IMac repair

October 11th, 2017

I have a 5 year old IMac that recently started to act up in a most annoying fashion. Every couple of hours it would shut itself off, with absolutely no warning.

Right from the first occurrence, this felt like a power supply issue. I could power it back up, simply by unplugging the unit, waiting a few minutes, and turning it back on.

Research on the internet, revealed that others had similar issues, and also how to get inside the unit. One symptom of this failure is that the sys log showed the last restart was done to loss of power, not some kind of crash.

Knowing how expensive Apple service would likely be, I decided to look into fixing it myself. I carefully worked a putty knife between the display and the chassis to get it open. What a horrible design, it’s so bad that it’s hard to comprehend that Apple would ship something like that. I managed to crack one corner of the glass getting it open, but it wasn’t a fatal crack, and the display remains perfectly usable.

Anyway, I removed and examined the power supply, looking for the telltale signs of a damaged component, but couldn’t find anything obviously wrong. I decided to reassemble the unit, hoping that perhaps one of the connectors was intermittent. I decided to use duct tape to reattach the screen, in case I might need to attempt something different, like replacing the power supply, altogether, in order to get the system fixed.

IMac held together with duct tape

IMac held together with duct tape

After powering up and running for a few days, the problem returned, so I knew it wasn’t a faulty connector. I decided to take one more look at the power supply before ordering a replacement. This time, I repeated the scan for obviously fried components, but found none. I did notice that the soldering on a few of the through hole components, just didn’t look that good. The solder job looked like the heat wasn’t enough to draw the solder down into the holes, so the solder was kind of balled up on the legs of these parts. Getting the heat right on a board like this, probably isn’t easy, as it contains a mix of small surface mount parts and through hole parts, that are connected to relatively large heat sinks.

I decided to reflow the solder on the suspect components and see if the system would work more reliably. I went ahead and took a shot at it and put the system back together (with duct tape). I’m still not sure if I fixed it, but three weeks later, I haven’t seen a reoccurrence of the random power off. However, it wouldn’t surprise me at all if putting up this post doesn’t cause an immediate reoccurance of the issue!

Operating an Original SCELBI 8B

September 26th, 2017

I recently spent a few hours with Mark Arnold and his original SCELBI 8B. This is the very machine that Mark used to develop SCELBAL with, way back, in 1975. Towards the end of our debug session, we had the machine in a largely working order!

Mark Arnold with his SCELBI-8B

Mark Arnold with his SCELBI-8B

We spent a lot of time, troubleshooting what appeared to be an intermittent EPROM issue. Finally we discovered that that the edge connector on the EPROM board wasn’t making reliable contact with the connector. After examining the board in detail, we noticed tarnish on the edge connector contacts. We used a soft pencil eraser to clean the tarnish off of the contacts and the intermittent operation completely disappeared. Fixing a computer with a pencil eraser was a first for me, though I expect others have done the same. Once the repair was done, Mark was able to reliably run a variety of MEA operations without any problems.

With only 4K of SRAM installed, we were unable to load the full floating point SCELBAL, but we were able to load tiny SCELBAL using a simple bit-banged RS232 port. Once loaded, Mark successfully entered and ran some small BASIC programs. Weeks later, I understand that Mark still has the system powered on and it responds to MEA commands, without any need to reset or reboot.

In order to declare that the machine is in 100% working order, a few things still need to be checked out. We had only 4K of SRAM installed, the other 8K should be added back into the system. We used a modified version of MEA with the page 76 EPROM changed. I created a version of page 76 EPROM with a 2400 baud driver, but we were having some issue with corrupted serial output characters. We don’t know if that was a ground loop problem or a baud rate problem with the modified page 76 driver. Serial input at 2400 is working perfectly, but the timing for output is done differently, so it could be either.

Finally, the cassette interface needs to be connected and tried out. I don’t really expect any issues with that, as I have used that particular cassette interface with a reproduction SCELBI 8B, back when I was first troubleshooting my reproduction cassette interface.

