End of an Era for Me

December 17th, 2016

As look back at some of the blog postings I made during the early days of the Mimeo project, the excitement I felt, could be easily discerned. For better or worse, after nearly 7 years, that excitement is no longer there, and it’s time to move the project over to someone who has a strong passion for that landmark system.

I’ve transitioned Mimeo 1 sales to Corey Cohen. I’m sure he will do a great job selling and supporting people interested in building reproduction Apple 1 Computers.

Since first making them available in March of 2010, I’ve sold 167 Mimeo PCBs, the first few as part of a kit, even a few that were completely assembled and tested. I have greatly enjoyed the entire process from creating the PCBs in the first place, to fixing a few boards that customers needed help with. Especially gratifying, has been the great relationship that I have established with all the people that I have made contact with over the years. One thing that I never expected, when starting this project, was the amazing contacts I made with so many people associated with Apple during those early years.

However, the learning part of the process, which is one of things that keeps me interested in my hobbies, has not been there over the past few years. It is time to move the sales over to Corey, who has unbelievable enthusiasm and knowledge for and about those milestone computers.


I will continue to sell SCELBI PCBs, as I am still excited about working on that system and learning a lot, while doing it. Blog followers will know that the Oscilloscope Interface PCBs, the last SCELBI boards that need to be reproduced, will soon become available to interested parties. I have some ideas for some even more obscure reproduction vintage computer projects. However, the Oscilloscope interface must be completed, before I move on to those projects.

Other Products

Other products I have sold in the past, will only be continue to be sold if I have remaining inventory in stock.

I presently have stock of the following

  • Brain Boards
  • Swift Cards
  • I am sold out of the following:

  • Apple II, rev 0 boards
  • SuperProto boards
  • PS/2 keyboard adapters.
  • Datanetics Keyboard PCB
  • The PS/2 Keyboard Adapter

    The PS/2 keyboard adapter is a little bit of a special item to me. When I had those PCBs made, because of economies of scale, I had 150 fabricated. I never thought I’d sell all of them. In the end, I sold over 140 those little dongles, using the others for a number of my own special projects. Though they probably exist, I have never heard of a PS/2 keyboard that it didn’t work with. It was designed to operate with Apple 1 and Apple II computers. As I originally hoped, people adapted the design to a number of systems beyond that. The firmware has had minor firmware features and improvements made over the years, but the basic design hasn’t changed over all these years.

    There have been a number of other PS/2 keyboard to parallel ASCII keyboard adapters designed over the years, some coming before, and inspiring my adapter. Though it could certainly be improved further, I sincerely believe my version is currently the best of the breed.

    When I get the time to add them to my web site, I will be releasing to the public domain, with no restrictions, all design files, including firmware and PCB CAD files. You will be able to do what you want with it, make copies for sale, improve it, or just study the design for your own purposes.

    SCELBI Oscope – Why the Y axis is upside down

    December 12th, 2016

    I finally took the time to figure out why the Oscope Y axis is upside down. It all stems from an error in the schematics.

    Analog Schematic Error

    Analog Schematic Error

    If you look closely at this cropped image of the schematic, you will notice that pin 4 of the Y axis op-amp is labelled (+) while pin 4 of the X axis op-amp is labelled (-). Both can’t be correct, as op-amps all have a negative and positive input. Here is the pinout of the SN72741 from the TI data sheet showing that the correct labelling of pin 4 should always be (-).

    SN72741 Package

    SN72741 Package

    The X and Y, plus and minus inputs to the analog board come directly from the digital board. If you look look at the digital and analog board schematics, the X and Y outputs match up nicely with the analog board inputs.

  • Xp is pin B-W on the digital board and connects to pin W on the analog board
  • Xm is pin B-Z on the digital board and connects to pin Z on the analog board
  • Yp is pin B-Y on the digital board and connects to pin Y on the analog board
  • Ym is pin B-X on the digital board and connects to pin X on the analog board
  • Digital Board X and Y Outputs

    Digital Board X and Y Outputs

    There is an image showing how I connected them in a previous blog post. Pins W, X, Y and Z are to the far right in the following image.

    Edge Connectors Wired

    Edge Connectors Wired

    Overall, it looks like a very clean solution. The problem is that the mislabelling of pin 4 and pin 5 of the Y axis op-amp caused the Y inputs to also be labelled backwards on the analog card schematic. Therefore, Y plus is really connected to Y minus and visa versa. The solution is to cross the connections for Y plus and minus between the digital and analog boards.

