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  • Allen ADC720 question

    Hi Folks,
    I attended a customers Allen today with the complaint of no sound from the organ. I found a +15v short circuit on the MADC-AP-1 module. Customer is reluctant to spend too much on the organ given it’s age. Can someone let me know the function of the MADC module, and if not working, would it shut down the entire organ.
    thx
    scott

  • #2
    "AP" stands for "audio processor" and that is the board where the audio comes after leaving the TG cage. The board holds a rudimentary mixer that was used in certain models with a separate percussion or rhythm unit, whose output needed to be blended with the organ tones before going on to the amps.

    This board also applies expression to the sound, and circulates a sample of the sound through the reverb spring. So, in a 720, which is a classical model without any other tone sources, you would only lose the expression and the reverb if you bypassed the AP board. To do that, simply disconnect the power input from that board so the electrical short on it won't be pulling down the system, and use a pair of RCA barrel couplers (the kind where you join two male RCA plugs end to end) to send the audio on to the amps or trem units or whatever follows the AP board. The organ will then play, but of course the owner may well miss the expression.

    You might get a little bit of expression if you use RCA "Y" cables to join the expression cables to the audio right at the amp inputs. It would work even better if you got the Harrison Lab attenuators, which are made for fixing Allen organ expression issues. They have a resistor in series, so they would make the cage output (which is in fact a low-impedance signal) appear to the expression cells as a higher-impedance source, so the cells could do a better job of attenuating them.

    The best fix of course would be to find a used AP board. They often show up on ebay, and other online sellers, when a person strips out an older MADC model for conversion to MIDI.
    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 JBird,
      That’s most helpful. I’m guessing the tantalum caps on the AP board are probably suspect, certainly on the +15 side. I’ll take the module out and see if I can fix it. I think the customer wants to replace it with a pipe organ, but it would make sense to get the 720 playing for the short or med term.
      cheers
      scott

      Comment


      • davidecasteel
        davidecasteel commented
        Editing a comment
        Purchasing a pipe organ would be a lot more expensive than any repair of the Allen found necessary, I think. Also, it takes a while to build one. (Of course, maybe could get a used one from the OHS.)

    • #4
      A follow up question, does anyone have a schematic for the MADC-AP-1 module. Also, I assume the organ has speaker protection relays, I noticed no audible ‘click’ when switched on.
      cheers

      Comment


      • #5
        The muting for the speakers is done at the amps, not in the AP board. It is a simple circuit that grounds the incoming audio for a few seconds at startup and instantly when the power is switch off, to prevent turn-on or turn- off thumps.

        Here is the schematic of the AP unit. You may be correct about one of the tantalum caps shorting out. I've seen that before.

        MADC-AP-1.pdf
        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
          Thanks John, that’s great. So should there still be a relay click from the muting circuit? Definitely no click that I can hear.

          Comment


          • #7
            Scott,

            Sometimes you can find boards for sale here: https://www.dafferorgans.com/product...cessing-board/ However, it is currently out of stockk. I've also seen them on American eBay (usually the Florida Flipper).

            Hope that provides other options.

            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

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            • #8
              Thanks Gents,
              I’m going back to work on the organ tomorrow. I’ll provide an update then.
              cheers

              Comment


              • #9
                The failure(shorting) of the tantalum caps is a common thing that I have seen. In many cases you can leave them off without noticing a difference, Or I have replaced them with a more generic electrolytic of a similar value and volts rating.
                Can't play an note but love all things "organ" Responsible for 2/10 Wurli pipe organ, Allen 3160(wife's), Allen LL324, Allen GW319EX, ADC4600, many others. E-organ shop to fund free organ lessons for kids.

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                • #10
                  Tantalum’s replaced with conventional electrolytics today. Organ working fine now.
                  Thx for your help
                  cheers
                  scott

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