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  • Allen MOS-1 making questionable noise

    Hi everyone,

    I have an Allen MOS-1 organ; the entry-level 120C. After installing a new 12V power supply (the time-delay circuit blew, but I am not confident at all with repairing power supply components; thankfully it is working now with a remote control), I decided to follow a few of jbird's tricks in his handy sticky-thread (Thanks by the way to jbird, those were awesome!). Those went really well (nothing like some good celeste to remind one of choirs of heavenly angels, and speaker-blowing 32' stops to remind one of the eternal fires of Hell); and as such, I decided that not just the "Main" but the "Flute" channel should have reverb (I was annoyed that switching to an all-flute registration made things sound very uncharacteristic).

    So, I used a couple of hand-spliced RCA cables (cringe, I know) to join the outputs of the DAC (dangerous, but I trusted that a diode was somewhere along the line; I think jbird said it was ok on another thread), which I fed into the reverb, and then into the "Swell to Great Attenuator" (following how things were connected beforehand). It was working very well, and I was getting nice reverb (albeit the flute vibrato was made unusable), when I decided to harmlessly change the bass levels. I changed the "bass" dials on the DAC (I thought it was an original and not a DAC 2/3/4 but I could be wrong). I probably fiddled with other stuff when I was in the back of the organ too, being a little too ambitious with the old thing. And now, I get this horrible hissing-ish static-like sound when the power is on (i.e. when I turn on the 12V power supply with my remote). The noise is of a high pitch, as it comes almost exclusively through the presence projector speakers (though it is audible through the lower console speakers as well).

    I thought perhaps something was wrong with the random noise generator, but when I turn it off, the noise persists. I don't think the problem is the amps (though I have yet to unplug their inputs to be sure), because when I turn on the flute vibrato (a digital one by the way; I think this was one of the later organs, or else was renovated in the past), the noise is clearly tremulated along with the stop sounds. The noise stays the same volume no matter how many stops are down. However, it is affected by the expression pedal, like any stop sounds would be. I am not sure if the noise is going through the reverb (which would indicate a problem with or before the DAC); it is hard to tell.

    So I removed one of my questionable hand-spliced cables and, after learning that the reverb has in fact two inputs, I routed each DAC output to its own reverb input. So now, my only hand-spliced cable is splitting the single reverb output to the two inputs of the attenuator. However, the noise still persists. I have yet to rule out the other cable. The only reason this noise is a problem is because, when the expression pedal is high enough that the 1' and 1 2/3' Swell stops are audible, they are drowned out by the noise.

    Some things I will try:
    • disconnecting the amp from its inputs, to rule out an amp problem
    • eliminating the flute-vibrato from the circuit, since it is unusable anyways
    • taking and showing you some pictures!
    Does anyone have thoughts as to what could be the problem? This organ actually sounds surprisingly good (well, most of the stops, anyways), and is a reliable practice instrument. Easy-to-register pieces like Bach's Trio Sonatas and chorale preludes sound awesome on it (though I would not try to play anything modern).

    Also: This organ was from a church I've never been to, from Craigslist (for $200 CAD), and I have no clue if it is wired correctly, or was repaired/renovated in the past. If the way I connected things sounds questionable, it probably is 😉

    Thank you guys very much! I am convinced that this forum is the best place on the Internet.

  • #2
    Xopowo,

    It sounds like you're heading the right direction with this organ and testing the right things. Let us know how all your trial-and-error works, and perhaps we can help from there. Just be sure you eliminate/change only one thing at a time and that will help you narrow things down.

    Please keep us posted.

    Michael
    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

    Comment


    • #3
      Okay! Problem solved! I wish I knew what it was.

      So what I did was, I initially disconnected the amps' inputs. Nothing besides a little static (they're pretty old amps) came through the speakers, as expected. So, the problem wasn't the amps.

      Then, I took out my questionably-spliced cable and re-spliced it to be a little less shady. I thought maybe some interference was coming through. I tried the amps connected to the flute vibrato, which was connected to the attenuator. No noise. So, the vibrato was not the culprit, and neither was (I think) the attenuator.

