Altair 8800c build thread

Discuss construction, troubleshooting, and operation of the Altair 8800c computer

Altair 8800c build thread

Postby Wayne Parham » April 5th, 2022, 3:30 pm

Hello World!

I've just started an Altair 8800c build. Even though all the "heavy lifting" has already been done by all you guys that came before me, it's still fun to see new builds documented with photos on forums like these. So here's mine.

I'm a pretty busy guy, so I only work on my system a few hours a week, sometimes just a few hours a month. So it may take a while to get to the "tested and working" post in this thread. And like many other hobbyist build threads, that may not be the last post anyway. I ordered a handful of assembled and unstuffed S-100 boards from various enthusiasts.

A bit of background first, some personal history. My name is Wayne Parham and I've been into microcomputers since just after the first microprocessor chips were introduced in the 1970s. You all probably remember those days, so there's no need for me to explain how it felt to be there then. I'm sure a lot of you here can relate, probably most of you are just like me.

I was a teenager in the 1970s, so the Altair was just out of reach because of its price. It wasn't expensive for what it was, but I still couldn't afford it. So I ended up getting a 6502 system because it was cheaper, specifically the Synertek Systems Sym-1. It was an excellent little computer, but I always lusted after that Altair. By the time I could have afforded one, the IBM PC had been introduced and so I ended up making a PC clone. After that, all my experience with Intel processors came by way of the PC and clones, and with the ISA buss and subsequent standards.

But I never got over that Altair 8800, so when I learned about Mike's clone, I immediately bought one. I love that thing! And now I want to build an 8800c, to get even closer to those systems of yesteryear. To tell the truth, I like this version even better than the original. It's sort of a "restomod" - to use a car term - and I just really like this replica.

Now onto the build.

First, the front panel. I soldered the resistors, capacitors and chip sockets onto the front panel board, just as Mike's instructions show. Then I used the front panel mounting bracket as a jig, again, as per the instructions. I used four flux remover spray can lids as supports for the bracket, and set in place all the switches. All the switches except the power switch, that is, because it hasn't arrived yet.

Altair8800c_frontpanel_assembly.jpg
Front panel assembly


One things I noticed - before soldering - was the switches have a slot in the threads. At first I had oriented all the switches with their stamped part numbers all on the same side. But this made some slots on the top and others on the bottom. So the stamped sides aren't uniform. No big deal, just something I noticed. I looked at Mike's build of my clone and saw that he had arranged all the switches with the slots towards the bottom. Nicely done, and I did the same. It looks better that way.

Once I soldered all the switches in place, I flipped over the mounting plate.

Altair8800c_switch_mount.jpg
Switches on mounting plate


Next, I'll add the LEDs. That's for another day.

Altair8800c_frontpanel_board_assembly.jpg
Front panel board without LEDs
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Re: Altair 8800c build thread

Postby Wayne Parham » April 5th, 2022, 3:33 pm

I got the 9-slot backpanel and connectors from Todd Goodman. I also got a few unpopulated S-100 boards from him, but they're projects for a later date. I haven't added any components to the backpanel yet except the edge connectors.

Altair8800c_backplane.jpg
Backplane board


And of course, the processor board. I bought a couple Altair 8800 CPU boards from Gary Kaufman. Only one is needed, of course, but I think it's good to have spare boards because it makes troubleshooting so much easier as time goes by. If something breaks, I can swap boards to find the defective board, and then swap chips to find the defective chip. Just makes maintenance easier.

Altair8800c_CPU_boards.jpg
8080 CPU boards for Altair 8800c


I have Mike's FDC+ disk controller board and Martin Eberhard's 88-2SIOJP serial board too. And I have a handful of old floppy drives so once I get everything ready, I will have a complete system.
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Re: Altair 8800c build thread

Postby BillO » April 5th, 2022, 5:12 pm

Nice start!

I too started with 6502 MACHINE. An OSI Superboard II. I still have it, and I picked up a Sym-1 along the way. Still have that too.
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Re: Altair 8800c build thread

Postby AltairClone » April 5th, 2022, 5:31 pm

Thanks for documenting your progress Wayne.

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Re: Altair 8800c build thread

Postby Wayne Parham » April 6th, 2022, 8:51 pm

Look what came in the mail today:

Altair8800c_18slot_backplane.jpg
18-slot backplane, edge connectors and card guide slots from Jerry Walker at JMprecision.co.uk


Now I need to decide which backpanel to use, the 9-slot or the 18-slot version.

Both appear to fit, but obviously the 18-slot takes up more space. It pretty much covers the whole inside. That's not a bad thing as long as I don't run into unexpected gotchas. Isn't that always the case?!!

Here's the test fit of the 9-slot backplane. Lots of room for that guy.

Altair8800c_9slot_backplane_test_fit_in_chassis.jpg
Altair cabinet with 9-slot backplane


And here's the test fit with the 18-slot backplane. Pretty much fills the cabinet side-to-side and front-to-back.

