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Rodgers 750B trumpet question

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  • Rodgers 750B trumpet question

    A technical question regarding the trumpet (or reeds in general) on the 750 and related series organs. Is there a way to brighten the trumpet tonality to get something closer to a festival trumpet or tuba mirabilis and not simply louder relative to the other stops?

    I have the factory manual and it covers voicing (really just level setting of indiviudal stops) but there are undocumented adjustments in the formant circuits and I am wondering if these can be tweaked in some orderly way (i.e. not randomly for try it and see type adjustments) or if even changing the value of certain passives would achieve this. Clearly, Rodgers had a system for doing this to achieve different results on these old analogs usuing substantively similar circuits with minor changes but it is not clearly documented anywhere I've seen.

    I have service manuals for the older 330/660/990 and a newer 850 microprocessor controlled model and they have a bit more detail than the 750B manual but still not a lot to go on. While I know these old analogs are generally considered obsolete from a sound quality standpoint, I find that with a bit of outboard signal processing and some other minor tweaks they sound quite good and I've chosen to retain the stock electronics.

    Now I have procured a few ranks of extra tone generators (oscillators and formant/keyer boards) some from a model 330 and some from a 750B and would like to make one of the standard trumpet sections - either the existing 750 swell trumpet or one of the spare reed ranks I have - a "brighter" (for lack of a better word) reed stop.

    Any insight from the resident pros would be greatly appreciated.

    Kevin Dierkes

  • #2

    You may have already played around with the formant coils, since you seem to be pretty knowledgeable about these circuits. Whenever I have done a complete tuning and regulation on one of the 700/725/750, I've experimented with moving those cores around, and have been pleasantly surprised at the range of tonal adjustment. When messing with these you are doing "trial and error" stuff, of course, and it works best if you have a good assistant at the keyboard while you tinker in the back.

    There are of course two different types of reed voicing boards in these organs. Your organ may have both, or may have just one. I don't remember just how those are parceled out in the various models in this group. The "basic" trompette voicer consists of just three formant coils that cover the entire range of the stop. In this version of the circuit, playing around with the three coils will result in a rather drastic change in the quality of the trompette voice. Each coil does something different to the tone color, and the three interact with one another as well, so you just have to play around and see what happens. You may discover something more pleasant than the factory default voicing. (It's wise to mark the original positions of the coil caps with a pencil so you can return to the factory setup if necessary.)

    The other trumpet voicing system uses one formant filter for each group of four notes. Each note of the rank get its own level pot, so you can freely adjust the scaling of the stop to make it taper toward the bass or toward the treble as you desire. There is only one formant coil in each circuit that serves a group of four notes, so it's somewhat easier to figure out exactly what is happening. You can certainly regulate the overall tone color of the trumpet stop using this system, as the turning of the coil will do exactly one thing -- make the group of four notes either brighter or duller. After settling on the desired tone color for each group, you can go back and tinker with the level pots to make the notes all fit together into a pleasing scale.

    Other than that, in the basic voicing network I described first, there are some capacitors that shunt the highest frequencies to ground at various points in the circuit. (C808, 809, and 810) Replacing any or all of these with smaller values will result in an overall brighter trompete stop, but you would have to do that by trial and error as well. A good value to try would be one-half the existing value, which should make quite an audible difference in the tone color.

    I think it's commendable of you to work on improving this organ. There is a charm about these organs that isn't found in the digitals, though not everyone will love one of them as much as you do. I hope your organ will continue to hold up and make music for a very long time, but not all of them are doing so well.

    We worked on one today that has been gradually falling apart over the past few years. Ciphers one month, dead notes the next. A couple of the amps have died, and speakers have needed replacing. Chips in the time-share have failed, resulting in ciphers or cross-coupling. Today we found that one piston memory circuit has flaked out, and pressing that piston evidently corrupts the entire memory level. Had to clip the wires off that piston so the organist couldn't accidentally use it. So this one is probably not long for this world, but some of them are no doubt still in mint condition.

    Good luck, and please post your results!
    *** 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!


    • #3

      Thank you for a very helpful and concise writeup. Pretty much what I was hoping to learn. I am expanding this 750BE well beyond it's original spec. I'm making it a three manual instrument with power drawknobs (not lighted) and a tab rail rather than just the two manuals and dual tab rail it started with. I also have a newish (1998 build) Peterson duoset capture action covering 5 divisions and four memory levels I'll be fitting to replace the exisiting solid state capture action - which works great but will not allow me to add any more SAMS for a choir division.

      Anyway, I have picked up all the tone generator boards from a retired 330 plus a few others here and there and will wire most (maybe not all of them) to use with the newly added choir division and a 32' Contre Viola to the pedal division (presently only has 16' flute and reed stops). Might divide up the exisiting divisions a bit more with extra oscillators to get a better ensemble. I have the three wood core manuals from the 330 and extra piston rails and pistons, though I actually favor the feel of the Loduca metal frame keyboards that are on it now.

      This is an ambitious project but most of the parts were inexpensive save for the Peterson combo action and the Syndyne power drawknobs (though I was lucky to get a lot of 36 NOS units from Matt at MCN systems in a surplus lot a while back at much less than normal price).

      As to reliability, this instrument lived a charmed life of infrequent use in a local church and was also fitted with some Walker upgrades as I came to discover later after I acquired it. Have had a handful of dead notes due to breaks in the coil wire where they solder to the oscillators or the diode slides but those are easy enouth to find and repair. Picked up a used Peterson VST to do the tuning.

      Really I just like the challenge of repurposing things others have written off, especially if they are perfectly workable. I thoroughly enjoy older electronics as they are easier to work on and understand. As for the signature Rodgers analog sound, I rather like it. With activity generators, judicious use of chiff and air puff as fitted from the factory and a good modern reverb running to properly placed speakers it sounds great. Won't fool a real pro but is still quite good and for a home practice instrument it will ar exceed the capabilites of most church installed organs I have ever played to date. I chuckle to think about the calibre of insturment I will have when finished. Even now in it's current state it is truly enjoyable to play and listen to.

      I recently picked up a 9 bell zimbelstern on ebay as well and have wired it up using a Peterson sforzando II reversible board - also an ebay find - just in time for some Christmas playing. This console was pre-fitted with a thumb piston and two terminal wiring block at the rear for connecting a zimbelstern so it was an easy setup. Now I just need to add a speed control for it on the jamb board as it is a bit aggressive at full speed.

      More to come...


      • #4
        What an impressive plan! I sure hope it all works out well for you. As you've discovered, these analogs can be quite fun to work on, and the operation is so straightforward that they are easy to upgrade and expand. Could be as much fun as scrounging up ranks of pipes and building up your own pipe organ!
        *** 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!