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Allen MOS stops: connectivity and signal type

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  • Allen MOS stops: connectivity and signal type

    My dad recently acquired an Allen 903 (MOS-1), and I have a couple of questions regarding the signal path from the stop switches to the computer:

    1) It looks like the stop switches (drawknobs) run to the center posts of the Tone Strip, where each stop is then connected to one or more of the upper and/or lower posts. Some posts on this forum seem to confirm this. Do the wires from the outer rows then run to the Stopboard Array cards, or somewhere else?

    2) What types of signals are present at (a) the stop switches, (b) the Tone Strip and (c) the output of the Stopboard Array card? My first guess is that the stop switches are DC, probably TTL level such as -5 to 0 VDC. However, the signal at the switches could easily be some other DC voltage range, or something else entirely - I've seen some comments about Allen MOS organs emitting RF due to low-MHz digital signals being routed around.

    Any information would be very helpful.

    Thanks,
    John B.

  • #2
    John,

    A large multi-computer MOS organ like that was a nice organ in its day, and they can still make good music. Glad to know your dad has acquired this one. In regard to your questions ...

    (1) The stop rail of a MOS organ is at system ground (0 volts). When the knob is pulled, the corresponding wire goes to ground. This wire from each knob leads to a nail in the center of the tone strip. A diode then connects it to one or both of the outer rows of nails. Each outer row corresponds to one of the two primary MOS systems (usually called A and B), and where a diode or diodes are present on a given stop, it activates the required tone in one or both MOS systems. Many if not most stops do in fact use both A and B computers, which gives the stop some "life" in the form of two independent sources, slightly detuned, coming from different speakers. When "celeste tuning" or "chorus tuning" are drawn, the tuning of the two computers is pulled further apart, enhancing the chorus or celeste in a given division of the organ.

    (On your 903, there is a third computer, often called "C", which only creates the choir division stops.)

    Most of the wires from these outer rows of nails lead to the Stopboard Array, which serves to pass along the stop-drawn data to the MOS board itself. However, a few stops bypass the SBA and are wired directly to the MOS board. If you can get your hands on a MOS system wiring diagram, you can see just how this is done. I believe those few stops that by pass the SBA are all related to a specific ROM on the MOS board and are the stops that are modified when a non-church type of voicing was specified on a certain organ.

    (2) So you can see that the stop rail itself, being at ground potential, sends nothing to the SBA or MOS board, but a drawn stop serves to ground the control wire for that stop. I don't know for sure, but I suspect the control lines are at +5 volts before being pulled down to zero by a drawn stop. At the tone strip, you can actually measure the voltage change by clipping a lead to a nail (center or either side) and connecting your other meter lead to the system ground. Measure before and after drawing a stop and see what happens.

    The output of the SBA card sends multiplexed data to the MOS board, and I have no idea what format it would be in. All I can say is that the SBA handles the inputs from about 32 stops and conveys that information on the MOS board, probably on a 4x8 matrix.

    The MOS system operates on a 4 mega-Hertz clock, so there could well be some RF radiated in that frequency range. Allen got away with that throughout the 70's and early 80's but found it necessary to put the digital stuff into a shielded cage when the ADC models came out about 1983 or 1984.

    The MOS system was the basis of every Allen digital built from 1971 until 1983, except for some elaborate custom models using a system called SDDS ("super-duper-digital-system?")... The MOS-2 models that were sold for only a couple of years in the early 80's were a revision and updating of the MOS concept.

    All the MOS and MOS-2 organs are capable of extensive customization and experimentation. Imagine what you can do if you tinker with the arrangement of the diodes on the tone strip -- causing a given tab or knob to draw as many tones as you like. In some very large MOS organs with several MOS computers in remote cabinets, they produced the effect of many dozens of unique stops by simply combining the stock set of MOS tones in creative ways, with and without percussion or chiff, for example.

    Your 903 has one "custom" stop on it, I believe. On the Great division you have a trumpet stop (maybe spelled some other way), but the MOS system does not in fact have a trumpet in the great. On the 900 series organs, the tone strip is wired so that turning on the trumpet stop draws two MOS waveshapes and blends them together -- a Schalmei and a Krummhorn. Makes a lovely trumpet.

    Good luck playing around with the MOS.
    John
    ----------
    *** 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!