Altair8800c_18slot_backplane_test_fit_in_chassis.jpg
Altair cabinet with 18-slot backplane


Questions for the group:

1. Does my power supply layout look OK to you guys? I think there's enough room for the fan, although I still need to confirm that. I don't really like it blowing over the little 16-volt supplies, as I doubt they'll need the cooling. The 8-volt unit would more likely need the fan. Then again, the shorter 16-volt supplies would allow flow over them and into the card cage. At least the middle boards would get some flow. What do you guys think?

2. This is a dumb question that really falls into the RTFM category, but I don't have a manual or schematic so I'll have to look around and see if I can find one. My question is do those switching supplies tie their chassis ground to any of the output lines, perhaps their negative outputs? I assume not since we're hoping to make +/-16 volts from the little pair of 16-volt units. I'll double-check with a meter, but I'm thinkin' you guys probably already know.

3. Unrelated to layout: I'd like to punch a hole in the cabinet rear panel opposite the DB-25 connectors to fit connector(s) for the disk drive(s). I read the suggestion to run the ribbon cable out the back, sliding it over the rear panel and just under the cabinet top. But what do you guys think about adding a connector or two for disk I/O on the rear panel?

4. Is there anything else you guys can see that might hang me up? Any preferences of backpanels? Any other advice?
Last edited by Wayne Parham on August 14th, 2022, 2:07 pm, edited 1 time in total.
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Re: Altair 8800c build thread

Postby toml_12953 » April 7th, 2022, 4:57 am

Thanks for the photos! I have an 8800c but I was hesitant to order the 18-slot board in case it didn't fit. Now I know it'll work fine.

1. I have my power supplies arranged just like you do and have no problems.

3. I have my disk cable laid over the edge of the back panel. The cover doesn't crimp or otherwise interfere with it.
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Re: Altair 8800c build thread

Postby AltairClone » April 7th, 2022, 7:29 pm

No, the switching supplies themselves do not tie chassis ground to any of the DC output terminals.

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Re: Altair 8800c build thread

Postby BillO » April 8th, 2022, 1:00 pm

With the modern cards you have (Mike's and Martin's) you certainly don't need the 18 slot MB unless, of course, you intend to try some 1970's cards. The shorter MB will be inherently less trouble electrically speaking.
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Re: Altair 8800c build thread

Postby Wayne Parham » April 8th, 2022, 3:35 pm

Thanks for the advice, guys.

Thanks for telling me the supply outputs are isolated from chassis ground, Mike. The supplies will be super-easy to install and connect. I like the old transformer rectified and filtered supplies - they're so simple - but I like the light weight of these little switchers.

On the backpanel, I definitely hear you Bill. I was worried about the same thing, and wondered if the signals would be clean enough with resistive termination or if the buss was so long it just had too much reactance to damp. I haven't studied enough to see what kinds of termination are used. Just glancing, it appears some use no termination at all - just whatever loads the boards provide - others add termination. So I'm guessing that big backpanel would probably benefit from a terminator.

Seems like I've seen 270Ω resistors used, both to ground and to +5v, making a voltage-divider that sets an unloaded line to around 2.5v. That would damp both current sources and current sinks. Might try something like that.

Then again, as you've said, there's really no need for 18 slots. Now days you can do everything with four slots leaving as many more open for extras. So I might just use that 9-slot board.

I'll let it ferment in my mind while I solder these LEDs to the front panel and build the front-panel interface.

I'm still pondering the drive connector too. I kind of like the idea of the DB-37 connectors used in the drive type 0 configuration, as described on page 9 of Mike's FDC+ manual. It's so period-correct.

But I think I like the IDC-34 connectors that support drive types 3 & 4, as shown on pages 12-14. It seems a little more convenient when connecting drives from the IBM PC era, which is what I'm sure I'll use.
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Re: Altair 8800c build thread

Postby Wayne Parham » April 8th, 2022, 8:11 pm

I had a little time this evening, so I decided to solder on the front-panel lights.

I pretty much just followed the directions, which worked beautifully. Inserted all the LEDs and then pushed the front-panel board onto the front face. Then placed the 5/8" spacers to set the spacing and soldered one lead on each LED. That way if I needed to adjust any of 'em, I could do that pretty easily. But all appeared to be sitting just right.

Altair8800c_frontpanel_board_installing_lights.jpg
Installing the lights


I "hosed off" the board really well with flux remover. I know I still need to solder in a couple of connectors and the power switch, but there's no telling when I'll get around to doing that. I don't even know when I'll get the power switch - It's on backorder. So I wanted to go ahead and clean the board because sometimes flux is harder to remove when it has sat a while.

Altair8800c_frontpanel_board_with_lights.jpg
Front-panel boards with lights installed


Now to do the final test-fit of the front-panel board on the chassis mount including the front face. I just "finger tightened" a few of the nuts to see the results. I also wanted to put it all together because it may be a few days before I get back to it, and having it assembled will prevent the LED leads from getting bent.

Altair8800c_frontpanel_test_fit.jpg
Test fit of the front-panel on the chassis mount
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