Repairing an Old Automobile Tape Deck

September 24th, 2017

Quite of a bit of the stuff I own and use everyday is, let’s just say old. This includes a 1988 Mustang convertible that sometimes becomes my daily driver. This car is what they call “bone stock”, and I like to keep it that way, though there can be challenges with it. One of the challenges is the sound system that gives you the option of a FM/AM radio or cassette tapes. Lately I’ve been relying mostly on the cassette tape player.

The problem with the cassette tape player is that after so many years, the pinch roller losses it’s grip, and the tape plays at too high a speed, giving you awful Donald Duck sounds. The last time that happened, I found an exact replacement tape deck on eBay and simply swapped it in. Well, eventually the replacement deck started exhibiting the same issue. This time around, though those decks can still be found on eBay, I figured that I would try to fix it. This became a multi-week adventure, as I stripped down both decks, swapping the best components of the replacement deck onto the original deck, which I had kept stashed in the attic. In the end, I ended up with a pile of parts and a working deck, but it wasn’t easy.

Radio in Pieces

Radio in Pieces

Here is some of the stuff that I did to get a working tape deck.

  • The original deck had chipped corners on the front bezel, so I swapped in the bezel from the replacement deck. In doing this swap, some of the wires on the flat flex cable connecting the front PCB to the main PCB were damaged, so jumper wires had to be added to repair this.
  • The backlight for the original deck’s display was gone, so I had to swap in the backlight from the replacement deck. This might actually be the light that was originally in that deck.
  • A power supply trace on the main PCB was found to be broken and had to be jumpered over.
  • I’m not sure, but I think the original tape deck had an issue with a switch, resulting in the tape playing at fast forward speed, similar to the slipping pinch roller sound. Rather than troubleshoot, I ended up swapping in the replacement deck, after cleaning the pinch rollers, which is was next step.
  • The pinch rollers on the replacement deck were cleaned, as best I could. In fact, if I had done this in the first place, that is all that I would have had to do.
  • Reproduction SCELBI 8B demonstrated at the Museum Of Computing History (UK)

    September 20th, 2017

    I just received this report from David Williams in the UK.

    Hi Mike,

    Just thought I’d drop you an email to let you know, I took my SCELBI 8B to the Retro Computer Festival at the Museum Of Computing History last weekend. Only a handful of people recognised the system initially but hopefully a few more people are now aware of the history behind the machine.

    I spent a good while describing the system to one chap who I later discovered is the guy heading up the EDSAC replica build at Bletchly Park.

    I was able to demonstrate the basics of the system using the monitor program to edit & display memory along with entering short programs. I couldn’t get SCELBAL to load due to a RAM fault (Only Identified when I got back home) which was a shame.

    I’ve attached a photo of the setup. Terminal being used is a TI Silent 745. The box below the SCELBI is the power supply, Teletype interface to the right and a fast-load box as described in issue 1 & 2 of the SCELBI newsletters.


    David William's 8B (reproduction)

    David William’s 8B (reproduction)

    SCELBI 8H Usability Reasessment

    August 2nd, 2017

    For a long time, I would say that the SCELBI 8H was the first practical computer marketed to the general public. The key word being practical, as certainly many of the other personal computers of the day, were not very useful machines, other than for educational purposes, at least without adding additional capabilities to them.

    From the time that I got my reproduction SCELBI 8H working, I’ve been able to demonstrate a number of interesting, practical applications on it. That I was able to do so, was justification for the claim that a SCELBI 8H was a “practical” computer. However, I’ve always used a second computer to assemble the source code and then downloaded the application to the SCELBI through a serial port. A few months ago, when I talked to Bob Findley, I asked about how they did application development for the SCELBI 8H. It turns out that Bob and Nat used almost exactly the same approach that I used. Only thing is that they used a PDP-8 as a platform for the cross assembler and a teletype with it’s paper tape punch/reader to download the program to the SCELBI. Though the cross development platforms are different, what Bob and Nat did, was essentially the same as what I’ve done.

