Embarking on an Altair 8800C build

[TheoAU]

Hello fellow Altair enthusiasts. Theo here from Australia. A little background story on me. I own an original Altair 8800 rev 0 that I purchased back in 2011 from a guy in Canada. The machine worked well from the front panel, and it came with quite a collection of S100 boards, but it didn't come with a serial board. So, I couldn't use the machine for anything useful. It took a couple of years to get a working serial board. Eventually I managed to get an SSMIO4 and also a MITS 882SIO. Both boards took a little work to get operational, but I'm happy to say both still work well. I also own two of the newer boards, Mike's FDC+ and Martin Eberhard's 882SIOJP. My FDC+ is currently hooked up to a TEAC 55GFR which works great.

I haven't done much with my Altair in years, but recently my enthusiasm was revived due to Mike's Altair 8800C Youtube video. Mike has helped me a lot over the years, especially with getting the SSMIO4 working. And his websites and Youtube channel are fantastic sources of information on the Altair. In particular, I was very excited about creating a new CPU board for the Altair. So, I've recently purchased the CPU board PCB by Gary Kaufman and the parts kit available on ebay through tpkirkp.

My plan is to first build the CPU and test it in my original Altair. Then move onto getting the S100 backplane and power supplies. And eventually onto the front panel kit and case. I'd love to have an Altair built out of new parts that I can use to diagnose some of the non-working S100 boards that I have.

So this weekend I'm assembling the CPU board. I'll add updates to this thread as I go. And I'm no electronics engineer either, just an overly enthusiastic amateur who's half decent at soldering.

Here are some pics of my CPU kit and my original Altair.

[BillO]

Awesome!

Welcome to the community and good luck with your build.

[Curto]

Welcome to the club. I am jealous of your original 8800!

[TheoAU]

The CPU board is built. I made sure I triple checked every component and connection while building it. Thanks to other users on this forum, I was able to view some high-res pictures of their CPU builds which helped a great deal. Thanks also to Shadowtron Blog Youtube channel for demonstrating the build and providing some good test procedures. Once my build was finished, I followed the same test procedures and all voltages checked out. I then inserted all the chips, plugged it in to my original Altair and what do you know, it works!

[TheoAU]

Here's the original Altair with reproduction CPU loading CP/M.

[Wayne Parham]

Absolutely wonderful!

The only thing better than an Altair is a couple of 'em.

[TheoAU]

Thanks BillO, Curto and Wayne. The posts on your 8800C builds have very helpful.

Here's a comparison pic of the reproduction CPU along side the original. I'm amazed that the original CPU board still works. The silver caps are showing signs of leaking. Now that it's out of my machine I'm going to replace them.

I ordered the power supplies today. I bought the ones recommended on Mike's page. I'll also look into getting the backplane and some connectors.

[Wayne Parham]

Definitely a good idea to replace those old electrolytic capacitors. Tantalums too. Swap 'em all, if they're old. They're really the only high-failure rate components. Other types of caps last forever, but not electrolytics.

Semi-related: I've repaired over a dozen tube radios, some with little problems, others with everything wrong, including an open field coil that I hand-wound to repair. But in all cases, the large value capacitors need to be replaced. Often times the tubes are fine but the caps are dead.

[TheoAU]

I've now managed to order everything for the 8800C except for the front panel LED's and corresponding resistors:

- Power supplies from Digikey, 1 x HRP-75-7.5, 2 x RS-15-15. Received.
- 9 slot backplane and EDAC S100 connectors from Todd Goodman.
- All switches and some other components from Mouser.
- IC's, capacitors, cables etc. from my local Jaycar store.
- Case and front panel boards from Mike.

The only thing I couldn't get hold of was the recommended HLMPD150A LED's. I see in Wayne's build thread they weren't available back then either.

LED's are quite easy to get, it's just a matter of getting some that are suitable. My local Jaycar store has these:
https://www.jaycar.com.au/red-5mm-led-8 ... =relevance

From what I've read, what matters with LED's is the MCD rating and the forward voltage. These are rated at 8mcd and forward voltage is 2.3v. The Jaycar page in the Specifications tab, recommends resistors at various voltages. For 5VDC they recommend 130 Ohm resistors for these LED's. I'll pickup some samples to test them before buying the whole lot.

[Wayne Parham]

I'm not sure the forward voltage is all that relevant. I even have blue LEDs in one of my builds - which require much higher forward voltage - and they look great. I think more an issue is the lumens per mA and the diffusion. And one more important factor: the lead length. Some LEDs have long leads but some barely make it.

All that said, in my opinion, the "good old" generic T1-3/4 (5mm) LEDs we were all familiar with in the 1970s are what you want. They are diffused and have a color and brightness that work well in the Altair. You definitely don't want a non-diffused LED but other than that, most of 'em will work.