Retrotechtacular: Donner 3500 Portable Analog Computer

What if we told you we had a computer you can take with you? What if it only weighed 28 pounds? This is a pretty hard sell when today you can get a 1.5 GHz quad-core processor packing computer to carry in your pocket which weighs less than 5 ounces. But back in the day the Donner 3500 was something to raise an eyebrow at, especially for tinkerers like us.

The machine was unveiled in 1959 as an analog computer. Instead of accepting programs via a terminal, or punch cards, it worked more like a breadboard. The top of the case features a grid of connectors (they look like banana plugs to us but we’re not sure). The kit came with components which the user could plug into the top to make the machine function (or compute) in different ways.

We’re skeptical as to how portable this actually was. It used vacuum tubes which are not fans of being jostled. Still, coming during the days when most computers were taking up entire buildings we guess the marketing claim holds up. If you’d like to see a bit more about the machine’s internals check out this forum post.

40 thoughts on “Retrotechtacular: Donner 3500 Portable Analog Computer

      1. Innnnteresting, most of the “programming” seems to be in the overlays, and what it can train the operator to do… But curious as to what actual smarts are in that little black clump in the wiring.

        1. Absolutely nothing, that’s just a wire nut. It looks like someone took it apart to make it do something else. There are no electronics whatsoever, it’s basically a bunch of light bulbs and ganged slide switches.

          1. I actually had one of these as a kid. Rather disappointing for a “computer,” but in reality it does perform logic switching as per the switches and the wiring. Sadly, the monkey is the system clock…

  1. Analog computers always fascinated me. I’v never used one, nor met anyone who has. But the idea of building an analog (in the sense of “simulation”) of an equation, then being able to vary the input quantities and immediately see the change in the output — well, it’s kind of like a “linear spreadsheet”, isn’t it?

    1. They’re getting to be in short supply, but find a Navy guy from around WWII. Targeting on the big guns was done via an analog computer, complete with roll corrections and everything. In some ways it’s more precise as there’s no floating point unit. And that’s the best pun I’ve come up with today.

    2. I remember reading about an analog computer used to solve a differential equation for finance. It involved water flowing into a tank at one rate, and out of it at another rate. The flow rate out was proportional to the height of the water in the tank. … Oh! Thank you, Google, Anyway, this is the sort of thing that a person could do in a workshop.

  2. “We’re skeptical as to how portable this actually was. It used vacuum tubes which are not fans of being jostled.”

    VTs don’t like to be dropped/smashed, that is true. But I think that their fragility is often overstated. Believe it or not, vacuum tubes were used in proximity fuses in artillery shells in WW2. Consider the G-forces a tube would experience being hurled out of the barrel of a cannon.

    More commonly, tubes have successfully taken a beating in “portable” devices like radios and amplifiers for many decades. Those heavy steel corner guards on your guitar amp head are not there because the manufacturer thinks the device will be babied.

  3. I don’t know about that claim that tubes hate being jostled. I can name at least a dozen cars from the 1950s and before which had tube radios that worked reliably for years. In fact, the most common failed part in the old tube car radios was called a “mechanical vibrator.” Plenty of tubes can withstand typical transport bumps.

    1. Yeah. Vibrators were how you got HV for the tubes, rather crude but effective. A lot of old novelty shocking devices like shocking lighters worked much the same way.

      1. I was trying to figure how to make a vibrator out of a mechanical buzzer…. but buggered if I can find anywhere to get a mechanical buzzer any more either. (it was gonna be a low buck workaround to something, so probably no point pointing me to $$$ vintage parts)

        1. Think “electric school bell” it is a relay/solenoid that when energized, pulls the plunger into the coil. The plunger opens a switch in series with the coil, shutting it off. A spring then pulls the plunger out and closes the switch and then re-activates the coil. Cycle repeats, and repeats…. The is also attached to the clapper for the bell. The frequency of the cycle is largely determined by the physical characteristics of the mechanism, as well as the inductance of the coil and the voltage it receives. A multi-vibrator is similar
          but it uses the on/off switching of the voltage in the input of a transformer or auto-transformer to produce a higher voltage in the secondary (the same principle is used to generate the thousands of volts for a spark plug from the 12 volt battery in a car.

  4. A long time ago, I programmed analog computers or school. They were no more difficult to use than a modern computer for modeling equations. Both magnitude and time scaling were sometimes needed but modern digital computers can also bite you for the same things.

  5. There was plenty of portable tube equipment, although lots of it would better be described as “luggable.” I have a tube-based VTVM that isn’t much larger than a high end modern multimeter. There were special sockets and clamps for securing tubes in vibration and shock situations. The giveaway that this device was meant to be carried around is the leather handles.

  6. Ha, tubes in soviet jets… There are still tubes in just about every fighter, warplane or rotary wing craft out there actually. Maybe not for computational needs exactly, but…
    And there are tube types that were built to be a little more rugged with better internal support structures and such along with a clip or can or external dampening. But yeah, filaments become brittle after being used, so don’t want to give them any rough loven’ or hard shocks for sure.

  7. I have owned several analog computer components of the years – all now passed along to others. You are correct about the banana plugs… that is indeed what they used for the patch bays. During WWII vacuum tubes were developed for tanks and aircraft that were literally indestructible by shock or vibration, and after the war the engineering world was fueled by war surplus components. I have many JAN tubes, and even some of those glass cased tubes are rated for over 20g’s.

  8. In the 90s, I once read a 50s or 60s book at a small Midwest collage with a HUGE library, that showed how to build a “computer” It was more of a demo computer, a bit disappointing, because it did no maths itself, as I understood it, and that was something I quite rely on computers for, and admire the simplicity of. I was into retro emulators at the time, & it didn’t seem worth emulating. I don’t remember much about it.

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