Mike's Hobby Blog

I just added the correct SCELBI power connector to my reproduction. Note that this was the factory setup. Back in the day, some people integrated power supplies right into the chassis, rather than using an external power supply. The plug is a 86CP4 and the socket is a 78S4. These days, they are kind of scarce. In this case, scarcity equates to price, as they are about 9 dollars each. Check inventory at alliedelec.com for 78S4 and at tedss.com for 86CP4.

Here are some images.

SCELBI Power External

You put the plug in the chassis so the cable coming from the power supply doesn’t have exposed pins.

Inside Power Wiring

The SCELBI manual says put +5 on pin 1, ground on pins 2 & 3, and -9 on pin 4. SCELI assembly directions have you position 1 and 4 at the top of the chassis. This makes wiring to the backplane pretty clean. The slip rings that are used to hold these connectors onto the panel are a pain to get on – be patient and don’t expect the rings to seat perfectly. I chose to connect the I/O port grounds directly to the incoming ground wire, rather than wire back to the backplane. Now I’m thinking I should have run both ground wires to the backplane, rather than join them together and run 1 wire to the backplane. I used 18 gauge wire, so it probably doesn’t matter.

Plugged In

The socket and plugs are keyed, in that two of the pins (positions 1 and 4) are larger than the other two. However, I think if you weren’t paying attention, you could plug this in far enough to make contact and cause problems. Be careful when plugging this unit in.

I have used electrical tape to protect the exposed terminals on the cable coming from the power supply. I need to pick up some of the hoods that are designed to cover these connectors. SCLEBI used gray clad multi-conductor cabling back in the day. I think it is still available, but like many SCELBI components, it is pretty expensive.

This process took me a couple of hours. Take your time to get things right and avoid costly mistakes.

Next up – getting the TTY interface built up and checked out. Other than schematics, and parts placement, we don’t have documentation for that board, so it might take a while.

7/7 is the 40th Anniversary of Nat Wadsworth running the first program on the first SCELBI. Thanks to Mark Arnold for reminding me of this.

Regards,
Mike Willegal

happy 4th of July..

SCELBI app

I think I have front panel controls working now

download the app with this link
Scelbi.zip

cheat – load scelbal and hit the run button to see blinking lights
Scelbal.hex

web page and some notes can be found here
http://www.willegal.net/scelbi/the8008andScelbi.html

Constructive feedback welcome.

regards,
Mike W.

I added some SCELBI comments from mid 70’s hobbyist magazines. Take a look.

I have a new version of the SCELBI/8008 OS/X emulator working that allows you to set the starting address after a reset, but I haven’t yet loaded it to the website. Hopefully I’ll get to that, this weekend.

I have the layout for a reproduction SCELBI TTY PCB close to complete. This is essentially a TTL to current loop interface originally designed for interfacing the SCELBI to a Teletype. The board is about 4.5″ x 3.5″ with a connector on a .156 spaced edge for plugging into to an edge connector with solder tail pins. This is very similar to a “Vector” prototyping board. Schematics and a drawing are available on Cam’s SCELBI website.

http://www.scelbi.com/documentation.html

Here is a photo of an original SCELBI TTY interface board.

SCELBI TTY interface

I’m not sure how many to make. I’m projecting a bare PCB price at $30 to $40, depending upon interest level. Let me know if you have interest in this board.

regards,
Mike W.

As I noted in a previous post, clock stability on my reproduction SCELBI was pretty bad. The system clock slowed down 10 or 15 percent as the system warmed up. This would have played havoc with my bit banged serial communications, but by the time I was able to toggle in the loader, it would be warmed up and fairly stable.

SCELBI cpu clocks

The timing circuit is made up of 4 interconnected 74121s one shots that control rising and falling edges of 2 interleaved clocks. The timing of each 74121 is controlled by a 5K trimmer resistor and a 300 pF cap. Figuring that the cap was part of the problem, I looked for some NP0 300 pf caps to replace the ones that i was using.

What I found was a modern 330 pF cap, Kemet part # C322C331J2G5TA. While I was at it, I decided to replace the 5K trimmers that I had been using since first bringing up the SCELBI. The original trimmers I bought had the wrong foot print and turned out to be a little jumpy while adjusting. I replaced them with Bournes # 3006P-1-502LF.

Though I don’t know which change made the difference, several trials have shown the new trimmers and NP0 caps seem to have stabilized the clock to the point, where I think I can forget about the “problem”.

The only thing I might do at some point, is try to find some old style NP0 caps to replace the obviously modern Kemet ones.