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 4×6

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

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.

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

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.

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.

SCELBI I/O Cable

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.

Take a look at these two images of original SCELBI boards.

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.

I’ve been working on writing detailed build instructions for the SCELBI Cassette Boards. I need to do this, because I haven’t found original build instructions. There have been a few challenges with the cassette boards, as the schematics haven’t always matched existing boards. Also, the initial version of schematics I had, were missing some labeling information on resistors and capacitors. I recently obtained images of some later schematics and got to examine a original board that help fill in some of those gaps.

My reproduction was built to match the earlier schematics and a couple of different boards that I had images of. As part of this documentation process I had a chance to compare an original board with my reproduction. Here is a side by side scan of my reproduction next to that original SCELBI write board.

Original and Reproduction Write Card Comparision

You can ignore the circuit in the relay socket. That was put there for debugging purposes, way back in the 70’s. It was basically using LEDs to indicate when the RELAY circuit was activated. The different color substrate is due to evolution of technology. The old green substrate is no longer made, because it fluoresces which affects modern photo based PCB fabrication processes. Some reproduction guys are dying their modern boards to give their reproductions the old time look.

Going through the later schematics and this board revealed a few things.

Here is my current version of the cassette write board build instructions.

8B- Cassette Write board – build instructions

I tried to make them match the style of original SCELBI build documents. Note that I need to investigate a few more things including that reversed 10uF capacitor before finalizing this document.

Here is the artwork I’m preparing for the front of the reproduction SCELBI cassette chassis.

Artwork Front Cassette Chassis

Dimensions are 6″ x 3.5″. I’ll either directly silk screen the chassis or make decals. The crosses are drill points for binding posts and Amphenol connectors. I believe the holes for the binding posts will be 1/2″. The Amphenol connectors also get 1/2″ pilot holes, and then will be punched out with a Greenlee chassis punch.

I tried doing a direct transfer from a laser printer printout, but I’m not satisfied with the result. This is essentially the same process as the laser printer PCB etch method. You print onto glossy paper then apply the image by melting the toner onto the target material. The problem is that with a PCB, you can roughen the surface with sand paper to get the toner to stick. I’m not interested in damaging the surface of the chassis and the surface of the chassis is too slick to make it stick consistantly.

I’m doing a fourth round on the front panel silk screen. The third round failed when I messed up registration of the two layers of transparencies that I used as a mask. I use two layers to ensure a good image. This time, I didn’t have them lined up exactly and I ended up with a nice screen, with a double image towards one end. The good news is that I think I have solved the other problems that I have been dealing with.

Bad Screen

Now that I have basic tape support going on the OS/X emulator app, I’ll get back to work on screen printing the front panels. My first two attempts to make a screen failed. On the first one, I didn’t get the screen stretched tight enough. The second attempt failed when I couldn’t rinse out the emulsion – I had probably left it dry too long. Hopefully third time is a charm.

I’m really excited to announce that this version contains an initial shot of cassette tape support. There are also some cosmetic UI improvements, a nifty application ICON and a memory configuration menu that includes built in MEA EPROM support.

Release notes and a download link can be found on my emulator web page: http://www.willegal.net/scelbi/scelbiapp.html