Archive for the ‘8008’ Category

SCELBI 1702 EPROM Contents Troubleshooting

Sunday, November 22nd, 2015

I’ve recently encountered some issues assembling the small program that I use for checking out MEA on my SCELBI 8B. 1702 EPROMs are notorious for “forgetting” data and my first thought was to verify the contents of those EPROMs.

Since the cassette write function still worked, it turns out that the verification process is pretty easy. Here is what I did.

1) I captured the contents of the MEA proms (060-000 to 077-377) to an audio AIFF file on my PC by connecting the audio output of the SCELBI to the mic in port of the PC. This assumes that the MEA cassette driver/utility to write the data out still works.
2) I converted the audio file to an Intel hex file using the utility that I had previously written.
3) I used an UNIX shell program named “diff” to compare this new INTEL hex file with the one that can be found at

What I found, was a couple of errors in the EPROM at page 73. I removed this EPROM from the PROM board, reprogrammed and verified it. After reinserting the EPROM and board, assembly of my little test program, once again, worked as expected.

As far as part 3 goes, I actually did a couple of additional verification steps by loading my OS/X SCELBI emulator with the Intel hex image converted from audio tape format and verifying that assembly of my test program also failed in that environment. I then dumped contents of suspicious memory locations with my emulator loaded with a good version of MEA and compared to dumps executed on the real SCELBI to confirm that those locations really had problems. Those extra steps aren’t strictly necessary, but helped confirm the issue, before I started the process of pulling boards and chips.

Note that I didn’t erase or use a new EPROM in this repair, but simply overwrote and verified contents of the existing EPROM with bad data. It will be interesting to see if these locations act up again in the future.

The whole repair process, including the extra verification steps and updating my ME1702 programmer with new firmware, probably took an hour or so. This was one of the easier vintage repairs that I’ve experienced, at least in recent memory.

SCELBI Keyboard Interface Design Notes

Saturday, November 21st, 2015

First of all, much to my surprise, the layout for a reproduction SCELBI keyboard interface is nearly complete, well ahead of schedule. I’m not sure when I’ll pull the trigger on getting boards made, as I think it really needs to be used in conjunction with the oscilloscope interface or something similar.

Using the interface with many vintage keyboards may present a bit of a problem, as it was designed to interface with the keyboard design in the April 1974 issue of Popular Electronics magazine. The basic interface design is described in the first SCELBI Computer Digest, which can be found at Cameron Cooper’s website. The PE keyboard is unusual, in that it is ASCII decoded, but does not latch data. This means that data is only present while a key is actually pressed, or shortly thereafter (the keyboard circuit has a RC network that will hold data for a little while). This is why the SCELBI keyboard interface has latches incorporated into it.

Since there is no latching of data on the PE keyboard, the actual latch mechanism on the SCELBI interface is triggered whenever data is actively being sent by the keyboard. This will not work with keyboards that have onboard latches, since they present data to the interface all the time. Some kind of modification with have to be made for interfacing with those kinds of keyboards. This is not a huge problem, but will require some additional thought.

Of course, a more interesting solution would be to build a reproduction PE style keyboard to go with this interface. I couldn’t entirely rule out the possibility of taking on that project sometime in the future, but first I’m going to finish the few remaining SCELBI boards that are begging to be built.

Cassette Tape Rub On Lettering

Friday, November 20th, 2015
Cassette Rub-on Lettering

Cassette Rub-on Lettering

Though I still have to add a protective clear coat, it looks like a successful trial. You can get your sheet by contacting at Since the film/plate is already made, the price should be $32 for a 4×6 1 color (black) sheet. You will have to tell him that this is the same sheet that Mike Willegal had made. I can’t make guarantees, but I think this sheet should have enough transfers to make a couple of complete SCELBI systems.

After trying a couple of different items for burnishing the transfer, I ended up using a cheap ball point pen.

New TTY Enclosure

Sunday, November 15th, 2015
New TTY Enclosure

New TTY Enclosure

After I clean the leaves off of the yard, I should be able to finish the TTY enclosure. I have been wiring the edge connectors first, labelling the wires with port and pin. After the edge connector is wired, I can put it in position, cut wires to final length and solder to the input and output ports. I think this works pretty well.

With this enclosure, the port for the TTY device exits from the reverse end. Here is a view of bundle of wires that will be routed out that end of the enclosure.

