I’ve been working on moving my SCELBI 8B to one of Corey’s custom chassis. Here is what it looks like at the moment.

8B Chassis

I figured I’d share some SCELBI chassis preparation tips here.

1) To prevent rub marks, use masking tape to protect any areas of the sheet metal that may experience contact with your tools.

2) Keep in mind that during this period, American engineers exclusively used imperial dimensions.

2) If you are converting a BUD AC-413 chassis you must first cut out the backplane area and drill holes for the I/O ports. If you have one of Corey’s chassis you can skip to step 6.

3) I recommend, making patterns for both top backplane opening and rear I/O connectors. Then attaching the patterns to the chassis when making the cuts.

4) The hole in the top for the backplane is set back 1/2″ from the front and centered from side to side. The dimensions Corey used for top cutout of the 8B is 6.375″ deep by 8.875″ wide. The 8H is 1 slot shorter in depth. Slots are spaced by .75″, so 8H would have .75″ less depth. 6.375-.75 = 5.625 There isn’t a lot of room for error when mounting the backplane, so try to be precise as possible. If done right, the edge connectors will just fit through the opening, when the backplane is mounted underneath. If you have doubts, you can add the edge connectors to the backplane and measure the area needed to fit them and work from there.

5) The I/O holes in the rear can be punched out with a type 732 GreenLee chassis punch, sized 1 11/64″. Be aware that type 732 comes in different sizes. Prior to punching you need to make a series of 1/2″ holes to mark the center of each opening. These holes are on 1 3/8″ centers. There is more on these dimensions on this blog posting.

6) Once you have pilot holes for the I/O connector and the backplane opening cut out, you can punch out the final I/O connector holes. When using the punch, the cutting edge should be on the outside of the chassis, so you don’t mar the exterior surface. Don’t mount the I/O connectors until all cutting on the chassis complete. I don’t know if it makes a difference, but I lubricate my Greenlee cutter with spray silicone lube.

7) If you have a front panel, you can use it as a guide for the toggle and push button switch holes. Simply clamp it to the front of the chassis. There should be even overhang on the sides and the bottom of the chassis should be flush with the bottom of the front panel. The holes for the toggle switches are 1/4″ and the push buttons are 9/32″

8) Mounting the backplane is one of the most critical aspects of building a SCELBI chassis. Solder on all your edge connectors before proceeding, but do not mount the card supports.

9) Note that the current version of my 8H backplane does not have predrilled mounting holes, so you will have to drill holes through both the backplane and chassis together, instead of using the backplane as a guide.

10) In order to drill the backplane mounting holes, position the backplane on the underside of the chassis. The edge connectors should protrude through to the top of the chassis. I use card stock to shim each side of the backplane so it is centered and doesn’t move. I use #6 screws to mount my chassis, so I used a 9/64″ drill bit. Don’t confuse the holes for the card supports with the chassis attachment holes. For the 8B, I start by using the backplane as a guide to drill 4 holes in the chassis. These holes are the ones on each back corner, and the ones on each side closest to the front.

11) I then move the backplane to the top of the chassis and use the holes I just drilled to correctly locate the backplane. Put 4 screws through the 4 holes you just drilled to keep it in position. If you have an 8B, you can use the backplane as a guide to drill out the remainder of the chassis support holes.

12) Note that the support of the front edge of the backplane is not by holes through the backplane, but by the nuts of 7 supporting screws which overlap the front edge of the backplane. These 7 holes must be located at the edge of the backplane, so that one edge of nuts will hold the backplane in place.

13) Any burrs left on the holes can be scrapped off by twisting a sharp, large bit in the center of the hole. Do this by hand, not in a power drill. The aluminum is soft and it doesn’t take much to remove these burrs.

14) Remove the backplane before you add the Amphenol I/O and power connectors. The rings that hold these on, do not seat completely and it is an exercise in frustration to try to make it so.

15) Now that all the holes are drilled, mounting the backplane to the bottom of the chassis is pretty straight forward. Just don’t over-tighten the nuts or you could crush the substrate.

It should take a little less than two weeks for them to arrive. During that time I’ll continue work on moving my 8B system from a temporary chassis to one of Corey’s custom chassis.

I started on the read card in December, so these two cards took about 7 months to do. I guess I might be slowing down a little bit. On the other hand, I’ve had a lot of other things in progress at the same time.

Assumming that these two boards work out, this leaves only the keyboard and the 2 oscilloscope cards left to do to complete the entire SCELBI board set. The keyboard and scope analog cards should go pretty fast, but the scope digital card is a fairly complex double wide card, so it will probably take a while.

I’ve started working on building up a second set of the “core” SCELBI boards. I call the boards that are shared between the SCELBI 8B and 8H, the “core” boards. These are the front panel, the cpu, the DBB, and the input boards. When I built my prototype 8B, I didn’t bother to make new “core” boards, I just borrowed them from my 8H. Now that I have a new set of the “core” PCBs in stock, I figured that I would make up a second set, so that I could run both the 8H and 8B at the same time.

As part of the building of these new boards, I figured I’d use the 8B manual that I am in the process of digitizing. Yesterday, after completing the front panel board, I discoved that all my LEDs were installed backwards. The manual states:

The anode of the L.E.D. (shorter lead) goes in the top most hole (furthest away from the card connector). The cathode of the L.E.D. (longer lead) goes in the bottom hole.

This is actually the reverse of how LEDs are made – the cathode has the shorter lead, not the anode. I didn’t encounter this issue when building the board for my 8H, because the 8H front panel instructions were written for the old style LEDs in the cans, so I had to figure out the right way to connect them without benefit of the SCELBI manual. You would think that with all the projects that I’ve done, that I would remember how LED’s are constructed and connected. However pinouts are the sorts of details that are easily found in reference material, so I don’t put any effort into memorizing pinouts of anything.

As far as the newly digitized manual goes, one of my intentions is to add content and clarification when it is missing, but maintain the “character” of the original manual. I think, in this case, I will correct the text and add a footnote that explains how the original manual was wrong.

Verifing the layout against schematics, is an important step in my reproduction process. My usual method of schematic/layout verification is to print a copy of the schematics then mark each trace as I verify the connection with a marker. This time I did it all digitally using photoshop as a replacement for the paper.

Tape Write Schematic Markup

The blue layer notes omissions, corrections or clarifications to the schematics. Most of these are due to the mistakes made in the initial rev of the board, which were later corrected. The version of the schematics I have access to, does not have those changes incorporated into them.

During this process, I did find a number of traces that were hidden under chips. Omitting these traces is an easy mistake to make. I missed one of these on the memory expansion board. This time, I am reasonably sure that I found them all.

The release of a new version of my OS/X SCELBI/8008 emulator was motivated to provide support of the Modified Creed Monitor for the 8008. To do this, I added menu options to support to optionally setting (input) and clearing (output) the most significant bit of input and output serial data. For the MCMON, the input menu for set the bit should NOT be set. The output setting doesn’t matter.