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Rodgers 750BE in and working!

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  • Rodgers 750BE in and working!

    Well, today was the big day. Moved the Rodgers 750BE I purchased from the church where it had been since new, and into my music room at home. Complete with two Walker speaker cabinets, which as it happens are configured for two channels each, so all 4 channels are working as designed. I do note the Rodgers amplifiers are a touch noisy at idle (slight hum/hiss) but some of this may be do to computer equipment in the vicinity of the console. Some line filtering and shielding may be in order.

    Right now I have done a quick and dirty setup, speakers are not in their final placement and there is some work to be done on the console. Remarkably, I have only one dead note, top octave G on the 8' swell oboe. I can hear it faintly, so it is doing something just not sounding at intended full gain. Could be the oscillator I suppose. Also have a burned out divisional piston #2 on the great. Anyone know if the bulb can be replaced or if I will need a new piston? These do not appear to be re-lampable by design.

    Everything else seems to work normally, though I see a fair number of modifications and repairs to various circuit boards on the two main racks in the console. Some of these look like factory corrections or updates, others look a bit hastily done as though they were done in the field.

    I note that this model does not have the mixing controls on the preamp boards to go from 4 to 2 channels, so I guess I will have to do mix-down to two channels for headphone use outboard with a small 4 input channel mixer of some sort. If I cannot get the factory amps quiet at idle, I will disable them and run pre-outs to outboard power amps that are higher spec. May do this anyway as I need to add a reverb; the factory spring tank is in there and sounds ok, but my music room is too dry and needs the magic of a DSP reverb module. I have a Lexicon MX400 on the way for this purpose.

    No pictures yet, but will have some up soon.


  • #2
    Hi, Kevin.


    It's typical for analog Rodgers to have a fair amount of background noise & bleed through--it isn't from the amps, but from all the electronic keyers that leak a little--that's not to say that adjacent equipment could not contribute to it, as well. I really doubt that you'd hear any difference at all by replacing the amps.

    I don't believe the piston lamps can be replaced. Syndyne should be able to sell you replacement pistons.

    The brochure for the 750B shows a headphone jack--look for it under the keydesk (you might have to lie on your back & look up to see it). If it is there, you have a 2 channel mixdown to drive the headphones in some fashion or another--perhaps from the amp outputs. The drive signal for the reverb tank is a mono-mix, and you could probably take that right into your digital reverb unit.


    • #3
      Ok, there is no headphone jack. The models with external speakers only specifically omitted the headphone feature (says so in the service manual). There is a place for one next to the reverb control, but it is not installed. So, I will employ a mixing console to take 4 in/2 out for headphone use. Can also pipe other synths into the mix this way if I so chose, and eventually Hauptwerk.

      On the background noise, I'm hoping I can do something to minimize it. I will test my theory by using outboard amps. If the noise is coming in on the keying or oscillator circuits, then not sure what (if anything) I can do to reduce it. The inboard amps should very likely be recapped anyway, looks like the big filter caps have date stamps from mid-late 1981. I have gear from the 60's and 70's with original caps, but don't like pushing my luck with something like this organ, as it has a LOT of 4000 series CMOS devices in it, and don't want to let the magic smoke out since the one amp is also the main power supply.

      On to a couple of technical matters: I have one [almost] dead note - G in the top octave of the swell 8' Oboe. The dead note moves with the transposer. I can hear it very faintly, but it is just barely there. What I can hear is in tune. So, should I be looking at the oscillator or keyer circuit (note 56 according to the service manual FWIW) and should I suspect a bad diode, resistor or capacitor. The generator rack on this thing is beastly large and a bit confusing to me still. I am starting to follow what is going on better, but am struggling a bit.

      2nd issue is the Cymbel II mixture on the Great division. Apparently getting only one of two tones that comprise it across the entire manual. What is odd is I am getting all notes of one tone, but they repeat the same pitches on all 5 octaves. So, effectively, I have only one octave of C through Bb that repeats 5 times. Not sure if this is right, and also (what I presume should be) the second tone is missing completely. I see on the diode slide board each section appears to have one small signal transistor on the backplane board, and I am wondering if I should suspect this is faulty for one of the two Cymbel II diode slides. If this is normal, I'd like to know as it seems a pretty useless stop in it's current state. In other words, I don't think this is correct.

      I have noted that the capture action memory appears to support 10 generals. Only 8 are implemented on the 750B by the components on the latch board #ROC 2563H. I am wondering if by simply adding the unpopulated MM74C08N IC, two transistors and a few resistors and diodes identical to the components in place for the first 8 General latches, if I can get the extra two generals by running the needed wiring and two extra pistons. Any thoughts on this? Not a really big deal, as 8 generals x 2 memory levels is already pretty generous IMHO, but if getting 9 and 10 would be that easy, I will probably do so.

      One final item, the main off/echo on controls are not functional on this organ. The tabs move, but have no effect. I see in the service manual there was an echo adapter PCB that was needed for the echo (antiphonal) function to work. Does this also enable the main off to operate? I only ask as it would be convenient to use it as a "mute" control for the main speakers when using headphones but as it presently stands, it has no effect on the output.

