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Rodgers 770 analog dead notes and cluster

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  • Rodgers 770 analog dead notes and cluster

    Well, well - I returned from an extended vacation, plugged my Rodgers 770 back in, and started to practice only to discover:

    Swell Eb2 on any/all stops plays a cluster of notes, like a handful!

    Swell D2 down to low C dead on all stops

    Any thoughts on what the problem is?

    The pedal and great are fine.

    Thanks

  • #2
    PS Swell to Great works as normal.

    Comment


    • #3
      Definitely a "keyboard encoder" issue, whatever that may be on that model. Since the notes play correctly when the swell is played via the swell to great coupler, the problem is not actually in the swell keying, but only in the swell keyboard itself.

      I am not an expert on Rodgers serial-keyed organs, but with a general understanding of how they work, I'll speculate... I think the keys may be tied together in groups of 8, with each group going into a "parallel to serial converter" chip. Second chip from the left on the swell encoder is bad, so playing the top key in its group (E-flat-2) results in sending a key down signal for all 8 at the same time, the "cluster" sound you are hearing. This chip is not sending data on down the line to the chip on its left, thus those eight keys are dead. The swell keyboard is probably the last set of inputs in that particular data chain, so the problem does not affect anything else.

      If you are able, you can replace the encoder chip, which may be available for a dollar or two. A tech familiar with 80's Rodgers organs can probably confirm the diagnosis and do the work quite handily.

      Another simpler possibility might be a shorted diode associated with the E-flat-2 keyswitch. I'm not sure diodes are used in this design though, thus my guess that it's the chip.
      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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      • #4
        On this series of organs, the 61 notes are connected in parallel into a serial data stream using shift register IC's--there is no multiplexing in the normal sense of multiplexing, and no diodes involved. The S/R IC's are 8 bit parallel in which connect to the keys and the serial data is connected daisy-chain fashion.

        If you have the metal and plastic keyboard, which seem to be more common on this model, the circuit boards for doing this are mounted under the keyboards, so you have to raise the stop tab rail and the upper keyboard (swell) to get to the circuit board. If you have the less common wooden core keyboards, the keyboard encoder board will be mounted elsewhere, but near the keyboard.

        The problem likely is in the shift register IC's from the malfunctioning key down to low C, the 3 lowest S/R IC's. They are probably soldered onto the circuit board--if you replace them yourself or a technician does it, install sockets. Sockets are low cost and make replacement much easier should they fail again.

        Another possibility is that some conductive material fell onto the circuit board and is shorting some of the key contact input connections. Look for anything stray that might do that--even, possibly, a broken key contact. It might even be shorting some of the keys to an off signal.

        If you are not using a surge protector, get a good one and use it. Seattle isn't an area where electrical storms are common, but they are possible.

        Comment


        • #5
          I have soldering skills so I think I can replace the chips (ICD, SHFT RG, 8BIT, P IN, 4021). Where do I purchase these chips (and sockets)? John mentioned in his post that one chip needs to be replaced and you mentioned several. Do I need to replace more than one? Thanks, Paul

          - - - Updated - - -

          PART NO. 1415-319

          Comment


          • #6
            Someone who has the actual "input data chain" chart can tell you for sure. I'd assume that the one that is playing all 8 of its notes simultaneously is a bad one. And it's always possible that the one before or after it in the chain is bad. Since they're cheap and easy to replace, I'd get 3 or 4 of them. And be sure to mount sockets on the board, then insert the chips, rather than soldering the chips directly to the board, as someone mentioned above.

            I get chips like that in bulk from Mouser.Com and there are other internet vendors who sell them as well. A quick googling just now (4021 shift register) turned up several sellers, at less than $2 each. Shipping is going to be more than the cost of the chips though, so look around and find a place that doesn't charge an arm and a leg for shipping.
            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


            • #7
              I concur with JBird on this--it might only be one chip that's bad, or it might be 3--if you're going to the trouble to replace one, I would replace all of them on the board. It's kind of a pain to get that board out, unsolder the old IC's and solder in a new one, and doing the others will only take another 10 minutes or so. Make sure you have appropriate unsoldering tools--solder wick, solder sucker, etc.

              I buy from Jameco (www.jameco.com) when I can because they have reasonable prices and are on the West coast so shipping is quick for me. The IC is $0.49 each, $0.39 if you buy 10. I buy machine tooled sockets, as they seem better built than the ones with stamped contacts. If you buy from Jameco, here are the part numbers:

              37402 for the socket
              12829 for the 4021 IC

              You might be able to find these locally at an electronics hobby store.

              Comment


              • #8
                Success!!! Replaced the offending IC with the help of JBIRD and TOODLES. Thank you guys for your expertise!

                Comment


                • #9
                  Congratulations--glad it worked out for you.

                  Comment


                  • #10
                    Good work! Glad you were successful. Keep up the good work.
                    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

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