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Mark-8 Restoration

I am no longer active in this hobby for the foreseeable future.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).

 

Mark-8 Restoration
Documentation
Other Mark-8's

Mark-8 Minicomputer
Apple-1
MOS Kim-1
V C F
Imsai 8080
Kenbak-1
TV Typewriter
LSI ADM-3A
ASR-33
The Digital Group

 

1974

In the fall of '74 I ran across a copy of Radio Electronics and the Mark 8 Minicomputer in my school library. For the next two years, owning my own computer was at the top of my goal list. I worked and saved, and in late '76 I finally had enough to join the computer revolution. Since power, and not nostalgia ruled, I had long since forgotten about 8008 based machines, and I spent my savings on a Digital Group system. The Mark 8 Minicomputer faded from my memory.
 

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The Mark-8 Minicomputer

April 1999

Lucky day! I found myself staring at an eBay auction and a photo of a Mark 8 circuit board. I could hardly believe my eyes. The seller had advertised (one board at a time!) a "circuit board for parts". The boards were listed under "Toys" or some similarly ridiculous category. The only reason I found them was a search hit on a different item the seller had listed. I checked the sellers other auctions, and ta-da! There they were. Conscience got the best of me; I emailed the seller to inform them of what they had. The seller had an "oh, well" attitude about it and the auctions proceeded. Although there were other bidders, I had no intention of being outbid, and now the Mark-8 is mine!! All mine!!! Aah-ha, ha, ha, ha, ha, ha, ha, ha, he, ha, he, he, hah, hum, ho, hee, he-heeeee.............what? 

After winning all the auctions, the seller contacted me and asked if I wanted the "whole box of junk", of course I said yes

The Boards

The Mark-8 consists of six boards, shown below. (Click on the thumbnails for a larger view.) The processor board is a new scan compared to the others. My original scan was without the C8008 chip, as I was testing parts of the board, and forgot to put the chip back for the photo. All the other scans were done soon after I received the boards.
 

m8reg2.jpg (54272 bytes)
Register Display

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Output Ports

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4K Memory

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Address Latch

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Input Multiplex

m8up2a.jpg (71966 bytes)
8008 Processor

 
At this point, I didn't have enough technical information on the Mark 8 to begin any serious work, but I started looking for switches and other items that were not in the 'box of junk"... What's that kludge circuit on the input Multiplex board?? At this point, I had no idea.
 

September 1999

I found an email address and contacted Jon Titus. He was generous enough to send me a copy of the plans offered by Radio Electronics in '74, and a few other interesting copies. Including a copy of the Intel manual describing the 8008 processor. Also included were the board interconnection diagrams that were missing from the original Mark 8 plans. My thanks to you Jon! I also found a copy of the Mark 8 RE magazine, a little rough, but not too bad. The images of my magazine are the ones that appear on this site.
 


Note from John Titus
 

I began restoration by learning how every circuit was intended to function, and then I tested every circuit in isolation on every board, one board at a time. This I did by building a signal source and signal display interface for each board, and then by simulating every signal the board might ever expect to see. Then I traced out each signal and verified the output signals. Every gate, flip-flop, and counter was thoroughly tested on every board. I tested the first hundred or so memory locations and a few from every 256 byte block in the bank. The remainder of memory locations were tested under program control when the machine was fully operational.

Spring 2000

Found a nice pretty C8008 to replace the all-scratched-up one that came with the boards. Finished analyzing and learning the circuitry, finished testing all of the circuits on all of the boards. To my surprise, not a single malfunction was detected in all the computer. I do have a few bad LED's on the Register Display board, and a few broken pins in the Molex connectors.

I have this great idea of putting the boards into a new cabinet to match the one on the cover of RE. Cool idea, huh? I am nearly finished reassembling the boards into a "replica" "original" wood card cage, and I have a full set of vintage toggles identical to Jon Titus' original, except that mine are blue.

 

September 2000

A full blown Mark 8 Minicomputer replica just sold on eBay. Steve, the builder, and I had exchanged emails earlier in the year, and I knew he was planning to build one, but wow! There it is!

Now that Steve has built a couple replicas, I have decided to restore my Mark 8 as close as possible to the original condition, using all the parts I received from the previous owner, rather than risk having it thought of as just another replica. 

Although I have fully restored the transistorized power supply, I chose not to use it since it has poor regulation compared to chip regulators, and no over-voltage or over-current protection. I have installed a chip based regulator in the main housing, side-by-side with the transistor based one. The original transistor based regulator is disconnected.
 

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Inside view of power supply--new on left, original on right

Spring 2001

My Mark 8 is fully restored. With the processor chip in place, hours went by before I had the nerve to flip on the power switch! I looked over the connections one last time, took a deep breath, and,  >CLICK!!<  Lights on, no smoke! It worked almost flawlessly! (I missed a bad connector finger on one of the boards that caused one of the lamps to flicker.)

I loaded a short program and single stepped through it a few times:

00 000   104       Jump
00 001   000
00 002   000

This single instruction program just jumps to itself continuously. By the end of the next day, I had it connected to my ASR-33 Teletype, and running a save and restore program from paper tape.

Ah! So what is the extra chip doing on the Input Multiplex card? It maps data from the front panel switches to an input port. It allows data entry from the front panel switches under program control. The original Mark 8 had no provision for reading the front panel switches while running a program!

September 2003

Tired of cleaning dust from the boards and barking at everyone to keep their fingers away, I had a plastic cover made by Hawkeye Plastics to protect the cards--it's beautiful!

The Mark-8 is complete, works great, has won many awards, and is now on display in my mini-museum, man cave, AKA the living room.

 


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