      Think that is it for now, as that is already a long list. I really appreciate any input or advice from the resident tech experts on this!



      • #4

        The main off tab only functions if you have the echo adapter chassis installed. It is a good sized little unit with LDR cells for each audio channel, and the main off would extinguish the lamp that lights up the main LDRs and the antiphonal on would turn on the lamp to light up the antiphonal LDRs. Very neat way to control audio and would work just fine for your headphone application. However, you can only get one of those by salvaging one from a junked-out Rodgers. You could also just follow the signal from the main off tab and perhaps use it to activate a relay to silence the main speakers. Since you have the service manual you should be able to figure this out.

        Your almost dead note on the oboe is surely a bad diode or capacitor in the keying somewhere. Note that the keying voltage follows a circuitous path before arriving at the pair of diodes that pass the oscillator output into the oboe voicing circuit. If you trace that path you'll find an open diode or else the capacitor right after the diode not passing the signal. Your problem is not in the oscillator unless every stop has the same pitch missing.

        The Cymbel is indeed supposed to sound exactly the same in every octave. That is how a cymbel stop is constructed. It may sound very odd by itself, but added to a chorus it adds just the right amount of sparkle over the entire keyboard range. If you are supposed to have two pitches and you only hear one, you'll have to trace the stop turn-on circuit to find out why it's not enabling the second pitch.
        *** 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!


        • #5
          As usual jbird gives good advice! Just a comment on the Cymbel: try it (one both pitches are working) with an 8' Flute on a small contrapuntal line--it adds clarity and interest, and is one of my favorite small combinations.



          • #6
            Regarding "sing-through": when you get that Oboe note fixed, take a look at the various keyers. Some had pots to be adjusted for minimum keyer bleed-through. I hope you have the 750B manual.

            . . . Jan


            • #7
              Thanks John, Toodles and Jan for the input. I do have a PDF of the 750B manual (not great but usable) so I am able to more effectively troubleshoot.

              Not being familiar with the Cymbel II stop (have never played an organ with this one before) I didn't know that one octave of pitches repeated for every octave of the manual compass was normal. I guess I need to look it up on the pipe organ stop encyclopedia. It is a very high pitched voice (probably 1' or 1 1/3' pitch) so it is difficult to tell for certain if there is only one "voice" per key. I am assuming that the designation "II" is the same as used in Mixture nomenclature; the roman numeral represents the number of "ranks" (oscillators in the present case) used to produce the voice. By that way of thinking, I figured the stop should be comprised of two clearly discernible voices per key. So far, it does not sound like it is to me, but I will give it a more detailed listen.

              In any case, it seems I have minimal issues to deal with; rather remarkable for a 30+ year old instrument that has seen virtually no use in some time, according to the church I purchased it from. The capture action back up battery pack seems to be newer. Code on the 2 cells seems to indicate a manufacture date of 2008, but is not quite clear as to how this code is constructed. It doesn't look like a week/year, nor day (in Julian format)/year code.

              Still wondering if anyone has an inkling on whether the 9th and 10th generals can be added. I have sourced the MM74HC08N chips rather cheaply (will buy a few as spares in case I ever need to repair the memory/latch board) and based on the open solder pads on the board, it appears only about 10 components (mostly resistors and diodes) and 1 of these IC's would be needed to implement support for two additional generals. Presumably the existing RAM can handle the full complement of registrations, and Rodgers left them off either as a cost savings or to offer another model with the added generals as a marketing ploy.

              Of course, adding the pistons and wiring would be required as well, but it looks as though this would be quite easy to accomplish. Almost too easy, which is why I ask if my thinking along these lines is correct. FWIW, based on un-populated component locations on the latch/memory board there is support for 2 additional divisional pistons on each division, which would give a total of 6 on each. Not so concerned about those as I usually register with only with generals or by hand.

              Looks like my Lexicon MX400 reverb will arrive tomorrow, then the real fun can begin.



              • #8
                Rodgers also had a Cymbel I stop, which was a repeating, single rank. Also known as a "Cymalflöte" or "Zimbelflöte". I think it was at 2/3'. The Tech Manual might give the mixture composition--it does on some models, but not all. Use is the same--high pitched stop to add clarity to a single stop or combination. I have a fondness for such stops.

                As to the combination action, I'd have to review the schematics. I don't know if the memory is installed for the extra pistons, and that would really be the deciding factor. Rodgers, like most manufacturers, used common circuit boards and assemblies across several models, and would add or not add components as needed to meet the individual model's needs. It's not really a marketing ploy--memory was, for a very long time, expensive to come by. Now it is cheap.

                In the 1960's it was not uncommon for a model of this size to have 4 or 6 setterboard divisional pistons, with cumulative generals. No "Memory B", etc. For 70 stops, that meant 280 to 420 individual slide switches to form the memory. We felt lucky back then to have a moving combination action at all.



                • #9
                  The 750B manual says the Cymbel II consists of 1/8' and 1/12'; it breaks twice per octave, as is common with the Cymbale, Zimbel, Cymbal, Cymbel stop.