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Allen 120 Sound Production Problem

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  • Allen 120 Sound Production Problem

    Hello. A friend of mine recently had an Allen MOS 120-C given to her church. She let me know that the organ is not working properly and asked me to come take a look at it. I can do minor repairs but cannot figure out what is wrong with this one. The organ looks like it has been very well taken care of. I removed the antiphonal speaker connections as the church will not be using them. I tried cleaning contacts and moving/unplugging/plugging any connection or pot that I could see. No luck. I unplugged each of the internal power connections then re-plugged them in, no success either. The organ makes a small static noise when on and a louder oscillating noise when any key is depressed. You can hear a sound difference when the stop tabs are down, but the sound is very distorted and always has a staticy oscillation to it. The problem is present regardless of tab selected so I don't think it can be attributed to the individual line (Flute or Main). I have a short video but can't figure out how to attach it, I will try to add it later. Thanks for any help!

  • #2
    Did you verify that the voltages coming off the power supplies are all correct? The -27 volts is particularly crucial and will cause extreme distortion if not fairly close to spec. Make sure as well to test each voltage for AC ripple. More than a few thousandths is too much, except on the unregulated 12 volt supply.
    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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    • #3
      How would I do that? If it is incorrect what is the remedy? Thanks!

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      • #4
        Use a voltmeter, preferably a digital type, even if it's a cheap one from Lowe's. Put the red lead on each terminal of the power supply, one at a time, with the black lead firmly planted or clip-leaded to the ground terminal of the power supply. Make sure the meter is set for DC voltages, and if it has more than one voltage range, set it for the range that includes the voltage you are measuring. For example, with the black lead on GND and the red one on -27 (steady or switched, with the organ powered up), your digital meter should read either exactly 27.000 volts or perhaps as much as a half volt above or below. If not, your supply probably has a little knob or shaft sticking out the front that says "-27 volts adjust" and you would turn it back and forth until the voltage settles down at -27. Same process for the +5 and -5 voltages. The 12 volts is not adjustable and isn't even regulated, so it may run as high as 15 volts if your power line voltage happens to be a bit above 120 volts.

        After confirming the DC voltages, move the meter to AC voltages, and set it for under 1 volt, if it has multiple ranges. Then put the red lead on each voltage terminal, black still on ground. You should read no more than about .009 volts of AC on any of the regulated terminals. The 12 volts may be as much as .050 and still be normal.

        If you aren't comfortable checking this yourself, you may need to get a tech involved. But it sounds like you've already done a lot of the things that a tech would do, such as checking all the connectors and plugs, pots, etc.
        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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        • #5
          Thanks very much! I'll give this a try with the next week.

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          • #6
            Thanks John, than was a huge help. I had a hard time finding the 'pot' to adjust the -27 volts because part of the power supply had been updated in the 90's. The normal adjustment knob was not there but after opening it up I found it attached to a greenboard. The organ mostly works just fine but there are some other problems if you have any suggestions.

            1. The swell pedal does not work. Does this have something to do with a light bulb?

            2. The stoptab connectors do not always allow a solid connection. Is there a recommended way to clean them? I seem to remember reading something about steel wool but wanted to check with someone who knows better. I opened the cover and used an electrical contact spray but that didn't change too much.

            3. The mixtures on the Great and Swell won't work. This may have to do with the above mentioned stoptab connections but not sure. The Pedal mixture works fine.

            Thanks again!
            Jon

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            • #7
              1. There is a bulb in the swell pedal. If it is burned out, volume will be at a maximum and you'll have no expression.
              2. Using steel wool to clean contacts is bad idea. It sheds tiny metal splinters that can cause shorts that will be very difficult to find. Use a good contact cleaner, such as DeOxit https://amzn.to/2WHitHS to clean the contacts. If the contacts are severely crusted with oxidation and the cleaner doesn't do the trick, you could use crocus cloth to polish them.

              -Admin

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

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              • #8
                Most of the time we are able to get MOS stop tabs working by cleaning the switch surfaces with a Q-tip moistened in alcohol or WD-40. Other good solvents can be used as well. Note that you may have to slightly and delicately bend the little metal leaves into position so that they contact the tongue of the tab when it is depressed. Since the MOS-1 system doesn't have separate EPROMs for the mixtures, it's likely that they are silent only due to contact issues on the stop tabs. It's remotely possible that there is a board problem, but I'd sure rule out easier solutions first.

                If this happens to be a model 124 instead of a 120, then it actually is a MOS-2 organ, and the mixtures are in fact in an EPROM, which might be missing or just needing to be reseated in the socket. But I'm betting on it being the tabs.

                As Admin states, there is a small lamp (a type 57X) in the swell shoe, held in place by a circular snap-in holder that you can pry away from the shoe with a small thin screwdriver. The standard #57 bulb may be found in some auto parts stores, or ordered on line. Allen turns them into a "57X" by painting a small dot on the tip to help shield the LDR cell from the direct glare of the burning filament. You might or might not really miss that little dot of paint. The expression may indeed be more extreme and abrupt without the dot, and if you wish, you can put a dot of heat-resistant paint there and let it dry before installing.
                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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                • #9
                  Thank you both for the help, I'll get to work in it soon.

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