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RE: [N8VEM-S100:1353] New/Old Project

Welcome on board Eric. Not sure what answers you are looking for. 

John Monahan Ph.D
e-mail: mon...@vitasoft.org
Text:    mon...@txt.att.net

-----Original Message-----
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On
Behalf Of Eric O
Sent: Saturday, February 02, 2013 1:23 AM
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:1353] New/Old Project

Andrew Lynch suggested I join this group and seek assistance with my

Back in 1975 I was a 20 year-old electrical engineering college student and
electronics hobbyist and saw the famous Popular Electronics article on the
Altair 8800 computer.  I ordered it, assembled it and it worked great as
soon as I powered it on for the first time.  Over the next year or three I
enhanced it with some additional memory, a homebrew parallel and serial
interface and the Processor Technology video card.  I wrote hand-assembled
machine code to "boot load" my own little monitor via a modem to the
mainframe computer on campus.  This involved an automated log-in to my
account, starting the listing of a hex file and then capturing and loading
that hex file into the Altair RAM.  Of course I had to switch a couple
hundred bytes of machine code into the Altair whenever I needed to "reboot".
I also wrote a terminal emulation program so I could then use it as a
terminal to that same mainframe.  Great fun and done on a shoe string
because I was a very poor college student.

Disaster literally struck out of the sky one day around 1979 when a very
powerful thunderstorm hit and a lightning bolt literally blew the top off
the power pole that fed the off-campus house I shared with three other
students. I should have unplugged the Altair when the thunderstorm arrived,
but I didn't want to have to take 15 minutes to reboot it.  Stupid!  Anyway
the power surge killed the machine.  It would still light up but it wouldn't
do anything approaching normal operation.  I did replace a number of the
chips in the weeks that followed, but I couldn't afford to do a proper job
of it.

Well, graduation came, then a job, then an IBM 5150, and then other
computers over the decades and now the Altair has been stored in a box for
almost 35 years.  I always meant to fix it someday but never got around to
it.  But now that I'm semi-retired from a career in computers I'm finally
getting around to it.  So a couple months ago I finally got it out of that
box and started doing a bit of research and I'm so happy to see all the love
that people have for these old machines.

One of the first things I learned was not to trust the original power
supply.  So I went out and got a couple switching power supplies from
MeanWell, mounted them up in the chassis, and leaving the old supply
physically in place, removed it electrically and replaced it with the new

I popped out all the boards, and turned it on.  I'm getting all the proper
voltages in all the proper places,  including regulated +5.13 on the display
board.  With the CPU in I get the proper regulated voltages on the CPU card:
(-5.25 on Pin 11, +11.69 on pin 28, +5.00 out of the regulator).

I've started working on the front panel and I've already identified two
inverters with the same logic state on each side of the gate, on two
different chips.  So I know I need to replace those.

Any and all suggestions welcome.

(I've seen the very good article at

Eric O

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