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Printer A & B

I am no longer active in this hobby for the foreseeable future. 
I will no longer maintain or update the website, but I will leave it accessible to the web for as long as possible (years).


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Printer A & B
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Mark-8 Minicomputer
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Imsai 8080
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The Digital Group


The Digital Group Printer

Two versions of this printer were sold by Digital Group in the early years, known only as the 'A' or the 'B' printers, they represent the bulk of all printers sold by the company. In later years, a Centronics 779-1 was introduced, but by this time the company was in trouble and it is unknown how many were ever delivered.

The printer is a 7-pin dot matrix printer that was manufactured by Practical Automation of Shelton Connecticut for the Digital Group. This 96 column printer advertised a speed of 120 cps, and a software based character set. Which meant that you could define your own characters: double wide, bold, etc. Although this was billed as a plus, it basically meant that the printer was "dumb"; it had no built in data processing ability. Your main processor did all the dirty work: timing, paper advance, printhead operations, etc.

One problem with this approach is the possibility of a software bug instructing the paper advance solenoid to engage while the printhead pins are extended... ouch. Hence the introduction of the "B" controller. :)

Since Dr. Suding, the designer of all things Digital Group, was not a big fan of interrupt programming, background printing was clumsy at best, but it worked well enough, bugs aside, and it was great quality printing for the day at a reasonable price. The printer sold complete for $495 in kit form, and $675 fully assembled. In today's dollars (2008), that is $1760 and $2400 respectively--Can you imagine? Competing printers of this era sold for as much as $2700. At the time, there were new cars selling for less.

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The first printer advertisement, Byte 3/77

This printer could also print up to four copies simultaneously, on standard size letter or form feed paper, making it immediately useful for business applications. Along with software available from the Digital Group, this became one of the first hobbyist computers to make the transition to business computing, where the big money was thought to be.

One other failing of this printer if used with the factory cabinet, was the wholly impractical requirement to remove the cover in order to add paper--and only half a roll would fit! Almost immediately, after-market kits were available to place the paper roll external to the printer.

I searched for one of these for a very long time before Bruce Damer of The Digibarn offered me his--knowing it would be carefully restored. Just as I was completing restoration of the mechanism, I was able to find TWO more of them! When it rains... Anyway, the other two I picked up were in just awful condition. I cannibalized a few parts from them to make the Digibarn printer complete. The comments and photos on this page detail the restoration of the Digibarn printer.

The Restoration

When I received the printer, the first thing I noticed was that the California humidity had done it no favors. Rust and corrosion were everywhere. Obviously, it was not operational. Fortunately, although the rust was pervasive, it had not rendered anything beyond salvation though some of the parts were in very rough shape and required some real creativity to deal with--read on!

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The Goal--from flyer 9

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Non-DG cabinet in this photo

The control electronics in the Digibarn printer was a version B. The other two printers I received later included an A electronics package in one of them, and I chose to use that one in this printer--preferring the earlier versions of stuff as I do. Eventually, I'll restore another with the B electronics. :)

I removed the printer from the non-original cabinet and began the disassembly process, photographing it each step of the way, as at the time I had no documentation to refer back to for reassembly. I also decided to log my time in the project carefully, just because I wanted to know how long a DG printer restoration would really take! Results? The progress on printer mechanism alone, shown here in these pictures took just over 95 hours--I did not time the interface board or cabinet restoration. It was a real pain keeping track of the time, and I gave up after the mechanism was complete. You get the idea though--my kids miss me.

The print head was in pretty bad shape. The pins had all rusted in place, and required great effort to remove, breaking a pin on one hammer in the process. The repair was difficult. Finding practical repair materials, settling on a technique, and doing the repair successfully on the first try, actually required a couple dozen hours of planning and setup. This was the most challenging part of the repair.

The next most difficult part was the rubber rollers. I picked up a half-dozen junk printers from a second-hand shop and reduced them to a pile of parts. Fishing through the pile, I pulled a set of rollers that had the same shaft size, but were too large otherwise. The oversized replacement rollers were re-sized by putting the roller shaft in a drill, and then running the drill in a jig over a belt sander. It took longer to set up the jig than to do the work, but the result were better than I had hoped--I'd call it a perfect repair! (I now have a lathe for such things--should I ever need to repeat this in the future. :)

The re-inker pads in the photos were an experiment. The original pads fell apart during restoration--as I suspected they might from my initial inspection. What I have in place now is made from (get this!!) Mr. Clean's "Magic Sponge". What can I say? Years later and it still works! 

After all of the repairs and rebuilds, I tested all of the printer mechanical actions, and everything worked fine. I then began work on the cabinet and electronics. The cabinet was in great condition and needed only minor paint work. The restore was pretty standard fare, taking a couple dozen hours to break down, clean, rebrush all the aluminum and airbrush the painted and anodized surfaces where needed to pretty them up a bit. Ahhh!

The fan seen in the photo's was destroyed. The housing had shattered, and the blades were bent terribly. I used JB Weld to cast the missing bits, and to repair the frame, with small brad nails strategically embedded in the mix for more strength. I have found this to be a great technique, and this time was no exception.

After connecting to my system, the completed printer took me forever to tune and adjust. The platen setting vs. the software settings were a pain. I must have done something a little off kilter in my restoration, since the printer will not work with the default software settings. The impact time had to be increased nearly to maximum, but with the altered software settings, the printer works great!

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Weight of the fan has broken the housing

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No lugs used on the caps--bad!

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Rust, dust, and spider webs

They say a picture is worth a thousand words--here's fifty thousand words. :) Starting with...


The Before Photos

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The printer, sans case

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Rust view

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Pressure rollers are destroyed

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The hand print is rusted in place

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Paper guide, that's not dirt, it's rust

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Print hammers were frozen in place

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Rust removal with trusty Dremel

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Reassembly--one hammer pin is broken

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Welding a new pin in place

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Sanding to remove that handprint

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Cleaning the paper advance solenoid

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Pretty rough

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Sulfuric acid bath removes rust

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Saving the springs




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The printer, After


Final Assembly


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Copyright 2008 Bryan's Old Computers
Last modified:
October 16, 2009