      So, I plugged in my freshly spliced cable from the Reverb to the attenuator. Everything was connected nicely. I turned down the levels of both channels on the DAC (which was a DAC 2 after all) and tried again. No noise at all. So, my guess is, something was wrong with that cable. Or, maybe just the motion of unplugging things solved something.

      Thanks everyone for paying attention! I hope this thread helps someone out in future. Good luck with your own repair endeavors!

      Zack

      Comment


      • #4
        It could have been that the connectors were dirty and when you put the cables back on that may have cleaned them enough to make good connection again. Just my thoughts.

        Comment


        • xopowo
          xopowo commented
          Editing a comment
          That's a good point. I'm not sure if those cables had ever been removed before, as they were quite difficult to remove.

      • #5
        My experience with the DAC board on my Allen Mos-2 225 HTC is that the trim pots on there don’t really seem to be meant for volume adjustments. At least, when I got them, they were both maximum loudness, and adjusting them didn’t seem to affect the loudness as much as I would have expected. So, my feeling is that it is best to leave them alone. The amplifier I have has trim pots that work closer to what I would expect, and on my organ there was a board that brings the organ output to line level - so it makes sense to adjust things downstream of the DAC board before tweaking there. Also, the sound on one of my channels was very weak before cleaning the contacts with Deoxit, so I would start there before adjusting the DAC board volume. Congratulations on your purchase!

        P.S. is that Tomás Luis de Victoria as your avatar photo? It looks very much like him to me.

        -Current Instruments: Allen Mos-2 225 RTC,1870's W. Bell Reed organ, 1890's Singer Chicago upright grand piano Former Instruments: Yamaha Electone E3R
        -Website: https://www.exercisesincatholicmythology.com

        Comment


        • xopowo
          xopowo commented
          Editing a comment
          I see... yes, I tend to stay away from those pots. Actually, have you heard of any way to adjust the volume of the Swell independently of the Great (it's a bit quiet)? I'm not sure I have so many boards as in your organ, but there are definitely a few ways to get the volume changed. Good thinking on the contacts, I might go back and start cleaning more often.

          Very close - my avatar is Giovanni Pierluigi da Palestrina. :)

        • myorgan
          myorgan commented
          Editing a comment
          Larason2,

          Those pots on the DAC board are for adjusting the high & low for each channel.

          Michael

        • xopowo
          xopowo commented
          Editing a comment
          Michael - I have seen the high/low pots, but there are also little screwdriver pots; one for each channel, that appear to adjust the volume - I think those were what he was referring to. I'm not 100% sure what they do though... ?

      • #6
        Unfortunately, for Mos-1 and Mos-2 organs, the relative volume of the stops is set from the factory, and there is no way to change it. However, you might get some good mileage from cleaning contacts and troubleshooting other problems. I feel like your swell volume shouldn’t be too far off the great. On my organ, the swell and great are pretty close in volume.

        Even though you don’t have the same boards as my organ, I’m thinking there must be some way to adjust the volume on your amplifier. Mine has two very small holes leading to two tiny little trim pots I need a very small screwdriver to access. If you post a photo of your amp, I’m sure someone on the forum will be able to help you, they are very knowledgeable about Allen’s. Does your model have a card reader? They were optional for Mos-1’s. That is one way to add louder stops to either manual.

        -Current Instruments: Allen Mos-2 225 RTC,1870's W. Bell Reed organ, 1890's Singer Chicago upright grand piano Former Instruments: Yamaha Electone E3R
        -Website: https://www.exercisesincatholicmythology.com

        Comment


        • you795a
          you795a commented
          Editing a comment
          What kind of amplifiers are in the organ? Could you post a photo of them? To my knowledge, the Swell should be a little softer than the Great. Does your organ have one expression or two expression shoes?

        • myorgan
          myorgan commented
          Editing a comment
          The 120C has only one expression shoe.

          Michael

        • xopowo
          xopowo commented
          Editing a comment
          I'll have a look as to the exact type of amplifiers. What I do know is that, as Michael said, there is only one expression shoe. Another thing is, the amps have 3 rca cables (no proprietary connectors).

