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Thread: Conn 650 bass pedals go out of tune

  1. #1
    ppp Pianississmo ukmusicman's Avatar
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    Conn 650 bass pedals go out of tune

    Hello everyone. Just found this website.

    I have acquired a Conn 650 Theatre Type One organ and have done a lot of restoration work on it, but have a persistent problem with the pedal generator on both 16' channels. Every so often, the second octave will jump out of tune by about a semitone. I've cleaned all the contact springs and the adjacent contacts and replaced two damaged springs, but this still keeps happening. Some times it stays in tune for a week or two, but other times it will go out after a few days, or the next day. Sometimes even while being played. I'm OK with all the mechanical jobs, but serious electronic fault finding is a bit out of my league. Any ideas guys?


    Cheers All.

  2. #2
    Moderator jbird604's Avatar
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    1 members found this post helpful.
    I believe you'll find a Master Tuning control just for that octave. On the 650, the 16' pedal stops come from two monophonic generators, one serving the bottom octave, the other one serving the second octave. (Notes above those are borrowed from the regular oscillators.)

    Each of the monophonic generators has of course 12 tuning pots for the 12 notes in that octave to be tuned individually. But there is also a master pot that brings the notes into roughly the right range before you make each one just right with the individual pots.

    So locate that pot, use a screwdriver to "exercise" it by rotating it through its full range about 15 or 20 times (slowly, gently). Then park it at the halfway point and try tuning some of the notes in that octave. If you have to turn the individual pot all the way CW or CCW, or nearly so, then re-adjust the Master pot until your individual pots are typically closer to the middle of their range. It can take some tinkering back and forth to get it to the right position, but you can find a good placement for it with trial and error.

    Other possible problems with erratic tuning include dirty tube sockets or a bad tube in the generator, as well as intermittent contact switches. But the master pot is certainly a primary suspect.

    Good luck! Welcome to Organ Forum, and please post about your organ.
    John
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    Church: Allen MDS-45 with Allen MIDI-DIVISION-II expander
    Home: Allen Renaissance R-230 with expanded four-channel audio and MIDI-DIVISION-II
    Shop: Bunch of organs in varying conditions, some good, some not...
    Half of an incredible two-man organ service team -- servicing all the major digitals in Arkansas churches
    https://www.facebook.com/pages/Birds...97551893588434

  3. #3
    ppp Pianississmo ukmusicman's Avatar
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    I've just written a long post about my organ. This is the second time I've done this. Why won't it go through?

    Simply because you're a brand new member - all your posts are subject to moderation at first. After a short while, you'll see your posts straight away.

    Although the forum does have an auto save feature for posts, I still tend to prepare long, detailed posts off line in Word or another word processor, and then paste them into a post. That way I can tinker with them and proof-read them before unleashing them on the world! Andy G - Moderator
    Last edited by andyg; 01-21-2017 at 08:57 AM.

  4. #4
    ppp Pianississmo ukmusicman's Avatar
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    Thanks for that. After several months of adjusting and re-adjusting, I've come to the conclusion that there must be something else causing this. I see on another post, that someone with a Rhapsody 628 had the same problem and it was suggested that it could be transistors going bad. On the circuit board concerned, there are several 2N2926 transistors, which I assume would all be the same, but they have different coloured dots on the tops. Does anyone know what the significance of this is, and if I get some new ones without dots, will this cause a problem? Just in passing, I replaced all the vinyl rods in the organ with Tibia rods that I cut the cranks off. Works fine. Others might be interested.
    Cheers all

  5. #5
    Moderator jbird604's Avatar
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    Before you replace anything on a circuit board, be absolutely sure that the spring-type switches are working correctly. The circuit flows from point to point through the pedal generator system, going through each and every one of those little springs before it passes on to the next little spring. In series, in other words.

    So a single point where the connection from spring to its stationary mate has developed a bit of corrosion, anything on either the spring or on the bar that introduces some resistance into the circuity -- anything like that will disrupt the operation of this sensitive circuit and make the tuning go haywire.

    Cleaning and de-oxidizing, cleaning again, vacuuming, dusting with a small brush -- whatever it takes, these little points of contact must work perfectly or else nothing else that you do will matter.

    And unfortunately, all this takes place down just an inch or so above floor level where you can't see it or reach it, where you must crouch and squint and moan and groan. So get a good light, get an inspection mirror, get a reliable ohm meter, and double-check all this touchy mechanism before you spend time and money on transistors or anything else.

    To be sure, there are components that can go bad in the circuit. I seem to recall having to change out one FET in particular to get one working sometime in the past 30 years. At the time, I think I bought a 5-pack of these FET's, and since then I haven't used another one! We don't see many Conn organs any more, of course, but truly, the vast majority of pedal issues were with the springs.

    Good luck!
    John
    ----------
    Church: Allen MDS-45 with Allen MIDI-DIVISION-II expander
    Home: Allen Renaissance R-230 with expanded four-channel audio and MIDI-DIVISION-II
    Shop: Bunch of organs in varying conditions, some good, some not...
    Half of an incredible two-man organ service team -- servicing all the major digitals in Arkansas churches
    https://www.facebook.com/pages/Birds...97551893588434

  6. #6
    ppp Pianississmo johnbeetem's Avatar
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    Quote Originally Posted by ukmusicman View Post
    I see on another post, that someone with a Rhapsody 628 had the same problem and it was suggested that it could be transistors going bad. On the circuit board concerned, there are several 2N2926 transistors, which I assume would all be the same, but they have different coloured dots on the tops. Does anyone know what the significance of this is, and if I get some new ones without dots, will this cause a problem?
    Colored dots can mean all sorts of things. Sometimes they're added by the transistor's manufacturer and may identify when the transistor was manufactured. Sometimes they're added by the product's manufacturer (in this case Conn) and may indicate results of testing.

    For example, a 2N2926 transistor is a current amplifier but the actual gain may vary over a wide range depending on purity of the silicon. The transistor's manufacturer guarantees a minimum gain (perhaps a factor of 10) but may ship you parts that have a much higher gain (say 40). The data sheet gives a minimum value and a typical value. All you can really count on is the minimal value.

    In digital circuits where the output is either turned on or turned off, you don't care if some outputs are turned on harder than others so it doesn't matter if the gain is higher than minimum. Analog circuits generally use feedback and other techniques to set the circuit gain using external resistors, which are much more accurate than transistors.

    In some cases, the product manufacturer needs to have a parameter such as gain spec'd more precisely. So they test each transistor and use colored dots to indicate the actual gain and then use only those parts in circuits that need a more precise value. The testing may be done by the transistor manufacturer or the product manufacturer. Heathkit (who remembers Heathkit?) tested all incoming components to be sure customers had working parts. They got a discount from part manufacturers who then didn't have to test the parts themselves.

    Now this is just an example and doesn't indicate what the dots actually mean on your Conn. If they do indicate a parameter such as gain and that gain value is important, swapping transistors without knowing what you're doing is ill-advised.

    In general, it's always better to have the schematics and test equipment such as a good multimeter and an oscilloscope. Then you can narrow down the source of the problem. Otherwise you're shooting in the dark.

  7. #7
    mp Mezzo-Piano twnelson's Avatar
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    As a former Conn tech...

    The dots on Conn transistors do indeed indicate grading for specific parameters, which included beta (gain), certain gain transfer curve properties (important for keyers), and a couple of more obscure things I can't recall. You definitely cannot perform arbitrary part substitutions without understanding the circuit and being prepared for some possible tweaking. Never was a big fan of their transistor-based designs as they were far too dependent on semiconductor properties that unavoidably vary.

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

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