Mike's Hobby Blog

After watching on ebay for a very long time, I finally obtained a copy of the March 1974 issue of QST magazine.

And on page 154, here is the small add.

I could probably have obtained this issue much sooner, if I had been willing to purchase a entire years worth of issues.

Nothing about the SCELBI is straight forward. The cutouts on the back of the chassis for the 78S type connectors is another example of this.

I puzzled over how to cleanly accomplish this for quite a few hours. Note that the power connector is a 86CP4 plug, so we are talking about 14 holes for the I/O ports and 1 hole for the power connector for a total of 15 holes.

After much research, I finally happened upon a tool that was expressly designed to punch out these holes. The solution is a tool made by Greenlee called a radio chassis punch. The size you need for 78S type sockets is the type 732, sized 1 11/64″. They appear to sell pretty frequently for under $50 on ebay, but I happened to find a NOS one on a nearby distributers shelf for under $25.

Here are some example holes made by the punch. There is a 78S socket in one hole and the punch in another. You need to drill a 1/2 inch pilot hole before you can punch out the final hole with this tool.

One thing I did to make punching out my chassis more accurate, was to create a pattern based on a 12″ by 3″ chassis back panel. I determined that the spacing between the 1 1/4″ diameter connectors is only 1/8″, so your layout has to be pretty accurate. My BUD AC413 chassis is only 3″ high, so you will have to adjust vertical dimensions a bit for a SCELBI 3.5″ high chassis.

The next step is to install all those sockets and wire them up.

I used the memory test in the user manual to test each page in each slot in the system and it appears fine. This test is pretty basic, but considering that it had to be toggled in each time I moved the memory card to a new slot, I’m considering it good enough. I did not check multiple memory cards together, but I’ll assume that Nat and others did that back in the “old days”.

Power consumption for a fully populated memory card compared to only 1 bank (8 chips) populated is a little over 2 amps versus 1.5 amps on the 5 volt rail and a little over .5 amp versus .3 amp on -9 volt rail.

I still need to checkout I/O ports and that may take a bit, as I still have to hand wire the chassis for the 78S11 connectors used for I/O.

Without card guides, the cards in my reproduction SCELBI are not as stable as they should be. They will rock back and forth a bit, even if just gently touched.

I just received a set of card guides for my reproduction SCELBI 8H. I installed the first pair on the front panel slot and they fit fine and hold the card in a very stable fashion.

The only cardguides that I could find that are still in production, are model BR27D from vector electronics.
http://www.digikey.com/product-detail/en/BR27D/V2027-ND/1886457

They are expensive, but at least they come in pairs. The screws for these guides appear to be size #4, not #6, like I had expected were used on the original SCELBI computers.

This demo can be found in the user manual. Took a while to toggle it in, as the front panel controls are pretty quirky. Once I got the program loaded in correctly, the system seems to run fine.

These days, this might not seem like too big of a deal, but in 1974, a SCELBI owner was likely to have the only “personal” computer in town. Running any kind of program in your own house would be a huge deal. Personally, I had to show it off to my family, so it still is a big deal to me in 2013!

I started work on putting together a cheat sheet, as the SCELBI user manual is kind of wordy, yet still is missing some important concepts, that I had to learn about, the hard way

I still need to check out memory addressing for the entire 4k memory range and I/O. This might take a couple of weeks. After that I’ll open up board sets for general sales.

preview my next (video) podcast to see first ever power on and the SCELBI equivalent of “hello world”

http://www.willegal.net/podcast2-2-13.mp4

Here is the SCELBI input card – ready to go. The SCELBI instructions were fine. The 65 10K ohm resistors take a while to solder in, but otherwise it is a very straight forward process. It draws around 200 milliamps in static, standalone condition.

Next up is 1 bank of 8 SRAM sockets on an SRAM card. Soon after that, I’ll be manipulating toggle switches!

Since the SCELBI was sold as assembled, kits or bare boards, what you do about sockets is largely up to you. However based on observations of original SCELBI mini-computers, here are some suggestions to make your SCELBI more authentic.

Suggestions for the Front Panel, CPU, DBB and Input Boards:
Many of the boards that I’ve seen, use the Molex strip connectors mentioned in an earlier post for all ICs on the board. SCELBI instructions only mention using these for the 8008 and soldering the rest of the chips directly to the PCB. I have seen one original system set up this way. Since the 8008 is the only rather expensive chip used on these boards, I have chosen to follow instructions and solder all parts to the PCB, except the 8008. If I need to pull a 74xx part, I’ll cut the legs off and remove the legs individually. A friend of mine managed to obtain a roll of the MOLEX pins and I installed those for the 8008. They are rather fidgety to install, and I suspect reliability might be an issue. Even if you can find enough stock to do an entire system, I wouldn’t recommend doing a whole system with those MOLEX pins. I have also seen one 8B that used regular 70s style closed frame solder tail sockets. You should be able to find a bunch of older closed frame sockets at surplus dealers, if you want to socket your boards. I haven’t seen any SCELBIs with machine pin sockets.

Suggestions for the Memory Cards
Though the instructions don’t call for sockets, all the memory cards I have seen, use either MOLEX strip sockets or old style closed frame sockets. Apparently memory was expensive enough and perhaps failed often enough in those days that soldering in memory was not considered wise.

Example Closed Frame Socket Used on SCELBI Memory Board.

I believe this one is made by SCANBE. SCANBE’s were also used in arcade machines and have a terrible reliability according to arcade system restorers. I’ve seen one page that suggests replacing all SCANBE sockets before proceeding further.

More on Molex Strip Sockets
Here is a page from an old (1973) MOLEX M-100 catalog