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Rodgers 755 Questions

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  • Rodgers 755 Questions

    I have just acquired a Rodgers 755. The previous owner gave it away and all I had to do was move it. I have some questions that I hope a knowledgeable person or persons can answer:
    1. The previous owner disconnected the speakers before I got there to get the organ, didn't take any photos of how they were connected, and didn't remember. So I am guessing at the connections. Fortunately, someone wrote on the back of an included manual: Ch1 - Red+/Grn-, Ch2 - Wht+/Blk-, Ch3 - Wht+/Blk-, Ch4 - Red+/Grn-. So I could get the pairs right, but I have no idea which speakers connect to which outputs. There are two 4-conductor cables coming out of the organ with the colors mentioned above. One has a piece of red tape wrapped around it at the end. The speakers I have are a) a large sub-woofer with 2 15" drivers; b) a cabinet with angled sides, about 10 speakers in it including a 15" woofer, and a single pair cable; c) an identically sized and angled-sides cabinet as b), but with two 15" woofers and two single-pair cables coming out; d) three small rectangular cabinets with several 6x9 speakers and tweeters. Can you shed any light on what would be the correct connections?

    2. I would like to know what the speaker cabinet model number sare. That might help me search for more information on them.
    3. With the speakers hooked up on a guess, it does play, but it sounds rather dry. I understand there is a way to add a reverb. Is anyone familiar with this process? I'm sure that would go a long way toward making this a great-sounding instrument.
    4. I found in the organ bench the following Walker manuals:
    A. DKAF24 Inharmonic Pipe Articulation Enhancement Generator for 32" & 16' Flute Extension Technical Manual B. DKAMP Inharmonic Pipe Articulation Enhancement Generator for Manual Principal Technical Manual C. DKAMF Inharmonic Pipe Articulation Enhancement Generator for Manual Flute Technical Manual Anyone familiar with what these might be all about?
    Many thanks in advance for any information you can provide.
    Bob



  • #2
    Speaker connections are identified on the connection panel in the organ--open it up, swing out the 2 racks, and the panel is on the back of the kneeboard. The outputs are labelled. See the tech manual for details. Neither I nor anyone else can know how the previous owner hooked it up except for that owner.

    There are 3 or 4 channels for the organ, as it was available in either of those options. Yours probably has 4, since you've got 3 Walker channels (with 2 being in one cabinet) plus a pedal cabinet. It doesn't matter which channel is which--the organ sends most voices through 2 channels for a chorus effect using phase shift between them.

    The subwoofer (pedal) channel is probably a Rodgers P16; the single channel Walker trapezoidal cabinet is likely a WM-128; I don't know the model number for the dual channel Walker. It probably won't help to know the models, though. There is very little on-line info. You can contact Walker Technical Company for specific info--they have usually been helpful.

    The cabinets with 6x9 speakers are probably Rodgers MC-10 or MC-10C--difference is a column format for the 10C version; you wouldn't normally use those with the Walkers unless you needed to supplement the sound for dispersion or for provided sound to an area that the other speakers didn't reach--like a choir loft.

    All I know about the Walker articulation generators is what is in the manuals, and you have those. From their name, I would estimate they add some noise based articulation to simulate pipe articulation.--i.e., chiff. The Rodgers inherently has harmonic based chiff, "inharmonic" would indicate noise or wind based sound.

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    • #3
      Much appreciate your help, toodles. I am learning much about this instrument, and am looking forward to being able to play it soon.

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      • #4
        All the great voices play at the same low loudness level regardless of the Great expression pedal. Where is the best place to start troubleshooting this problem? Is there a common failure I should be looking for first?

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        • #5
          The expression pedal position is encoded, processed through the microprocessor and the expression value is read out on the output preamp board to drive the expression lamps.

          It could simply be that the wiring on the expression potentiometer has broken. If that wiring is OK, then it is an issue with the output decoding for the expression lamp drivers or the expression lamp itself. See sheet 7 of the output preamp circuit. It covers the decoding and expression lamp driver circuits..

