Restoration of an IMSAI 8080 | Hacker News Reader

Restoration of an IMSAI 8080

retrocmp.de

61 pointsnickt27 comments

Even without Wargames, this is an iconic computer. Those colored paddle switches and dark bezel with LEDs behind them screamed “computer”. Only thing missing was a reel to reel tape drive.

In contrast to the Altair, it was actually more subdued and less substantial. “Just” a bunch of toggle switches and lights. Could have been anything. Maybe an elaborate model railroad control panel. Industrial, perhaps, but not necessarily a computer.

The IMSAI was my first contact with computers. The one we had was built as a student project the year before. It had a keyboard, video, an open chassis 9” monitor (no touching!), 8K of RAM, and the ubiquitous cassette interface.

My instructor taught me BASIC in a hour and left the lab, leaving me to flail the rest of the evening. I forgot something fundamental and couldn’t get anything to work. (It was expressions.)

He answered my question the next day, I “aha’d” and that was the last formal instruction I had for computers for 3 years until I took Fortran in college. All self taught through books, magazines, etc. Alone, I had no one really to talk to. For whatever reason I never really talked to my instructor about it again.

I ditched my first class playing with that thing.

We never turned it off, if you did you had to type in, oh, 30-50 bytes of hex code into the monitor so you could load BASIC from cassette. I tried to use the front panel to do it once, that got old very fast. It didn’t work.

Next year we had several PET computers donated, and I never touched the IMSAI again. I spent a lot of time on the PETs. I routinely broke into the room that housed them in the library, climbing up and in through the window.

But the IMSAI, that was my black monolith in the swamp, that just happened to be blue, that touched me first.

> Maybe an elaborate model railroad control panel.

Damn, now I want an IMSAI8080 to control a model railroad

Here you have a PDP1 : https://www.masswerk.at/nowgobang/2021/train-spotting-1

First, if you want to know more about the company behind the IMSAI, there's a long section in chapter 3 of Fire In The Valley [1] about IMSAIs founding through to its demise. It's an interesting lesson about what happens when salespeople run tech companies:

> Meanwhile, IMSAI had grown quickly and showed no signs of decelerating. The company already occupied two buildings on Wicks Boulevard in San Leandro; administration, sales, marketing, and engineering were located in one building, and production and support in the other. Millard had assembled a driven organization, a fact nowhere more apparent than among the sales team in Building One. Telephone salespeople like Bill Lohse came in promptly at 8 A.M. and, after a brief meeting, got on the phones and stayed there, logging every call until lunchtime. Lohse would take an hour off, during which time he compared notes with other sales representatives on how many thousands of dollars of equipment they had sold that morning. Then he’d jump back on the phones again, making calls until the end-of-the-day sales meeting. ...

> Building Two was where reality came in. Selling a million dollars’ worth of computers and building a million dollars’ worth of computers are two different things, and the production people had trouble keeping up with the orders. ...

Second, I highly recommend this playlist on YouTube from a guy who restored his IMSAI to working condition. I learned a ton about the basics of an Altair class machine - how it worked, what common boards were used, and how it connected to things like printers and displays, etc. [2]

1 https://www.amazon.com/Fire-Valley-Birth-Personal-Computer-e...

2. https://youtube.com/playlist?list=PLXDK0MeyK4ZjnpbqshdqWuBJL...

It's funny, I've watched IMSAI Guy's videos on electronics but hadn't actually watched the videos about IMSAI. Thanks!

The author has a separate page for rebuilding the large electrolytic capacitors that is really interesting:

https://retrocmp.de/northstar-hrz/ns-hrz_p08_capacitors.htm

> Why Do Capacitors Need to be Reformed?

>Whether capacitors are stored as a component or installed as part of an equipment assembly, the result is the same. Electrolytic capacitors that remain unpowered for longer than 6 to 18 months can experience degradation of their aluminum oxide dielectric insulation. Power applied at the capacitor’s rated capacity can exploit any weakness in this insulation. This will allow the voltage to punch through to a short circuit condition.

>The resulting short circuit of a capacitor results in physical and electrical damage to the capacitor. It also leads to catastrophic failure of the equipment. A severe failure of a capacitor can also cause collateral damage to surrounding equipment. For example, built up pressure inside a failed capacitor can cause physical damage in the surrounding area from capacitor material being expelled. Or, combustion damage to fragile circuitry and other components is possible. This type of failure can possibly pose a risk to personnel in the area in the event of a failure.