      • #7
        If your great is totally over-powering the swell, there could be some imbalance in your audio. In particular, the Main channel could be turned up too high. It is simply a matter of taste, for the most part. I usually adjust the two channels on an original MOS so that the principal 8 and the 8' flute on the great are fairly similar in strength, though some people might prefer to have the principal quite a bit more powerful than the flute (while some Gospel-music-oriented players would want the flutes to dominate).

        But if the balance is good, you should have a sturdy foundation chorus (8/4/2/mixture) on the great that is approximately balanced with a chorus on the swell made up of the flutes 8/4/2 + the gemshorn 8 and the 4' principal + the mixture + the 8' trompette.

        Since the stop levels in MOS are totally fixed by the MOS board chips, you may have to adjust your stop selections in order to get a balance between the two divisions, if the music you want to play requires such a balance. If the swell is still not loud enough, add the other 8' reed, or add the strings or even the nazard. Or take away something from the great.

        On a more voiceable organ, we'd take pains to balance the 4' principal stops in the two divisions, giving the great slightly more weight throughout the scale. Then we could make other subtle adjustments so that the divisional balances would match the organist's expectations. But that simply isn't possible in MOS, so you have to do your own balancing with your stop selections.
        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


        • myorgan
          myorgan commented
          Editing a comment
          John,

          From his description, it sounds like the Swell doesn't have enough power even after adjustment. I have never seen a MOS-1 manual, so I'm not so sure I can help further. I DO have an MOS-2 manual, if that will help. Did Allen make a comparable 125 MOS-2 organ?

          Michael

          P.S. Xopowo, have you switched the RCA cables between channels to see if the issue moves? I almost have the feeling we're trying to solve 5 problems at once rather than taking them one at a time.

      • #8
        That's definitely something to consider, John... thanks for that. My organ came with a few copies (thankfully) of the owner's manual, which, though vague, gave the same advice as you did for the great principal 8 and flute 8 to be balanced, so I have definitely adjusted that. Since I joined the channels with a Y-cable, both channels tend to come through both amps, and I tend to turn the flute amp up higher because all the Pedal stops go through the flute channel (due to its large subwoofer), otherwise the 32' stop is inaudible.

        Wow, all those stops on the swell??? I suppose the imbalance is by design then, because I tend not to register things so heavily.

        Well, thanks for your advice. I saw in the owners' manual that my organ was already "perfectly voiced from the factory" and as such I wouldn't need to "worry" about such things ;)

        Comment


        • #9
          There are many things about a MOS organ that seem strange at first, but make perfect sense once you understand the whole picture. For example, the three RCA cables attached to each amp --

          (1) one carries the actual audio signal straight from the DAC board (or possibly going through the attenuator relay and/or the trem generator or other optional accessory, but in any case, this is the actual audio signal

          (2) a second cable runs from the "voicing" knob on the front of the console, usually at the left end of the swell manual. This is a simple "treble" control, and is the only provision made for regulating the tone. The muting relays are attached to that control (just for convenience, they could've been anywhere), and they keep the amplifier's input muted for the first few seconds to avoid start-up noises

          (3) a third cable runs to the expression pedal and connects to the LDR cell. This cable plugs into a special RCA jack, which is normally marked "EXP" and which does not connect directly in parallel to the other two jacks, but connects to one end of the expression capacitor, which is mounted right on top of the amplifier -- a little capacitor about .1 mfd or so that jumps across two metal terminals right above the jack area. This expression circuit works by grounding out a small amount of the audio signal, the capacitor serving to force the circuit to work more effectively on high frequencies than lows (for musical reasons -- in a real organ the bass is barely expressed, while the highs are heavily expressed).

          Other things about the system that seem odd to you right now will eventually be clear.
          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


          • #10
            i am going to throw a thought out here. Could it be a defective CDS cell in the expression shoe?

            Comment


            • jbird604
              jbird604 commented
              Editing a comment
              I have seen a few of those go bad. The usual symptom is very little expression range -- not much difference in loudness between closed and open positions of the shoe. And it will only affect one of the channels. If it affects both channels, it's more likely to be the lamp or something else wrong with the expression mechanism.
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