    I’ve come to realize that though you can run some interesting, practical applications on the SCELBI 8H, it really can’t do it, without either help from a host computer, an impractical amount of time, or the addition of features like a built in monitor. After talking to Bob, and thinking about it a bit, I guess I’m ready to say that though the SCELBI 8H, as shipped, can run practical applications, it was not a very practical stand alone computer. However, that still doesn’t remove the significance of the SCELBI 8H as the first offering from one of the very first personal computer startup type companies.

    Note that the limited capabilities of the 8H were recognized by Nat and Bob, who developed the follow on SCELBI 8B, with it’s extensive built in software suite. More about the 8B in a follow on post.

    How Many SCELBI’s Were Really Sold?

    July 15th, 2017

    According to Bob Findley, whom I spoke to earlier this year, the often quoted number of 200, is an exageration. Bob said they sold more than 40 completed systems and over 100 board sets.

    One interesting thing that Bob said, is that until the Altair came to market, they felt that they had the market to themselves. The Mark-8 wasn’t considered a competitor, as it wasn’t sold as a functional system.

    I also found out why there are so few Oscilloscope interfaces remaining (only one is known to exist at the CHM). Bob said that they only sold about 4 of them.

    More information from Bob in a follow up post.

    ENIAC in Action Book Review

    June 22nd, 2017

    First of all, let me tell you that I’ve had a hard time writing this review, as I enjoyed reading the book quite a bit, but I think that the book is not for everyone. I really wanted to write that everyone should read this book, as the reader will learn a lot about what happens in new product development, which is more relevant these days, than ever.

    Anyway here goes the rest of my attempt of a review.

    Eniac in Action by Thomas Haigh, Mark Priestley and Crispin Rope is a different kind of book in some ways. First off, it goes into a significant amount of technical detail. Much of this detail would be hard for a “lay” reader to understand. This is the part that I think may cause problems with some readers. Technical detail aside, where the book really shines is how it describes in detail, the process that it took to create, maintain and eventually enhance ENIAC, a fairly complex implementation of a new technology.

    The development of ENIAC was filled with many challenges and obstacles, which the authors describe in a very engaging style. The details of ENIAC development are unique. However, in my opinion, when compared to the process of developmenting other complex systems, there isn’t much that is really different about the ENIAC. That is why I think everyone should read this book. You will get an understanding of just how hard it is to “change the world” with revolutionary new products and systems. Development of complex, new systems are always problematic and take time to sort out. Certain people in the process will get most of the credit and many, many people will be forgotten.

    Reading ENIAC in Action will give you a glimpse of one such project. When reading it, keep in mind that there are many engineering teams around the world currently engaged in similiarly daunting tasks.

    Early Draft SCELBI HW Construction Manual Available

    June 10th, 2017

    This is a very early draft. Consider it to be full of errors, so compare to original documents that can be found at, before using. The main reason I’m posting this, is that I have created and added a couple of chapters for assembly of the oscilloscope interface. Any original documentation that existed about the oscilloscope interface has been lost. I still need to create a chapter with technical information on the oscilloscope interface, including a bill of materials, schematics and theory of operation, but this is a good start. I also need to add a section on PCB rework for this interface. The other chapters will also get revised, as I find time, with photographs of the boards and any notes or errata that I have discovered.

    SCELBI Front Panel redo in progress

    May 27th, 2017
    SCELBI Front Panel

    SCELBI Front Panel

    This SCELBI front panel had some issues, so I decided to remove the existing anodizing, polish out the defects and completely redo it. Removing the anodizing took about an hour. There is a term, “hard anodizing”, and I found out why. It was clear that the anodizing clearly made the surface extra hard. I still need to go over this panel and remove any remaining imperfections before redoing the anodizing. I have a few more panels that I will need to polish out, before taking the batch back to the anodizer for the redo.

