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MDS 41S Reverb Issues

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  • MDS 41S Reverb Issues

    I am having some issues with the reverb on my Allen 41S.

    Problem 1. When the organ powers up, my organ is normally completely quiet and you wouldn't know it was on except for the power light. However, when you flip the reverb toggle switch, there is a light static noise which grows fainter after several seconds. Adjustment of any of the reverb voicing controls (treble, bass, input, output) have very little effect. Turning the input control does make minor improvement, but of course, I loose the reverb effect at some point as I retard it. Turning the reverb off again makes the problem go away. With the reverb on and listening very carefully, imbedded with the static is a very faint beat or pulse of about one beat per second.

    Problem 2. The reverb works ok, as far as Allen reverb systems go. But there is a staticy noise after notes are played that sound similar to the noise the snare attachment makes on the bottom of a snare drum, except much quieter. Another analogy might be a very light drizzle on a tin roof. Indeed, when playing, I am not actually aware of the noise, but have the impression it might be raining. If I quit playing and just play a note or chord on any manual, I become aware of this noise which seems to dwindle slower than the notes as the reverb effect fades out. Again, adjusting the reverb voicing controls have little effect on the situation.

    Thoughts: I know there are reverb dip switches, but I do not know the proper setting. I have not changed them, but someone in the past may have.
    Is it possible that the problem is upstream of the reverb, a failed filter or something?
    There are two different set of reverb voicing controls. Are there actually two different reverbs and can they be switched to see if the problem moves. Currently, it would appear that
    the static plays on two channels as I hear it on both sides of the organ.

    The problem is not that annoying. The reverb works as advertised. I just worry that it may be a messenger of an impending bigger problem.

    Anyone care to share their thoughts?

  • #2
    Bill,

    The problem may indeed be upstream from the reverb, but unfortunately all that stuff is inside the cage and not separately replaceable. So let's hope some typical maintenance and adjustment will get you some relief. I don't know that it will get worse with time, unless it's a component such as an op-amp on the AP board that's trying to fail. If that happens, you'll probably know it right away.

    The DIP switches in the reverb section serve to set the length of the reverb. Longest reverb is with all in the off position, shortest with all in the on position. Various combinations of on and off give reverbs in between these two. But should not affect the noise. Try them like this: Off-Off-On-On (starting with #1). That should give a medium amount of reverb time.

    I found a general guide to setting up W5 reverb on the Allen tech site. It says to start with the reverb's bass and treble pots at maximum, the input gain at 3/4 and both output gains at 1/4. I suppose these are the settings they arrived at when adjusting organs at the factory for typical applications. You might need more reverb in a home setup, so try moving the output gains to 1/2 or even 3/4 to see if you get a more satisfying reverb.

    Since the reverb system is contained inside the W5 cage, and each cage has its own system, you should be able to figure out which cage is giving you the noise. If it's only on swell stops, for example, it will be the "B" cage. Turn down amps #1 and #2 to kill the swell completely and see if the noise goes away. If so, it is coming from the "B" cage. OTOH, if the noise comes through channels 3 and 4 only, it is coming from cage "A".

    You may have to take off the cage cover and do some routine maintenance inside. Be sure the power is off, but do NOT unplug the power cord from the wall, as it is essential to keep the system grounded through the ground plug of the AC receptacle, to prevent static damage. Before touching anything inside the cage, ground your hand to any bare metal surface in the console. Don't shuffle around on the carpet, and preferably don't have the air in the room too dry. Boil some water on the stove if necessary to relieve the dryness a little while you're working in the cage.

    Now, in the cage you will see that there are numerous socketed chips, mostly EPROMs, on the generator board. There may also be a daughterboard attached to the W5 board which has additional socketed chips. Your goal is to gently but firmly press down on all the socketed chips to make sure they are securely seated in their sockets. Most of them will go "crunch" as you press down the first time, because they will have worked loose to some extent.

    I assume you've already done the Vaseline thing to the RCA cables throughout the system. Just be sure that every place where something is socketed or plugs into something else the connections are very secure and solid. Wires on fanning strips must be checked individually to make sure the screws haven't loosed. If everything is snug and tight, you will have done all you can do to chase out the problem.

    The pots in the reverb section need "exercise" just like the tone control pots in the TG section, so don't overlook them. After you do all of this, you will have done all a person can do without calling in a tech or getting some new boards. I hope this will help you.
    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
      Thanks John,

      I have exercised all the voicing and reverb associated pots. Played with all the connections external to the cages. So, I may have to go inside after isolating which channels the problem is on. Appreciate your taking a look on the tech site. Thanks...Bill

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      • #4
        Well John, playing with the amp, the problem was primarily on the great and choir, but the hissing could also be heard on one channel of the swell with the organ not being played. I did find that turning the gain down to about four on all four channels made the noise go away. So, now it is quiet with the organ on whether or not the reverb switch is also. I also didn't notice the rain falling while playing it.

        I am playing in a small room (10x20) and wonder if perhaps like the mechanical sound of bass pedals, perhaps some 'noise' is inherit in this small space. I doubt in a sanctuary that the small amount of whatever would have been noticeable.

        I still haven't opened up either cage, but perhaps should do so and reseat stuff as you suggested. Would you recommend a wrist chain or something similar to keep me grounded while working inside? Also, am I to assume that the highest voltage inside this organ, outside of the main amplifier is 15 volts?

        Lie to me John. If voltages exceed 25 volts, this would be a great way to get rid of one of your pests!

        Comment


        • #5
          There is some 120 volt AC inside the cage. You'll notice that an AC power cord is plugged into it. But following the transformer and power supply, the highest voltage would be 15 volts. So just keep away from the power supply section and you'll be safe.

          I don't use a wrist strap, but I do always ground myself by touching a metal object before I touch any boards or connectors in the organ. Your body can pick up a large static charge just brushing against the carpet on the floor or other fabric in the room. And a high voltage zap might destroy an IC before you know it.

          As to the noise, I'm sure it would be less noticeable in a larger room, but it's still curious that you can hear anything like that at all with the amps above a "4" mark. However, now and then I run across an Allen with more hiss or noise in the audio than usual. I don't really know why that is, but it might not ever cause any trouble. I wouldn't worry about it if it's not too objectionable. Does turning down the Treble control make any difference? Perhaps the treble is turned up too high on some channels. Could also be the Wind controls.
          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


          • #6
            No, nothing I did to the reverb affected anything. Since I've turned the amps down, I don't here much of anything and perhaps the organ is a bit clearer. So, perhaps the amp was distorting. I had previously exercised its volume controls and cleaned the contacts. But, for the moment, it sounds great so we will see. Its back up against the wall and I think I will just leave it there for a while and get on with my practice...Bill

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            • #7
              Good plan. My console is backed up to the wall and in a corner too, so I really have to tear up things to work on it. Right now I'm having an expression issue -- volume seems to suddenly rise and fall at times without any movement of the pedals. But I can't bring myself to move all the other furniture around (including a piano) to make room for pulling out the organ! Some tech I turned out to be!
              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
                jbird604--know the feeling. I had rather take a beating than pull my style C Allen console out.

                Comment


                • #9
                  Well, mine isn't too bad, but I have a bank of speakers arranged on both sides that have to be moved each time I need access to the back. Then, of course, its out in the middle of the room and in the way. Whine, whine,.....

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