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    • #3
      Originally posted by jbird604 View Post
      (2) So you can see that the stop rail itself, being at ground potential, sends nothing to the SBA or MOS board, but a drawn stop serves to ground the control wire for that stop. I don't know for sure, but I suspect the control lines are at +5 volts before being pulled down to zero by a drawn stop. .
      Definitely not +5 vdc to SBA. One of the biggest pains in developing my MOS Midi encoder boards was the fact that this is PMOS technology and the signal voltages are negative. I'm sure the voltages on the SBA pins are negative, and I think they're -12 vdc, but don't recall off the top of my head. The combo action sense signals on the stops are +5 vdc however.
      -Admin

      Allen 965
      Zuma Group Midi Keyboard Encoder
      Zuma Group DM Midi Stop Controller
      Hauptwerk 4.2

      Comment


      • #4
        Thank you both for the information.

        -John

        Comment


        • #5
          You are most welcome, John. And welcome to this forum!

          I serviced a MOS Allen yesterday, a 600 series (classical two-manual model with two computer systems). The 600 series would be basically the same as your 903 without the choir division.

          Just for fun, I measured the voltages at the tone strip and found that each stop control line is approximately -5 volts with stop canceled. When the stop is drawn, the tone strip nail goes to zero, of course, as the tab grounds the line.

          Admin, you may be correct that the lines are -12 on the SBA, as there are no doubt dropping resistors involved.

          Whenever I work on a MOS organ I'm impressed with the simplicity of the system (compared to a lot of older analog organs) and the longevity. How much of today's whiz-bang digital stuff will still be working in 40+ years?
          John
          ----------
          *** 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!

          https://www.facebook.com/pages/Birds...97551893588434

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          • #6
            After taking some measurements, first with a voltmeter, then with an oscilloscope, here is what I found:

            1) A few stops are at -27 VDC when cancelled. These include: (Pedal) Contrabass 32 and Untersatz; (Swell) Contratrompette 16 and Plein Jeu III; (Great) Fourniture 4 and Quintaden 16; (Choir) Fagotto 16, Celeste Effect and Chiff, as well as the Swell to Great coupler (all of the coupler rocker switches run through the tone strip as well as all of the stops)

            2) Two of the couplers (Choir to Great and Swell to Choir) are +12V when off.

            3) The Alterable stops carry a repeating sequence of -5 V pulses, which appear to be different for each stop (I only looked at a couple of them).

            4) The majority of the stops (measured at the tone strip) are around -4.5 V when cancelled, and 0 V when drawn. However, there is an interesting non-DC waveform present on these wires - see the attached scope capture. I don't know if this is a power supply issue, some other signal being coupled onto these lines, or intentional. The waveform is around 35 Hz, just under 5 V in amplitude, and fades away over a period of approx. 40 seconds, then builds back in. The higher-frequency sawtooth never goes away (approx. 1 V amplitude). When the corresponding stop is drawn, the signal is flat at 0V, as expected.

            Click image for larger version

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            If anyone has any insight into the nature of this signal, I'd love to hear it.

            -John B.

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            • #7
              Interesting. I can tell you the reasons for some of this, but not all.

              Most stops wire to the Stopboard Array, and these are the ones that measure about -5 volts when not drawn. Since the stop rail is at ground and there are dropping resistors on all lines coming out of the stopboard array, turning on a stop will change the measured voltage to virtually zero.

              The stops that measure -27 when not drawn are the ones that wire directly to the MOS board (through some dropping resistors on the motherboard), bypassing the stop-board array unit. Because of the way the MOS system is designed, certain stops are "custom" in the sense that their standard chip on the MOS board could, back in the day, be swapped out for a special chip. These customizable stops included most mixtures and the 16' and 32' stops of the pedal, among others. Because these stops were produced by a distinct sub-system on the MOS board, they could not be addressed by the SBA and required a control line going directly to the MOS board, which happens to operate on the -27 volt power supply.

              Some of the controls you mention (chiff, celeste effect) are not actually MOS board functions, so I can't tell you why they are at -27 without digging into the wiring diagrams for that model. On the choir computer of your 903, "chiff" is a completely different thing from the "chiff" on the swell. The choir chiff is a percussion applied to one or more stops of the great stop selection (great stops are otherwise not used since the choir stops are actually the swell stops of the MOS stoplist). This percussed sound is coupled via a natural coupler to the other stops of the choir. And the celeste effect of the choir is a complicated rigging of both natural and synthetic couplers along with a function called "sharp tuning." Both of these stops have a drawback in that they cause each key pressed to consume two keying slots, thus limiting the polyphony of the choir to only six notes when either of these is drawn.

