1 bank of 8 chips on 1 SRAM board for a total of 8 chips and 256 bytes.

You need 6K of memory to run SCELBI BASIC, so it is not possible on a normal 8H. I’ve already started development of the additional/different 4 boards needed for a SCELBI 8B, which can run SCELBI BASIC. The 4 additional boards are a different backplane, 4K SRAM board, memory expansion board and a PROM board. The CPU, DBB, INPUT and front panel are exactly the same for both 8H and 8B systems.

I’m currently looking at edge connector options and will post information on that and other chassis parts in a few days. The edge connector is .156 pin spacing and .140 row spacing, same as Apple 1, though you want ears and longer pins, if possible. Some wires are connected to the edge connector from the back side of the backplane, which makes longer pins desirable.

SCELBI Parts List

BOM for SCELBI-boards

Here is an interesting bit of trivia – not including the chassis components, the 8 boards making up a fully populated 4K SCELBI 8H require 587 individual components compared to 221 making up an Apple 1 motherboard populated with 8K and 278 making up a 48K Apple II, rev 0.

I’ll bet that some of you thought that this would never happen. I just ordered a batch of six different PCBs that will enable building the SCELBI 8H. I started on the first board about 15 months ago. Comparing to the Mimeo, which I did in 4 months, this has been a huge task.

CPU
DATA BUS BUFFER
INPUT
FRONT PANEL
1K SRAM
BACKPLANE

I plan to offer a limited number of prototype sets at a significant discount for people that have built up one of my other boards in the past. These will be untested boards that will be shipped to you as soon as I get them in, before I have done complete bring and test, myself.

I’ll announce a price in a few days.

Now to get my act together and get the rest of the components that I need for bringup.

Stay tuned,
Mike W.