Old computers: Altair design issues

This Web page last updated June 04 2008. For more repair issues, check this linked Web page.

early Altair 8800 design issues

After Allison Parent's October 2006 description of clocking the Altair 8800 with a pulse generator, I asked about her specific design issues with the Altair. She told me how early her Altair was, and then how she or MITS improved it. The "8800a" model came out after the original Altair as offered in 1974-75. The "8800b" models were a major re-design sometime later. Allison's reply is below. - Herb

Mine was early, I got a copy of the January 1975 PE [announcing the Altair] in the mail on the 13th of December 1974 as subscribers generally did then. As I'd been working in the industry with the 8008 and had read some stuff from Intel on the 8080, I was a fast leap on the bandwagon. First order was box, CPU, 4K, and PIO card ($1157 as memory serves). I would later add two more 4k memory [cards], ACR and MITS 4k basic and 8K basic.

To put that [price] in context my first truck cost $2500!!! My parents thought I was really off my rocker. My mother was agast, it's spend that kind of money? My father was surprized but curious: he felt there was a computer industry out there, beyond the white coat TV depictions, especially for USED computers. This was in 1974!

Because I was fast on the trigger I got mine in January and it was assembled in a weekend. Those that were even a few weeks slow would have delivery times sometimes measured in months. I was young/crazy then and projects like that meant working Friday and not sleeping till Monday say 2:00AM. That would give me around 50 hours of working time by time you factor meals and needs other than sleep. So I made time to solder it all up. Since I had gotten my engineering degree by carrying a bag as a land-mobile tech, [my] soldering was up to stuff and I had good temp controlled irons and even a desoldering station. Unlike other RF and audio projects it was a kit, populated [with parts to] solder and [needing] some limited mechanical assembly.

[But] by 1977 I'd retired it! I still have it but haven't powered it in years. The last time it was powered was 1979 after a lightining hit to the house. I used the front panel to troubleshoot the NS* Horizon system. During the repair effort I got a Netronics Explorer 8085 system and used that to check and test cards that was far better than toggle switches. That Netronics system still live and gets used as does the NS* Horizon.

The [later] 8800A was an improved machine all the way around.

Things the basic early 8800 needed fixing...

120VAC switching was brought to the front panel and was unprotected against contact by user. Most relocated that switch to the rear panel.

8080 Clock circuit. MITS found some one-shots behaved better than others. I found I hated one-shots and ripped them up and put in a 8224 in spring of '75.

Power [supply]. The stock transformer could not support the CPU, PIO and 12k of S4K dynamic ram. At that loading the rail was barely 8V and the ripple was below 7.4V. [Also the] rectifiers on PS card would melt at high current on the 8V bus.

If you extended the bus (it came in 4 slot segments) and wired them as directions said the interconnecting wires for the 8V bus would melt between the first and second card. If you fixed that the 8V bus trace would burn up with 4 8K static cards!

If the bus was extended to two or more 4 card slots bus ringing was a severe problem. The 4-slot cards were single sided etched! [No ground plane to reduce noise. - Herb] It was much worse if your front-panel interconnect was the specified length. Those leads remade as short as possible helped. At that point a WAMCO Quiet Mother (backplane) was assembled and installed.

Various mods to the S4K dynamic 4k rams were followed by a reduced cost kit for S4K owners of a 88MCD card sans rams. The 88MCD was a cleaner design but didn't work well with all Z80s (card dependent).

Cooling was always an issue, as the fan is in the middle of the rear and can either blow or suck air at the cards but the cards in the middle of the 80 or so slots had no place for air to go to (blow mode) or for air to come from (suck [air out of the case] mode) due to the card having one end abutting the front panel. Solution was bend a new cover with inlet slot two inches by 18 inches on top behind the point where the cover meets the front bezel. Didn't hurt to use a Rotron fan with 120CFM capability either!

FYI: my I/O then was via a PIO card to SWTP [TV Typewriter] (modded for 64x16) and keyboard later replaced with [Processor Technology] VDM1 64x16 video card.

When the system [reliability] wasn't "the project" du jour. I was working on getting off the d@** flaky ACR [cassette interface] and moving to a saturation recording system (Redactron cassette drive) FM (phase encoded) at a more palatable ~20kbaud. My goal at the time was a block replaceable directory based storage. I was also an early adoptor of the NS* MDS controller and a single drive as that represented the Ne pas ultra of the time (1977). Imagine 82k of storage and it was fast! It did however underline the flaky altair as needed retirement for something without painful switches and zillions of oneshots.

From Jan 1975 to late fall of 77 I spent banging my head against the wall with the early Altair. A while ago I got a complete 8800BT [turnkey] with floppy disks and every thing Altair even the desk with rack. It is designed better and still works as well as when the owner got it back in '79. However, the one thing they didn't fix was the lousy airflow. It has one very noisy fan working hard to get air across all (it's a full box) of the cards.

When I saw the first IMSAI I was greatly impressed as it solved so many of the Altair problems and was also easier on the eye and fingers!

Herb Johnson
New Jersey, USA
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Copyright © 2009 Herb Johnson