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RE: [N8VEM-S100:1357] New/Old Project (Altair restoration)

With advice from Tom Lafleur and others I've been making some progress on my
Altair restoration.  This machine was killed by a lightning induced power
surge many years ago.  I've installed new, modern power supplies, and I've
removed all the chips from the front panel and the CPU.  These were replaced
with sockets.  I learned that apparently, you can't always replace 74xx with
74LSxx.  I learned this when the system clock would not run when I tried two
different 74LS04 chips in the clock circuit but works fine with a 7404.

Anyway, after repopulating the chip sockets with mostly LS parts, the Altair
exhibits very strange and unstable behavior.  I almost don't know where to
begin to describe it.  Let's just say that the results of a reset are fairly

I thought I'd ask a few specific questions:
If I "stop" the Altair and check the voltage of an address line on the bus
that is high, I get about 3.4 volts.  The only boards in the system are the
front panel and the CPU.  In this configuration, all that's going on is the
8080 address lines are buffered through 74LS367�s onto the bus.  The front
panel is involved only insofar as the address lines each go through an LED
and a 220 ohm resistor to ground to display the address line's state.  Hard
to imagine a simpler situation.  The output from the 8080 is a healthy 4.9V
where it goes into the 74LS367, but on the output side it is only 3.4V.
This low voltage value for a high logic state seems like a potential problem
to me.  Am I right?

Lest you think it might be a bad 74LS367, be aware that I previously had the
functionally equivalent 8T97 chips in there and had essentially the same

Also, the CPU and front panel regulators were replaced and I get a healthy
4.96V on the +5 side of the regulators.

So the immediate questions are:
1. Am I right in saying that the 3.4V level is an issue?
2. If so, any ideas what could be causing this?
3. I'm attaching the oscilloscope trace of the system clock as seen on bus
line 49.  Does that look OK, or is there too much ringing?

CPU Schematic here:

Thanks for your ideas.

- Eric Osman

-----Original Message-----
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com]On
Behalf Of Eric Osman
Sent: Saturday, February 02, 2013 12:48 PM
To: n8vem...@googlegroups.com
Subject: RE: [N8VEM-S100:1357] New/Old Project

Douglas -

Thanks for the introduction and summary.  I'm on Andrew's list for an
extender, and I'll probably be looking to obtain a prototyping card as well.
These are driven by my initial goal, which is just to get my Altair working
again.  Longer term I'll be looking to enhance it a bit.

I was intrigued enough by the Raspberry Pi to get one, but of course, that
is not the focus of this board.

I'll be looking over the Wiki's and mail group archives that you mentioned.

Thanks again.

- Eric

-----Original Message-----
From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com]On
Behalf Of Douglas Goodall
Sent: Saturday, February 02, 2013 10:52 AM
To: n8vem...@googlegroups.com
Subject: Re: [N8VEM-S100:1355] New/Old Project


Our group consists of lots of old-timers and youth as well.

Apparently all of us like to fiddle with hardware, and there is quite a lot
of fun going on.

The major interests within the group are focused on a range of hardware, as
simple as a single
board computer (See the Zeta), and more sophisticated buss oriented systems,
both S-100 and ECB.

Andrew supplies bare circuit boards for us, mini boards, SBC's, and a highly
integrated machine we
started calling the N8 (originally named "Home Computer").

John sells S-100 bare boards, CPU cards, memory boards, ...

There are two main Google mail groups, one for Andrew's focus
(n8...@googlegroups.com) and one
for John's (n8vem...@googlegroups.com). There is another one recently
formed for the scsi to ide
project, aka S2I.

Information about the boards, schematics, board layouts, etc are found on
the wiki (n8vem-sbc.pbworks.com).

Building these boards is a learning experience, and we gain knowledge about
sourcing parts, building  up
boards and then debug them. The community members are very happy to help
each other get things working
the google groups are a constant stream of questions and answers about
aspects of the hardware and

There are a number of different BIOSs written by community members, some of
which are more specific and
some of which are more productized and full featured. If you want to find
out more about the boards, look
under board information on the wiki. There is a software information section
as well.

Welcome to our community, and don't be shy to communicate with us via the
lists or privately.


Douglas Goodall

On Feb 2, 2013, at 1:23 AM, Eric O <ewok...@gmail.com> wrote:

> Andrew Lynch suggested I join this group and seek assistance with my
> Background:
> 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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Douglas Goodall, http://goodall.com

Note: I don't use messenger, or skype, or facebook, chat programs in
general. Having always-on open communication links through massive public
servers I don't have control over seems like too much of an invitation to be
infected by a virus or bot. It is bad enough that my Mac wants to stay in
periodic contact with Apple's cloud. Skype was tempting before Microsoft
bought them. There have been too many examples of remote session links being
abused by vendor employees. Even "back to  my mac" makes me nervous. There
was a recent episode where Apple cooperated with a social engineer and
compromised someone's entire electronic persona. If you want to speak with
me, calling me on the phone works well, and you don't have to wonder if the
electronic mail got through or not. When I say "Hello, this is Doug", you
know who you are talking to. Just in case you were curious.

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