    I would have to assume that this swapping was done when constructing the original SCELBI Oscilloscope Interfaces, but I don’t have access to an original unit to confirm. The images I have, don’t show the connections between boards, so I can’t go on that. I would also assume that the intention of the design was to make connecting the boards easy, and if enough units were built, that Y minus and Y plus would be swapped on the PCB, so that connecting the digital to analog board, would be more straight forward. My guess is that so few oscilloscope cards were made, that this change was never made.

    Fortunately, I’m pretty sure that I will be able to swap these connections, without dissassembling the entire Oscilloscope Interface.

    More Scope Progress

    December 11th, 2016

    Here, I finally have multiple lines of star bursts displayed.

    Multiple Lines with SCELBI Oscope Output

    Multiple Lines with SCELBI Oscope Output

    My problem was that I had the input port data bus hooked up wrong. When I connected it, I assumed that the edge connector would be wired from data bit 0 to data bit 7 in order, but that wasn’t the case. I also had to turn off the invert switch on the scope. The problem with inverted characters will have to be investigated further.

    I also have a good understanding of adjustments for Horizontal Width (R10), Horizontal Speed Balance (R6), Vertical Speed Balance (r17) and Tilt (R20). The speed balance adjustments are necessary to make sure that movements left and right are at the same speed and movements up and down are at the same speed.

    I still need to figure out what’s happening at the beginning of each line and at the beginning of the page.

    The driver implementation is being investigated. It promises to be a serious challenge to fit it in 256 bytes, as half of it will be taken up by an ASCII to 2 byte output table.

    One Line Of Star Bursts

    December 7th, 2016

    The SCELBI Scope project is not revealing it’s secrets easily. With some tweaks of the pots and some new software I now have a line of star bursts. Dumping the entire pre formatted buffer in one shot seems like the way to go. I inverted the display by using the invert button on my oscilloscope. I’ll look into the implementation more thoroughly later on.

    One Line of Star Bursts

    One Line of Star Bursts

    It looks a bit better in real life. Next step is to figure out how to make multiple lines work. The software I wrote is supposed to print 5 lines, just like the MEA manual says it supports. One page is reserved for the scope output buffer. With 2 bytes per displayed character, 20 characters per line and 5 lines, that takes up 200 bytes. Right now, I’m automatically adding 2 extra bytes per line for the LF and end of page functions. That brings buffer consumption up to 210 bytes.

    It’s going to be a real challenge to fit the software for the both the scope and keyboard displays in 256 bytes. Currently almost half of the 256 bytes is consumed just by the ASCII to a 2 byte code table in my implementation. I do think that with some clever coding, I should be able to free up a few bytes from that table and I’ll probably need it.

    SCELBI Oscope Progress

    November 30th, 2016

    Read the article in the previous post in order to get an overview of the Oscilloscope Interface design. Here is an expanded image of a single starburst character as my implementation currently stands.

    SCELBI O-scope Drawing

    SCELBI O-scope Drawing

    This is clearly not what is desired in displaying text, but you can see how the scope is attempting to draws a starburst image. In order to make more sense of what was happening here, versus what I thought the design should do, I drew a series of lines over the top of actual image.

    Expected Oscope Display

    Expected Oscope Display

    The software writes two bytes “A” and “B”, which controls which lines are “on” and which ones are “off”. The line starts at line “A0” and proceeds to draw through “A7”, then B0 through B7. A0 represents bit 0 of the first byte, A7, bit 7 of the first byte. B0 through B7 represent the second byte. NA represents lines that are never “on”, those lines are simply used to reposition the cursor in order to draw the next line. For purposes of debug, I had all lines turned on for this image.

    Next I moved the arrows to match what is seen on the oscilloscope.

    Oscope Display Trace

    Oscope Display Trace

    This technique clearly revealed two big issues, and a few smaller ones.

    The first major problem is that the Y axis is inverted versus what was expected. I’ll have to investigate hardware connections to clearly understand what is going on with that. It could be as simple as incorrect hookup between the digital and analog board, or perhaps an incorrect analysis of the design. In any case, this can be handled by reversing line segment assignment in order to flip the characters.

