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Thread: Allen T12A -- "Dead Key" Problem

  1. #1
    ppp Pianississmo lexicon's Avatar
    Join Date
    Nov 2006

    Allen T12A -- "Dead Key" Problem

    <FONT size=2>Hello All,</FONT></P>

    <FONT size=2>I am new to the group and I have an Allen T12A organ with 4 dead keys; the 4 dead keys are duplicated on both the Swell and the Great manuals. In addition, I have some basic technical know how like: What are capacitors and resistors, and how to identify their values; I also know how to use a voltmeter and a soldering iron. I bought a scope the other day but I have not yet fathom its operation.</FONT></P>

    <FONT size=2>Can anyone with knowledge of Allen T12A analog organ give me some instructions about correcting the "dead key" problem. Any help will be greatly appreciated. Thanks.</FONT></P>

    <FONT size=2>Lexicon</FONT></P>

    <FONT size=2>Allen T12A and a </FONT><FONT size=2>Rodgers 750I</FONT></P>

    <FONT size=2></FONT></P>

  2. #2
    Moderator jbird604's Avatar
    Join Date
    Sep 2006
    Arkansas, USA

    Re: Allen T12A -- "Dead Key" Problem


    I assume that you mean dead "pitches" that are the same on both manuals. The T-12 has a single set of oscillators producing a pure flute tone. A unit scheme lets you sound those flute notes at all manner of pitches, combined into synthetic diapasons, strings, and reeds. The oscillators are "keyed," which means they are not powered or running unless a key is depressed which requires a given oscillator to sound.</P>

    So, a dead pitch is simply an oscillator that does not"start up"when it is keyed. The culprit 90% of the time is the "keying capacitor" for that pitch. The giveaway is that the oscillator gives a tiny little "gulp" when keyed but never produces the steady tone. That symptom always indicates a bad keying cap.</P>

    You should contact Allen Organ and get the schematic. They will gladly sell you one for that analog organ. (Though they won't sell digital organ schematics to anyone besides their authorized techs.) The keying capacitors are located in a neat row, often covered over with a strip of masking tape, inside the metal generator chassis. You remove some screws, flip open the "lid" that holds the tuning coils, and see the keying caps lined up on an inside edge of the metal chassis.</P>

    (Be aware that there are two distinct styles of generator chassis. Most T-12 organs will have the old-fashioned metal pan with most components hidden inside. But the last ones built, from the late 60's into the 70's, have the "in-line" style, with the components laid out in a "staight-line" fashion on top of a phenolic board.)</P>

    Find thekeying capthat corresponds to the dead note. It will be 2, 5, 10, or 15 mfd. Replace, and the note should sound. You can even check it by clip-leading a new cap in without removing the old one, just to be sure that's going to fix it. Be sure to observe correct polarity. Electrolytic capscan EXPLODE if put in backwards.</P>

    Church: Allen MDS-45 with Allen MIDI-DIVISION-II expander
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  3. #3
    ppp Pianississmo lexicon's Avatar
    Join Date
    Nov 2006

    Re: Allen T15A -- "Dead Pitch" Problem

    Hi John,

    Thanks for replying to my "SOS." Yes, you are right, it's a dead pitch. It's the 5th octave G, the 6th octave C, the 7th octave F, and the 7th octave G#.

    In regards to the Organ Model ID, I made a mistake, it's an Allen T15A. To be sure, I looked into the unit and saw the Model ID marked "T15A."

    I have not checked the keying capacitor for those dead pitches yet. No doubt, I will have to purchase a few caps and trouble shoot the problem. In addition, thanks for pointing about the danger when using Electrolitic caps. I will keep you updated on my progress. Thanks.

    PS; I did purchase a schematic for the T15A from Allen and now know where the Keying Caps are located.

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