TTY Enclosure Reverse View

TTY Enclosure Reverse View

Cassette Interface Enclosure

Wednesday, November 11th, 2015
Cassette Interface

Cassette Interface Hooked Up

I have my reproduction cassette boards mounted in the enclosure and functioning. I still have the adjustment and read LED holes to drill and lettering to do. I have rub on lettering ordered and should have it in about a week. If it works out well, I’ll provide ordering information from the vendor, and you will be able to order directly from them. I made use of some white space and added lettering for keyboard interface, power supply and oscilliscope interface to the sheet.

A couple of hints on construction/assembly and wiring of the cassette chassis. The brackets are mounted about 2/3″ back from the end of the chassis. It’s a pretty tight fit in there. Mount the edge connectors on the brackets, but don’t connect the brackets to the chassis. Connect wires to the edge connectors. Then feed the wires through the holes in the panel and cut to length and solder to the connectors. Power and ground lines all go to the power and ground connections on the edge connectors. Power and ground of the two edge connectors are bussed together.

The cable to the cassette tape recorder is only four wires. They are write, read, and two ground wires. Both ground wires connect to pin 11, read is pin 1 and write is pin 2. Tie-wrapped to the outside of this cable are the 2 wires for the motor on/off control. I think that SCELBI might have used the same 4 conductor cable that was normally used for connecting the power supply to the main chassis for this application.

I’m still working on perfecting my silk screen making technique for the front panel. I’m determined to get to the point where I can get near professional quality results. Unfortunately I still have some learning to do.

Lettering Woes

Saturday, November 7th, 2015

I need to take another shot (fifth) at the SCELBI front panel screen. I should have time this weekend for this attempt. The last failure was due to the transparency and screen not being perfectly flat, so the exposure was not sharp.

Decaling the Tape Enclosure failed in two ways. First failure was due to the toner in my laser not transferring well to the decal film. I made an attempt at using the result anyway, just to see how a good decal would look in this application. It went on OK, but the decal melted when I tried to clear coat with a heavy coat of clear satin lacquer. I think a lighter coat of testers gloss coat would work OK. In any case, I’m exploring getting dry transfers made, which would be more authentic anyway. I made up artwork for a double set of transfers for the Cassette Tape, TTY, and the back of the main enclosure and sent it off for a quote.

Decals/Transfers 4x6

Decals/Transfers 4×6

Cassette Enclosure Progress

Tuesday, November 3rd, 2015

The front has been drilled and punched. Note how I used a printout of the drawing as a template for drilling. I still need to drill a hole on the bottom for visibility of the read board LED and one on the back for access to the read board adjustment pot. I’ll do that after I get the edge connectors installed to make sure the holes line up with the board correctly.

Cassette Enclosure Progress

Cassette Enclosure Progress

The binding posts that I ended up with fit nicely into a 7/16″ hole, though I’ve seen drawings for others that take a 1/2″ hole. I’m not happy with my workmanship on the edge connector brackets and am going to make another set before installing. One of the downsides of using Amphenol connectors is that once you solder in the wiring, you cannot remove the connectors from the panel. Since the decals will be clear coated, I need to make and install decals, then clear coat the front of the enclosure before installing those connectors.

To save time, I’m going to reuse most of my original, temporary edge connector wiring by cutting the wires to an appropriate length. I will have to remove the connector ends of the wires from the connectors. Then it will be a simple matter of reconnecting the shorter wires, this time with the connectors mounted in the enclosure. I might redo the power and ground connections so both sides of the edge/connector and board get connected.

Expansion Chassis Bracket Dimensions

Saturday, October 31st, 2015

Here is a drawing with dimensions of the “L” brackets used to hold the edge connectors in the expansion chassis. Note the .75″ spacing of the cassette cards. That is a pretty standard dimension that was used for card spacing back in the old days. The backplane uses the same spacing. SCELBI used #4 screws and bolts with no washers to hold these in place. I’m thinking of using 1/8″ rivets instead. A machinist chart shows the clearance hole drill for a #4 screw is from #32 (.1160) to #30 (.1285). I think a 1/8″ drill bit would do the trick.

Expansion Chassis Brackets

Expansion Chassis Brackets

Since I have laser printer decal paper on hand and a laser printer, I’m going to try to make the lettering with decals. If it doesn’t work well, I’ll try getting some custom dry transfers made.

SCELBI I/O Cable Fabrication

Wednesday, 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

Monday, 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.