The SCELBI cassette and oscilloscope interfaces both use a rather unusual variation of the jelly bean 741 op-amp. This is the TI SN72741, which puts this op-amp in a 14 pin DIP package. Though they can be had on e-bay, I recently discovered that expediters.com has them for $5.50 each. They can be found at this link:http://www.expediters.com/index.jsp?path=product&part=845153&ds=dept&process=search&qdx=0&text=Sn72741
Archive for the ‘8008’ Category
This SCELBI 8B power supply is built in a shadow box type chassis that is approximately 10″x5″x6″.
The internals are simply two Power-One linear Supplies, a switch, a fuse, 3 binding post type connectors, an amphenol 78S4 type connector and a fan. Here is an image of the interior. The fan is connected to the top, so isn’t visible in this view. The 5 volt supply is a C5-6 and the -9 is a modified B15-1.5. Be aware that the -9 supply is not powerful enough for a fully loaded 8H chassis.
Looks like I have enough parts on hand to build one of these babies up. It expects a decoded but unlatched keyboard. I don’t have one of these, so I’ll modify the firmware on one of my PS/2 adapters to act like one. With that I should be able to hack together a simple 8008 driver that will eventually work with MEA on the SCELBI.
Here is a complete, original, SCELBI 8B system that I was recently able to examine and operate. In the past, I already had examined the TTY and Cassette interfaces. This time, I was able to examine the power supply and will put up some posts in the near feature, including interior shots.
This document provides rework instructions that will bring the PCB up to the revision seen in the schematics. I now have a pretty good understanding of how this interfaces works and will provide more details in a future post.
This is an image of the current state of the back sidelayout of the long awaited (by a few) digital board overlaid over an image of the actual board. Base image is curtesy of Jack Rubin, the only person I know, that has seen an SCELBI oscilloscope up close and personal in recent memory. This one is in the Computer History Museum’s off site storage facility.
I haven’t worked on any of the lettering, but I’ve gone through multiple passes of the rest of the board. Based on file size, this board is actually more complex than the SCELBI CPU board.
One other thing that I’m going to do is more study of the rework on the only known Oscilloscope digital board. The arrows in the image point to areas where rework was applied. Initial review seems to indicate that the schematics found at scelbi.com match the board after rework was applied. There is also a bit of rework on the front side.
The analog and keyboard layouts are ready to go, though I’ll probably give each of those one more quick design review pass before ordering a batch.
Link to SCELBI’s app on Apple’s App Store This is the same version (2.7) as available for download from my SCELBI web page. If you have used this version it would be great if you would review this Application on Apple’s App Store.
I now have everything that I normally stock on hand – except SCELBI front panels.
I was a bit behind on things, but today I shipped a few items that I owed people, so am caught up, with the exception of those SCELBI front panels.
In addition, due to popular demand, I made a new run of Brain Board kits. I tested an example earlier in the week, and except for a bad 74LS74 IC, I found they work fine. I’ll have to go through my stock of 74LS74’s and test them before finishing putting together kits. With luck, I’ll have kits ready to ship by next weekend. Watch for an update in the next few days before sending money.
Here is an image of a SCELBI analog board. Overlaid on top is my current front copper layout and a silk screen layer showing component mix. As you can see, the layout is very far along, considering that I’ve only worked on it for parts of two weekends. There is still plenty of tweaking and checking to be done, but there is good chance that the layout, as it stands, would work.
Note that this board has several component locations without components stuffed and a couple of 56K resistors tacked on. There are no pads for these resistors, but they show up in the schematics and on the SCELBI placement diagram. I wonder if there was a second batch of cards made with corrections?
The 130 ohm resistors are listed as 100 ohms in the schematic. It would be hard to know why there was a difference without doing some experiments on actual hardware or a circuit analysis. This change could be functional in nature or do to using components that were close enough and on hand. The 3K resistors are listed as 3.3K in the schematics, but I think they used what was on hand, and there probably isn’t a functional difference in that case.
The pads in the lower left are for decoupling capacitors. I don’t know why they weren’t stuffed in this example. The component mix is pretty basic. There appear to be some film and mica capacitors. Based on what I’ve seen on other SCELBI boards, the transistors are probably 2n2222 and 2n2907 types, but I don’t have confirmation on that. Probably the most expensive parts to source are those pesky 72741 op-amps in 14 pin packages. They seem to be available on eBay, but the asking price is very high. I’m going to be on the lookout for a better source.
One final thing that threw me for a loop on this board. When compared to all other SCELBI boards, the DIP packages are mounted upside down. I’m going to have to pay attention when assembling this board.
Over the weekend, I started work on the SCELBI O’Scope PCB layout. There are two boards that make up the package, a standard size analog board, and a double width digital board. The standard size analog board looks like it will be pretty easy to layout. However the double width digital board looks pretty complicated. It has 35 chips, and around 220 VIAs. By comparison, the Apple II, rev 0, which has to be the most complex PCB that I have recreated has about 380 VIAs. The chip count is the second most for any SCELBI board. It trails behind the 4K SRAM board, which has 37, 32 of which are SRAM chips. All the SRAM chips on a SRAM board are connected the same way, which simplified things quite a bit on that board.
The good news is that, so far, I haven’t run across any components that will be excessively difficult to find.
Another complication on the digital board, is that I only have three good images of two different O’Scope boards. The front and back of one board, and the front view of a second. The board that I have front and back views of, has quite a bit of rework on it. I don’t know if these are design issues or customer modifications. It will be interesting to sort through these changes and see if there are any differences between these two boards.
The final challenge will be figuring out the details of the power supply that is integrated into the O’Scope chassis. At this point, all that I have to go on, is a picture. When you put everything together, the SCELBI O’Scope interface turns out to be far more complicated than many single board “trainer” type computers.
The keyboard interface layout is complete. I have done design checks and have a quote in hand. I probably will not order boards until the O’Scope layouts come together, or at least until they get further along. I think of the keyboard and O’Scope boards as kind of a matched set, the keyboard for input, and the O’Scope for display output. One, without the other is kind of a half solution, though I will be able to test them out independantly.