    A CHM Youtube Video Worth Watching

    May 21st, 2017

    The CHM’s youtube channel has this video, which caught my attention.

    I’ve always been an advocate for the behind the scenes “little guy” that do 90% of the work that really make new products possible. My mom taught me, when I was little, that “words are cheap”. The same can be said for the new ideas that lead to breakthroughs in technology. There is always a foundation for these new ideas and they usually aren’t giant leaps, by themselves, but almost always, incremental steps in understanding.

    In my mind, the hard part, isn’t coming up with the idea, but implementing it. In fact, many products are described decades before they can be implemented.

    The stories on Andy Hertzfeld’s website, demonstrate the hard work that goes into implementation, as well as anything that I’ve ever seen. The thing to understand is that the effort that went into making Macintosh, isn’t unique, but rather the norm for almost all of the gadgets that we take for granted, these days. How do I know this, you may ask? Well, I’ve been involved in new product development for something like 38 years. Some of the products that I have worked on have been failures, but many of them, successful. In either case, it’s always a struggle for those involved. A rewarding struggle when it goes right, but still a struggle.

    It seems Thomas Haigh understands this. I’ve ordered his book on Eniac and will write a review after I read it.

    Thomas’ comments on Isaacson’s book “The Inovators”, made me pull out Isaacson’s book, “Jobs” and review the sections on Apple 1 and Apple II. I think I understand those products and what went into making them, very well. Thomas’ comments made me want to review the book for faults. I knew that Isaacson had Job’s and Wozniak soldering Apple 1’s, which was incorrect, but wondered what else I would find, if I reviewed those sections. Before I comment on what I found, I will say that I greatly respect anyone that can write such an engaging book, as I don’t have the patience or talent to do it.

    Here are the mistakes I managed to find during a quick review of those sections.

  • Page 62: The guy who drew the up the circuit boards didn’t work at Atari. His name was Howard Canton, and he was an independent consultant, who did contract work for Atari and other companies in the valley.
  • Page 67: Woz and the gang didn’t solder Apple 1’s. They were wave soldered in a factory. Assembling the boards really meant stuffing the chips into the factory soldered boards. Daniel Kottke tested the boards, and if they failed, put the failing board in a “bone pile”, that Woz would debug during occasional visits to the Job’s home.
  • Though there are no real mistakes with the Apple II section, there are, in my mind, serious omissions.

  • Page 74: The real problem with the first Apple II PCB layout, was that Howard Canton, instead of doing it himself, had hired someone to do the layout, and that person did a horrible job. The layout was redone digitally, which took, if I remember right, three months.
  • The first Apple II PCB’s didn’t work, do to noise on the address lines that were connected to the DRAM. Rod Holt fixed the problem by adding termination resistors to those lines. This fix was more important than the implementation of a switching power supply. By the time the Apple II was released, switching power supplies had already been in use for 6 or 7 years. In any case, I doubt that use of a linear or switching power supply would have made a great deal of difference in the success or failure of the Apple II. Proof of this, is that the switching power supply is only mentioned in the first Apple II sales brochure, as a one liner in the last page’s technical overview section.
  • Allen Baum had a significant role in developing the monitor for the Apple II, which is not mentioned.
  • Page 84: Though Apple had venture funding, the company was on very shaky financial ground through it’s first year or two. It was not an instant success.
  • In scanning this section, I could find no mention of the Disk II, floppy disk interface. This was a critical item that enabled the success of the Apple II.
  • I understand why Isaacson, in his story, emphasized Job’s interaction with a number of significant personalities. It is a shame that the struggle to develop a new product by a team of talented engineers gets so little “ink”. I also wonder how accurate the depictions of the interactions between the significant personalities really is. It seems that, in terms of the technical stuff that I understand pretty well, Isaacson would grab a fact and elaborate upon it, kind of putting his spin on it, without doing serious fact checking.

    In any case, it’s an entertaining read, that I recommend, in spite of the errors and omissions.