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Two card slots?

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  • Two card slots?

    So I recently found this:

    I never heard of Allen installing two card slots on their units. How common is this?

  • #2
    The System 600 MOS-1 (1971-1973) had two card readers, one for each of the two computers. I can't remember if the System 632 did also, or if they combined all -8- alterables into one card reader like the System 601-2-3 did IIRC.
    R, Bill Miller


    • #3
      MOS organs above the 300 series have more than one "computer" system in the console. Obviously these larger organs sound better, have more stops, more features, etc. The advantages of having two or more computers in one organ were many.

      A problem created by having two systems in the same console was the possibility of interaction between the two systems, each of which had its own clock, with the clock frequencies usually slightly offset from each other. When you're in the megahertz range, you could hardly expect two clocks to be synchronized, no matter if you wanted them to be or not. Anyway, there would also be some difference in clock frequency. Any electrical connection between the two systems had the potential to create a wild and weird "strumming" phenomenon in which the system would spontaneously begin to emit a continuous output of notes running up the scale. To hear this, all you have to do inside any two-computer MOS organ is put a clip-lead between the two isolated leaf switches behind a key.

      Because the two computers had to be fully isolated, they couldn't share anything. There would be two power supplies, two layers of switches on the keyboards, and of course two card readers, each one supplying half the alterables in each division.

      Isolation between the two systems at the stop switches was done by having diodes on each stop line so that each line could sense the position of the switch but was unaware of the other line being connected. The pedal switches also had to be finessed, as it was impossible to have two reed switches on each pedal magnet. A similar diode system was used.

      It wasn't too long before they figured out a way to do something like that with the card reader, so a single reader could be used to communicate with both computers without "strumming".

      While they never created a keying solution for MOS-1, the MOS-2 organs had isolation built into the system for the keyboard contacts, and they never had to use the two layers of contacts. (The single-computer MOS-2 model that also had an analog celeste did in fact have two layers of switches, but that was for an entirely different reason.)

      So, if you see a MOS organ with two card readers, it is quite an early one. I suppose they built hundreds of them that way, so it is probably not as uncommon as you might think.
      *** Please post your questions about technical service or repair matters ON THE FORUM. Do not send your questions to me or another member by private message. Information shared is for the benefit of the entire organ community, but other folks will not be helped by information we exchange in private messages!


      • #4
        Thanks... by the way, do you know if I could see a 5 manual model with the two card readers? (I was just thinking about my other thread: )