    The second issue is apparent without the process of analyzing the drawing sequence. That is the bright spot to the right of the character. That problem is actually due to the software algorythm used to draw a series of characters. An ASCII character is sent to a lookup routine that searches through a table of characters in order to find the matching bytes that must be sent to the “A” and “B” registers of the digital board. Once the lookup is complete, the 2 bytes are written to the digital board. The problem is that the scope is stuck on a single spot while the lookup is done. Even though the “Z” input should dim the scope, it isn’t turned all the way off. The solution to this problem will be to look up all the text and create a buffer of all the data to be sent to the “A” and “B” registers. That should avoid burning in one spot.

    The other problems are uneven and tilted lines. I’m hoping that this can be eliminated though carefull adjustment of the variable resistors on the analog board. I’ll discuss this further in a future post as I work through those issues.

    The good news with all this analysis, is that it is clear that the majority of the hardware must be working correctly in order to display this much of the starburst, as distorted as it is.

    SCELBI Oscilloscope Board Hardware Description

    November 29th, 2016

    Last year, Paul Robson sent me this document, describing how he thought the SCELBI Oscilloscope Interface worked.

    SCELBI Oscilloscope Interface Description

    Paul’s reverse engineering efforts turned out to be right on the money. Having this document has helped me substantially with software design and bring up of real hardware.

    There are some aspects to the design that I will elaborate on in future posts. Some parts of Paul’s document will not be obvious to some novices, even though they are described well enough in Paul’s document for many people to understand. Also, I’ve discovered some practical limitations to the implementation, as I go through physical hardware bring up that need further elaboration.

    SCELBI Oscope Interface – “First Light”

    November 19th, 2016

    Astronomer use the phrase “First Light” when they use their telescopes for the first time. I decided that turning on my SCELBI Oscilliscope interface on for the first time is, in a sense the same kind of event, so I’m borrowing the astronomer’s phrase. I’ll bet some followers of my blog figured that I’d never get around to hooking this interface up, but they would be wrong.

    Here is the test setup. I’m using my reprodution SCELBI 8H, and the “scope”, (not pictured here) is my Tektronics 465.

    SCELBI Oscope Test Setup

    SCELBI Oscope Test Setup

    I wrote a simple test driver that should have repeatedly printed two lines containing “ABC”. This is what I saw when I ran it.

    SCELBI O-Scope Interface First Light

    SCELBI O-Scope Interface First Light

    The bad news is that this is unintellegible. The good news is that I’m getting signals on both X and Y outputs. The Z output has no signal. Next step will be to figure out if this is a hardware, software or adjustment issue or combination of these.

    O-Scope Repair

    November 12th, 2016

    After a few evenings chasing this intermittent fault, I thought I finally had solved it when I sprayed some Detoxit gold on a couple of the channel 2 input preamp selector switches and it snapped back to life. The next day while moving back to bring up of the O-scope interface, channel two failed again, this time appearing to be completely dead. This was actually good news, because a solid failure can be easier to debug than an intermittent failure.

    This morning I figured out that there was a short between an input capacitor case ground and chassis ground.

    O-scope Intermittent Short

    O-scope Intermittent Short

    Scraping the debris from between the capacitor ground and nearby chassis ground seems to have rectified the issue, hopefully for good. Having a good working channel to compare the bad channel with, made this repair easier than it otherwise might have been.

    Oscope – analog power supply working well enough (I think), however…

    November 7th, 2016

    The 1N5355 helped with the -18voltage, but the positive voltage still was too low. I resolved the positive voltage by changing the dropping resistor down to a value of 20 ohms. I also connected the center tap of the transformer secondary to the ground. With these three changes, the positive and negative 18 volts seem pretty stable.

    However, my next problem is that the second channel on my Tektronix’s 465 oscilloscope has become intermittent. Looks like something is wrong on the vertical preamp board. A few hours of debugging hasn’t revealed any obvious component fault. Seems like an intermittent connection somewhere close the input jack is the culprit. I can’t very well test the SCELBI oscope interface until I get both channels working again, so I’ll have to keep working on it.

    Oscope Power Supply Part ??

    November 2nd, 2016

    I’ve now created a few posts about the little split power supply that is used to power the SCELBI Oscilloscope Analog board. Here is another, and most likely, not the last.

    After wiring the chassis and powering it up I found the positive side of this supply was 4 or 5 volts low with both digital and analog boards installed. After spending a few evenings fooling around with it, I decided that the main part of the problem probably is the relatively high zener impedance of 45 ohms of the BZX79C18 zener that I had installed in the circuit. I believe that the zener’s impediance is dominating the voltage regulation, not the zener effect. I’ve decided to try an alternate part, the 1N5355, with an impredance of 2.5 ohms. Hopefully it will work better for this application.