Sunday, March 9, 2014

Vintage Computer -MITS Altair 8800 restored by Josh Bensadon

Popular Electronics,January 1975
Popular Electronics,January 1975
Another ALTAIR 8800 is working once again.  This system was bought from David Larsen in August of 2013.  When I first saw it, immediately, my curiosity and imagination began to ponder what kind of service did this machine provide.  Some of the labels were worn out, so that speaks of a lot of switch use.  However, the switches themselves did not look worse for wear, so I'm guessing this system was used by one person that was careful to not damage switches over time.  Normally, the display LED's and Switches are grouped in sets of 3 to facilitate the octal numbering system.  This numbering system fits well with the 8080 instruction set and was used in many of the mini computers that came before the ALTAIR.  Interestingly enough, this system had the Switches and LED's grouped in sets of 4 for the hexadecimal system.  This might have been done because the CPU card that came with it was a Z80 CPU, but I am just guessing here.

Of course, the system was not operational when it got on my bench.  This is a good thing, since it meant I would have to open it up and learn more about it.  Before I go any further, the one thing I found to be very odd about the ALTAIR is the direction of the Power Switch.  Down is ON?  That was a very European thing, but I don't know a whole lot about Ed Roberts history.The problems were found quickly and without getting into trouble shooting, I'll just say the Bridge Rectifier had a bad connection and the 7430 chip that put the CPU into wait state was not working.  About 12 months earlier, I fixed up an IMSAI 8080 system which
IMSAI 8080
IMSAI 8080
was told to be a copy of the ALTAIR BUS, but what I didn't realize was how much of a copy it really was.  While studying the schematics to trouble shoot, I could see the IMSAI front panel design as practically a chip for chip copy.  The ALTAIR 8800 was brought out to market as a construction article in Popular Electronics, Jan 1975.  This article was driven as a response to the competition's magazine (Radio Electronics) article to build the MARK-8, which came out 6 months earlier in July 1974.  But what made this computer more significant was how others like the IMSAI 8080 copied it.  They say imitation is the sincerest form of flattery.  By copying it, they claimed the bus structure was a standard that everyone could build to, and people did.  I recall reading PE and RE magazines and everyone was either operating an S-100 bus or they provided a module to interface to it.  Well, I must bring this blurb to a close, but before I do, I would like to say as a Popular and Radio Electronics collector and enthusiast, this now working ALTAIR 8800 is the center piece of my collection.  Thank you David.

Josh Bensadon

Josh Bensadon his "MITS 680"  "IMSAI 8080"
Josh Bensadon his MITS 8800,680  & IMSAI 8080 

David G Larsen
David G Larsen
 I am delighted to see Josh has this MITS Altair microcomputer operational again. It had been in the Bugbook Historical Microcomputer Museum warehouse for at least 25 years.

"by David Larsen"  KK4WW Computer Collector Historian   
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