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Altair 8800 restoration

W

Worblehat

Member
Joined
Feb 28, 2017
Messages
36
Location
Victoria BC Canada
A while ago a couple of Altairs were donated to a collection that I'm in charge of, an early model 8800 and an 8800b. Neither work. Of the two, the 8800b is in the best shape, but missing a couple of key components (CPU card and RAM). I'm going to set that one aside for now. Instead I want to focus on restoring the 8800 as best I can.

It's not in great shape. As you can see from the photos, the power supply has been pillaged and is missing a transformer or two. Even more oddly, it has a large capacitor which doesn't seem to be in any of the photos I've seen of other systems of this type. The big capacitor looks to have been grounded to the chassis. Anyone care to speculate on what might be going on there? Any reason why I shouldn't remove it?

This one at least has the CPU card, although no RAM.

So what's my goal here? Well, it's not going to be a museum quality restoration; my skill level isn't that great. I have a JAIR 8080 card; at the bare minimum I would like to get this Altair to the point where it can run on that. If I can get it to run on the original CPU card, so much the better.

Before I dive in, does anyone have any sage advice to impart? Any obvious gotchas in the photos that I might be missing? I'd be especially interested if anyone has tips on where to find replacements for the missing transformers.

Thanks,
John

altair-front.jpg altair-cpu-1.jpg altair-cpu-2.jpg
altair-cap-ground.jpg altair-power-1.jpg altair-power-2.jpg
altair-serial.jpg
 
Well, the front panel is at least very nice! The big cap was likely added to help make up for how inadequate the original 8800 power supplies were. If you're not going for a 100% authentic type upgrade, just remove it and the original capacitor board, and replace with modern modular switchmode supplies. They'll run cooler, and the boards will run cooler too.
 
Yes, still nice relatively nice looking on the outside, although some of the lettering has worn away. I've been back and forth on whether to go with a modern power supply, but I think you're right, for what I'm doing a modern power supply would be the better choice. Thanks for the advice!
 
Ok, so doing a bit more research, I've come across some switch-mode power supply recommendations at:

http://www.vcfed.org/forum/archive/index.php/t-37091.html

and

www.s100computers.com/N8VEM_Archive/docxgm0EI0ecm1.docx

They recommend a couple of Mean Well power supplies, those being:

Mean Well NES-100-7.5 power supply (13A at 7.5 volts,
Mean Well NET-75-C (+15 at 2A, -15 at 0.5A, +5v at 6A)

and

Mean Well SP-100-7.5, adjusted to 8V @ 13amps
Mean Well T60-C, adjusted to +16V @ 2amps, -16V @ 0.5amps. (+5V@5amps unused)

The models listed in these docs don't appear to be available anymore but would these would be contemporary equivalents?

RSP-100-7.5 (https://www.meanwell.co.uk/media/productPDF/RSP-100-spec_8.pdf )
RT-65C (https://datasheet.octopart.com/RT-65C-Mean-Well-datasheet-20724245.pdf)

thanks,
John
 
Worblehat said:
And - sorry - another question, since I'm pretty new at this game. If I'm using a modern regulated power supply, is there any advantage to using one that puts out +16, -16, and +8 volts? Or can I get away with a power supply that puts out +12, -12, and +5 volts as shown in this video:
Click to expand...

The S100 boards have regulators on them that provide the required voltages, and those regulators will not work without supply voltages at least as high as their margin voltages. The 7805 +5V regulator requires a minimum input voltage of +7V. The 7812 +12V regulator requires a minimum of +14.5V. To do what you suggest, you would need to remove the regulators from the boards and bypass appropriately with jumpers. This would obviously leave your boards in a non-original state and mean you couldn't use them in another S100 chassis.
 
The Mean-Well 7.5V output supplies are an excellent choice for the +8V supply. You can often get away without adjusting the voltage up, which will run the regulators cooler.
 
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