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1971 Allen system 300 tuning problem principal, reed and string stops

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  • 1971 Allen system 300 tuning problem principal, reed and string stops



    Hello, this is my first post to the organ forum.</p>

    We have an Allen system 300 which is exhibiting an unusual tuning problem. On the prinicipal, reed or string stops, the notes from middle C down on any of the stops in these groups are not playing the right pitch. It is around the correct pitch but not quite there. The weird thing is that the problem seems to come and go intermittently. The flute stops and celeste always play the right pitch even when the other stops are acting up.
    </p>

    I replaced all of the electrolytic capacitors in the 12 volt supply and the +5 volt supply which was the original 1971 unit and also replaced the diodes in it. The -5V and -27 volt supplies were replaced in 1992 and all of the output voltages are correct. I also replaced any other old electrolytics on the circuit cards and replaced all electrolytics in the pre-amp module. I've got more than 20 yrs. electronic repair experience but have not worked on an Allen before beyond replacing the amps...most of my experience is with Hammonds and Baldwin tube units. </p>

    I unseated and reseated the MOS board and clock circuit board.
    </p>

    I'd rather actually repair this problem at the component level if possible rather than having to get a whole new circuit card. It seems like there must be a thermally related problem somewhere. Is there any service literature available or block diagrams showing how this organ works? It seems like it uses a master oscillator on the clock circuit card but beyond that I'm not sure what functions the MOS board performs.
    </p>

  • #2
    Re: 1971 Allen system 300 tuning problem principal, reed and string stops



    Greetings Retrochad! Are you also Retrochad on You Tube? I've seen those postings. You are indeed an experienced service tech!</P>


    Unfortunately, most Allen literature is available only through authorized dealers. You can contact the factory for a referral to your area's dealer. They alone can order the service manual for your organ.</P>


    Two very knowledgable tech's who work with Allen organs are members JBird and Arie. Perhaps they have encountered this problem. One of them mentioned dead spots in trim pots recently and since you say your problem is intermittent it might be worth your while to examine the trim pots.</P>


    Is this your personal instrument or one of the ones you play in church?</P>

    Comment


    • #3
      Re: 1971 Allen system 300 tuning problem principal, reed and string stops



      Hi,</p>

      This sounds like a MOS-1 board problem. In general, service techs do not do component level work on this board. </p>

      This is likely a problem that will have to be diagnosed and fixed at the factory.</p>

      AV
      </p>

      Comment


      • #4
        Re: 1971 Allen system 300 tuning problem principal, reed and string stops



        Sounds like you've done about everything anybody can do. As Arie says, sounds like a MOS board issue. The only other components that could affect tuning would be the Keyboard Array (KBA) board and the Tremulant Gate (if this organ has one). The KBA has some discrete parts, especially the old original version of the board. You might be able to poke around on that board and see if a diode or transistor has gone intermittent. The Tremulant Gate can be temporarily pulled too see if the problem goes away (the entire organ will shift upward in pitch with the Trem Gate pulled).</P>


        I still am guessing you'vegot a bad MOS board. That board must be factory serviced. There are no in-field repairs recommended, although I have heard of people working on the "clock-level-shifter" section in the middle of the board -- the components covered by the plastic shield. I guess with your level of expertise you might carefully check out some of those discrete components.</P>


        Allen simply didn't want people replacing those big IC's outside the factory walls. No schematics of that board have ever been released AFAIK. The MOS service literature has a chart in it somewhere that very vaguely identifies the function of each chip, but there isn't enough info to make troubleshooting possible. And even if one could identify the bad chip, there are no replacements available except from Allen, and they won't sell you one.</P>


        Good news is that Allen still, after almost 40 years, will exchange or refurbish the MOS board for a nominal fee. You'll have to ask your local Allen tech or dealer to send it in for you, but you will be pleasantly surprised at how little they charge for the exchange.</P>


        Bad news is that you cannot be 100% sure that the board exchange will fix it unless you also have the local tech bring out his kit and sub in the test board to see if that solves the problem. So you will probably wind up having to pay for a service call.</P>


        Nice to have you on this forum. I havent' see the youtube videos yet, but plan to check them out. Best of luck with this repair. Let us know how it turns out.</P>


        John</P>
        <P mce_keep="true"></P>
        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


        • #5
          Re: 1971 Allen system 300 tuning problem principal, reed and string stops



          Thanks a lot John for your expertise! I wasn't aware that Allen still did factory service on these modules.</p>

          I'll try and see if I can find anything wrong with the keyboard array board.
          </p>

          We have a technician who works on the Rodgers Cambridge series 600 organ I play at another church so I'll see if he can test it with a new MOS board and/or send in the MOS board for repair. The problem seems like something you could find with freeze spray on the bad component but knowing the parts are proprietary and that Allen still has service for them it does seem better to have it sent in.</p>

          I made some videos of repairing the power supply and other modules I could work on with "off-the-shelf" components. I haven't uploaded the one of the problem actually occuring but will try to tonight.
          </p>

          Comment


          • #6
            Re: 1971 Allen system 300 tuning problem principal, reed and string stops



            Hello, yes I am Retrochad on youtube. Thank you very much for your reply. This is actually one I play at one of the churches I work for.</p>

            We do have an Allen at home which is a little bit later model but unfortunately it is behind a lot of other equipment in the barn...someday would like to get it set up in a better environment but it is hard to find a good place for it as it is so big!
            </p>

            Comment


            • #7
              Re: 1971 Allen system 300 tuning problem principal, reed and string stops




              </p>

              Allen simply didn't want people replacing those big IC's outside the factory walls...</p>


              Good news is that Allen still, after almost 40 years, will exchange or refurbish the MOS board for a nominal fee. You'll have to ask your local Allen tech or dealer to send it in for you, but you will be pleasantly surprised at how little they charge for the exchange.</p>

              I would hazard a guess that outside of very specialized, and probably classified applications, i.e., missile launch systems...nobody in the world is still servicing the type of chips seen on those prehistoric looking MOS1 boards.</p>

              </p>

              Comment


              • #8
                Re: 1971 Allen system 300 tuning problem principal, reed and string stops



                Hello Retrochad,</P>


                Welcome to the group!</P>


                I've seen you many times on youtube and I subscribe to your channel, under a different name.</P>


                I don't know how to fix your organ, looking at your youtube videos I thought you could fix anything, however, I hope you can fix it!</P>


                I have a 301-C and I think those Allens are great organs!</P>

                Comment


                • #9
                  I know this is from a while back but I wanted to post the results. The problem was indeed a bad MOS board. We were able to trade in the old one for a good second-hand one from Tyson Church Organ in Dallas. When the new MOS board was put in the problem never came back. I also upgraded the audio with a small 2-channel Mackie preamp and 100W/channel amplifier and the sound is much, much better.

                  Comment


                  • #10
                    Thanks for the update! I love to hear the end of a story!
                    Home organ, same as church's organ - Rodgers 940

                    Sign on my work toolbox that effectively keeps people away:

                    DANGER!!! 1,000,000 OHMS!!!

                    Comment

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