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Allen Organ Polyphony and Crackling Speakers

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  • Allen Organ Polyphony and Crackling Speakers

    I've recently acquired an Allen MOS1 organ. When I ply it, the output channel for flutes works perfectly. For the other voices, principals and reeds, a single note sounds right but multiple notes cause severe crackling. Does anyone have any ideas for this?
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  • #2
    Whatever your problem is, it's not related to the normal polyphony limitations of these models. When you hit that limit notes drop out, they do not crackle. Apart from that, it could be many things. MOS models have a minimum of two channels - one for flute, one for main. First thing to do would be to swap the inputs to the amplifiers to see if the problem moves. If it doesn't the problem is in the amplifier or speaker for that channel.

    Knowing the actual model # of your organ would be helpful.

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


    • #3
      I will get the specific model number when I get back to work. One other comment that may help or may not. I can use the flute stops on either manual but other voices get crackly above a single note. If I push the SFORZ button, I get a large sound that seems to contain reeds and principals with no crackling. The output from the organ seems to come from 4 TV style cables that connect to the amplifiers. There is also a brown 12v cable but it's unclear what this one might do. I'll take more photos.


      • #4
        It's a S 600 from 1972- 2 computers. The next model, 601, changed the combination action so it is before that.
        Nice organ- sold for $15K installed way back then.
        Crackling *could* be a bad MOS board but there are a lot of other things to rule out first. I'm sure jbird604 will give you more info. Good luck.


        • beel m
          beel m commented
          Editing a comment
          BTW the original Allen amp rack was blue and had the amps horizontal. I hope someone didn't modify other things in the organ...

      • #5
        The amps and speaker cabinets are all constructed the same way. To the best I can tell they are Allen construction and the silver amp is dated 1992.


        • #6
          I got your PM, and I'm glad you went ahead and posted on the forum, and that you are getting some attention from some good guys. You sound like possibly a novice when it comes to tech stuff, so you may have to take baby steps. As noted, the 600 has two "computers" -- two identical MOS systems in the console. This greatly improves the sound quality, and also provides you with two of every board and just about every other part. So once you get just one of the systems up and running correctly, it's a simple matter of swapping parts back and forth to determine what isn't working, and what is causing your problem.

          Troubleshooting should start with the simplest things to check, and in this case, it's the MOS power supplies. They have critical voltage outputs at -27, +5, and -5 on each of the two supplies. Before you do anything else, check to see that these supplies are putting out exactly these voltages, then put your meter on the AC scale to make sure there isn't more than a few thousands of a volt of AC on any DC voltage. The -27 is especially critical. If it's off more than a volt, it can often create extreme distortion and noise.

          Another vital thing to test would be the four audio systems. The 600 is a four-channel organ with four amplifiers and four speakers. You should use one of the flute outputs (since they seem to be working perfectly) to test all the channels by swapping one of the flute signal cables around to all four amps in turn. That will tell you if you have four good working amps and four working speakers. If any amp or speaker is bad, you'll know it. BTW, an amp can sound "bad" when it only needs the volume pot "exercised" -- turn it back and forth a number of times through its range. So don't overlook that.

          Beyond power supplies and amps, you are getting into more complicated issues such as bad boards or the need for extensive maintenance. A lot of seemingly serious troubles will actually clear up with "routine maintenance" in the form of meticulously cleaning and lubricating every socket, plug, jack, terminal strip, and every other point in the organ where metal contacts metal to pass electrons along. These connections need to be clean and secure for the digital and audio signals to pass data reliably and output good sound.

          So run some tests and report back and we'll know better how to advise you. Good luck and welcome to the forum!
          *** 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!


          • #7
            The crackling described sounds like an amplifier may be overloading a speaker. If the AM-100 in the amplifier rack is a replacement, could it possibly be that is the wrong code amplifier for the organ? I know if I were to use my AM-100 (coded for MOS-2) on an ADC or MOS-1 organ, the result might be the same.

            I really need to get my Monitor II speakers connected sometime so I can get a better idea of what they sound like when compared side-by-side with the HC-12 speakers.

            Good identification, Bill! The organ is definitely a 600-series MOS-1. The ID plate on the back rail where it is hinged will give the exact model.

            Way too many organs to list, but I do have 5 Allens:
            • MOS-2 Model 505-B / ADC-4300-DK / ADC-5400 / ADC-6000 (Symphony) / ADC-8000DKC
            • Lowrey Heritage (DSO-1)
            • 11 Pump Organs, 1 Pipe Organ & 7 Pianos


            • #8
              This advice is spot on. The voltages were more than 1 volt off, approximately 1.5 off from the 27. Once I adjusted to 27 the problems seem to have gone away. I'll take photos of the other mystery tomorrow as I begin to check the two older amplifiers. As things stand now I have what I'm told is one principal center speaker, two bass speakers and two flute speakers, but 4 organ outputs and a brown wire that appears adjusted for 12 volts. Just continuing along. Thanks everyone for your helpful advice so far. I feel so much more positive this evening.


              • myorgan
                myorgan commented
                Editing a comment
                Glad to be of assistance. Please keep us informed of your progress!


            • #9
              So, we have moved the organ into our school auditorium, not an easy task that involves at least 10 people and several steps. So here is the situation at the moment. Our 620 has 4 coaxial cable audio outputs. Two are flutes, two are principals and reeds. They all work properly. I also have 6 speakers. Two are labeled flutes, two are for the principals and two are 15 inch bass speakers. At the moment, the replacement amp is working on both Channels, but as of yet, the two older ones are not. I'm trying to figure out how to connect all of these 4 ohm speakers without damaging anything? Series or Parallel perhaps? I could also feed the principals into an external amp if that's ok.