              The two couplers that measure +12 volts are the couplers that couple one MOS system to another. In a big MOS organ like the 903, these couplers activate the "remote keyer" boards, and I suppose those boards use +12 as their operating voltage. The other couplers are "synthetic" couplers (in MOS-speak) which work within an individual computer system, and all the native voltages in the MOS boards are negative.

              The other stuff you found is beyond my knowledge. If you suspect some kind of power supply anomaly, all you have to do is put your scope on the various outputs of the MOS power supply. You should see a very steady and clean DC voltage, free of AC ripple. If you see ripple, the large filter cap on that leg needs to be replaced. But this is unlikely, as those power supplies fail very rarely, and when they do the organ will probably stop working altogether. I think the pulse trains and odd effects you see on your scope represent some kind of activity within the system.
              John
              ----------
              *** 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!

              https://www.facebook.com/pages/Birds...97551893588434

              Comment


              • #8
                Originally posted by jbird604 View Post
                ...The stops that measure -27 when not drawn are the ones that wire directly to the MOS board (through some dropping resistors on the motherboard), bypassing the stop-board array unit. Because of the way the MOS system is designed, certain stops are "custom" in the sense that their standard chip on the MOS board could, back in the day, be swapped out for a special chip. These customizable stops included most mixtures and the 16' and 32' stops of the pedal, among others. Because these stops were produced by a distinct sub-system on the MOS board, they could not be addressed by the SBA and required a control line going directly to the MOS board, which happens to operate on the -27 volt power supply.

                Some of the controls you mention (chiff, celeste effect) are not actually MOS board functions, so I can't tell you why they are at -27 without digging into the wiring diagrams for that model. On the choir computer of your 903, "chiff" is a completely different thing from the "chiff" on the swell. The choir chiff is a percussion applied to one or more stops of the great stop selection (great stops are otherwise not used since the choir stops are actually the swell stops of the MOS stoplist). This percussed sound is coupled via a natural coupler to the other stops of the choir. And the celeste effect of the choir is a complicated rigging of both natural and synthetic couplers along with a function called "sharp tuning." Both of these stops have a drawback in that they cause each key pressed to consume two keying slots, thus limiting the polyphony of the choir to only six notes when either of these is drawn.
                Makes sense. It is interesting to see the different customization paths that they put into the design. (Or rather, I'm beginning to see bits and pieces - so far, I haven't succeeded in digging up any schematics or other documents that would be invaluable for understanding the system. Without the expertise on this forum, I don't think I'd even attempt it.)

                Originally posted by jbird604 View Post
                ...The other stuff you found is beyond my knowledge. If you suspect some kind of power supply anomaly, all you have to do is put your scope on the various outputs of the MOS power supply. You should see a very steady and clean DC voltage, free of AC ripple. If you see ripple, the large filter cap on that leg needs to be replaced. But this is unlikely, as those power supplies fail very rarely, and when they do the organ will probably stop working altogether. I think the pulse trains and odd effects you see on your scope represent some kind of activity within the system.
                I agree that the pulse trains are probably a normal part of the system. The 35 Hz waveform on the "DC" stops is very puzzling, though. I don't think it is a PS issue (but I do have some power supply questions that I'll post shortly), but I can't think of any reason why a signal like that would be used.

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                • #9
                  JohnB, I'm not sure if you're asking about the stop signal levels out of mere curiosity, or whether you're planning to interface to them for a practical purpose. If it is the latter, as I've already alluded to in this thread, the signals to use are the capture sense signals that go to the DM board, not the ones on the tone strip.

                  As for the signals on the tone strip, I don't have any information for MOS 1 models, but for MOS 2 models the voltage will be -16 vdc with the stop off and 0 volts with the stop on. There are no pulse waveforms on these inputs.
                  -Admin

                  Allen 965
                  Zuma Group Midi Keyboard Encoder
                  Zuma Group DM Midi Stop Controller
                  Hauptwerk 4.2

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