I recently got a near perfect condition IMSAI 8080, and took it apart to make sure everything was good to go. The power supply is indeed beefy! One of the interesting things to note was the only fuse was on the primary side, and it is 5 amps. With no fuses on the secondary side the 8V rail would not break the primary side fuse until ~70amps! I would guess you would have diode failure before that point, but certainly something to be careful with when poking around in there.

It is also interesting to see the 120V AC going to the front panel switch with no separation on the PCB. Those old times were good times.

It is otherwise such a fantastic part of computer history and a reminder of a time when assembly of a computer was within reach of so many skill levels.

(I did do a small video about the unit: https://www.youtube.com/watch?v=cICD6j4HJMg)

Nice clean unit, and floppy drives too! Good haul.

Okay, going to hook this up with a couple 8" disk drives and an acoustic coupler, then we can find a BBS with lots of games to play. What could possibly go wrong?

It’s not obvious today, but at the time that movie was released, David Lightman’s computer hardware was already “retro”. By 1983, the IMSAI was old, and 8” floppies had been replaced with 5” floppies.

The intention was to show that David, as a school kid, couldn’t afford high-end equipment and was using hand-me-down, or second hand equipment.

To most viewers today, that distinction is lost, as it’s all almost equally in the past.

That's true when it was released, but was it an intentional choice (ie the movie is explicitly set in our 1983 where this gear is old)?

Or is it just that between writing and releasing technology had changed so quickly?

Intentional choice.

Indeed we know it was deliberate, the producers have talked about it before.

That film had the best technical advisors that money could buy. I saw it in the theater as a child, and a few months ago. Every technical element rang true, whether it was yesterday's, today's, or plausible future scope. I often recount how I loved the way David busted out of the office by using alligator clips and a microcassette to replay tones for the cipher lock.

In a world where every other film is "hacker in a dark room wears a hoodie and cranks up EDM while CSS scrolls on the screen", WarGames appealed to nerds as an exploration of current tech, and a cautionary tale of a possible future.

To me, at the time the movie came out, it showed that David was someone interested in technical matters, IMSAI stuff has never been cheap, and the speech board re-enforced that notion in my mind.

Well in 1986 his new parents bought him a IBM PC XT (with mouse!) so he got a pretty nice upgrade. :)

Interesting, because at the time I didn’t get that either. His rig was a lot more impressive than my VIC-20 and cassette drive. A bit less impressive than my Intellivoice for speech/games but good enough.

The movie is probably what taught me modems existed however.

Recently I’ve watched it and wondered why he’s using an (expensive but old) 70s style computer rather than a cheap 80s home computer and now I know, thanks.

Backdoors are not secrets!

Miniature IMSAI 8080 - https://news.ycombinator.com/item?id=35960604 - May 2023 (1 comment)

IMSAI 8080 computer (2004) - https://news.ycombinator.com/item?id=23176125 - May 2020 (15 comments)

IMSAI 8080 replica - https://news.ycombinator.com/item?id=21876042 - Dec 2019 (5 comments)

IMSAI – The Computer Film Star of War Games - https://news.ycombinator.com/item?id=10033325 - Aug 2015 (5 comments)

That power supply is really something. I'd be tempted to replace it with something modern, but it's so crazy and massive it's really a major part of the machine's aesthetic when you open it up.

It's a linear power supply and uses twice as much electric as a modern switch mode power supply, if I remember correctly. But very reliable as long as the capacitors are in good condition.

Yeah, seems to be over-engineered. But I guess with all those slots, you never know what the current demands on the system are going to be.

I just googled to find out a bit more about those huuuuge caps and found this: https://www.theretrowagon.com/imsai-power-supply-rebuild/

I had gotten a bit of nostalgia reading an earlier article about the Altair (which this is a clone of) back in 2020. I can't find them on eBay now, but at the time, you could get an Arduino Due based clone of the Altair, so I bought one:

https://imgur.com/a/qntqSMG

Haven't played with it for a while, but I did get it to run Fuzix...a pretty minimalist unix-like OS.

You can still get something like this. Did a search on Tindie:

https://www.tindie.com/search/?q=altair

Yeah that first one is similar, but more ambitious and more expensive...with the replica case and all. I found the email receipt for mine...it was $107 USD:

RETRO ALTAIR 8800 S100 CLONE *FREE SHIPPING* Total: $107.17

I only recently learned that a late friend of mine (Lloyd Smith) had a business in the early days of taking these systems, splitting the power bus, and adding a second power supply to beef them up to be able to run with all the slots full.

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