          You can check the expression lamp by pulling off the rubber boot cover and seeing if the lamp changes intensity with the expression pedal or even if it illuminates at all. make sure it's the great/pedal cover you remove and not the swell.

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          • #6
            I suspect the Great Expression lamp may be bad (I assume its an incandescent lamp, so highly suspect). Does anyone know how to remove the cover from the lamp/LDR assembly so I can check the lamp?

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            • #7
              The cover should just pull straight off, carefully.

              --- Tom
              Rodgers 660 with additional analog rack sets (practice), 36D/C in digital conversion, Yamaha CVP-107

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              • #8
                What Tom said is correct. The cover is rubber, but stiff. Gently pull to remove, and, if it is stubborn, try pulling to one side to get it started.

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                • #9
                  I discovered that none of the lamps are burned out. I also discovered that the intensity of the lamps is inversely proportional to the loudness of the division. In other words, the dimmer the lamp, the louder the sound. So if a lamp would burn out, that division would play at maximum volume, not minimum.
                  The swell lamp varies in intensity with the position of the swell expression shoe, but the other two do not, always showing maximum brightness. This leads me to look for a common component between the two lamp control circuits, or something physical with the Great expression shoe itself or the potentiometer that senses the position of the shoe, or the wiring between the potentiometer and the A-D converter on the CPU board. In the lamp control circuit, the only common component is the 6551 dual op-amp IC. I think replacing that with another will be the next and easiest step.

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                  • #10
                    OH, BTW, the op-amp is in a socket, so very easy to try.

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                    • #11
                      I was wrong about the op-amps. When I tried my theory, I discovered that the 6551 op-amps are soldered to the board, and they are in a very unusual (for me at least) 9 pin SIP package. So I could not pursue that test.
                      Instead, I decided to visual the expression pedals themselves and the connected pots. What I found was this: someone added a mod that switches the Great expression lead that goes into the CPU to the Swell pedal by way of a toggle switch that was mounted and glued to the top of the cardboard shoe "cage". With the switch in the position is was as received, the Great pedal does nothing, the swell pedal expresses only the swell. The great is minimum loudness as previously reported. By toggling the switch, the Great expresses along with the swell. So this proves that there are no problems with anything on the circuit boards. There could only be two reasons for the switch: 1. Someone wanted a "All Swells to Swell" feature, because in the one position, that's what it does. 2. The Great pot is bad, and this was a convenient/temporary fix. I will need to get a meter and see if the great pot does output a varying voltage on it's slider. This will be tomorrow, perhaps. At least knowing I now have great expression, the organ is almost playable. Yay!

                      I also discovered that the Celests to Great coupler is not functioning, but that is minor compared to the great expression issue.

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                      • #12
                        It might also be that the mechanical linkage on the pot isn't right, or the set screw is loose.

                        As to the Celestes to Great, if the stop doesn't turn on and the LED doesn't light, it might be a simple adjustment of the hall effect transistor not seeing the magnet, or the hall effect switch has failed.

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                        • #13
                          Does anyone know what the kick-switch does that is mounted on the top left of the Great Expression shoe? It shows on fig. 5-10 of the Tech Manual.

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                          • #14
                            I got my Great Expression shoe working. The problem was in the mod that someone made. One of the solder joints on the switch was bad. I just completely disconnected the switch and returned the wiring back to the way I believe it was originally. No more problems.
                            Interestingly, the Celeste coupler is back in operation now too.
                            So aside from a minor repair I have to do on the music rack wiring (the wire was cut to remove the music rack for moving.) and two woofers in the speaker cabinets that need repaired or replaced, I think this instrument is back to full operation.
                            I plan to bring some music in next week to see how well it sings.
                            Thanks for the help from everyone.

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                            • #15
                              The kickswitch is used to provide MIDI sustain pedal functionality.

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