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Thread: Conn ST-11

  1. #1
    Senior Member Euthymia's Avatar
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    Conn ST-11

    I just picked up one of these at a swap meet in Cupertino. I've wanted one for about 30 years.

    The guy said that he had gotten as far as plugging it in and that it powered on but did not spin up.

    I found that the power transformer had come loose and broken its connections, and repaired those and got everything to come on and got the wheel to spin up.

    When I switch it to "Calibrate," though, I can't get any of the rows to "freeze."

    I found an old post where Enzo mentions that he's fixed hundreds of these, so perhaps there are some places to check first on this thing?

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    Lifetime Member Enzo's Avatar
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    Cal puts 60Hz on the lamps, That is like a low note, so usually it syncs in only the lowest band near the hub and it won't have a real sharp edge. Does the vernier knob affect the pattern motion? Check that pot. You usually get a sort of glob that stabilizes.

    Does it work otherwise? Can you get patterns to freeze on any of the notes?

    The thing is not all that complex. Ther are two parts. The drive and the strobe. The drive is a powr oscillator for the motor. Each of the 12 notes of the scale spin the wheel at a different rate, selected by the rotary switch.

    The strobe is just a little amplifier stage driving the neon bulbs. Whatever signal you input turns into light. if the signal pattern is the same ffreq as the spinning disc pattern, they sync up. If the mic makes the neons light, that part works.

    I almost always find the power transformer coming loose. Add a little locktite or use nylock nuts. Or dope the threads with nail polish or something.

    For the most part you will not want to be using the X2 setting.

    The neon lamps darken with age, just like in trem bugs. I usually need to replace the lamps for a brighter display.

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    I'm sure Enzo can add to this, but in my experience when the tranny comes loose (and it seems like they always do), it smacks some components on the vertically-mounted PCB right next to it and either breaks the solder joints or the component leads themselves. Actually there are other spots on the other larger PCB that seem to get cracked solders too just from thermal cycling, and the power supplies are getting old enough that the filter caps can be suspect.

    I've got a project one (via ebay) that I'm going to try some high-brightness neon lamps in if I ever get around to it...

    Good luck!

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    Last edited by Mark Black; 05-13-2008 at 01:28 AM. Reason: Oops - Enzo beat me to the reply - guess I should learn to type...

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    Lifetime Member Enzo's Avatar
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    I use real bulbs from Conn/Peterson, they are not just NE2s. But others will work. Brightness is a function of current through the things. One thing that helps is to turn the volume down. The higher the volume setting the more extraneous noise the lamp driver amplifies, so the cloudier the light patterns get. I had one fellow send me one once - an older accordion guy - and I could not seem to convince him of this. He kept complaining that the lights wouldn't com eon unless the mic was all the way up. He expected a steady glow from the lamps instead of only responding to the music. They are never what you would call bright. peterson eventually went to LEDs, which are brighter and clearer. If yuo want to experiment, you could probably make that conversion. Without looking it up, seems to me the neons run from a HV rail with a simple transistor to turn them on. That transistor could just as easily control LEDs running from a lower voltage in the unit. Or so it would seem to me. But remember to keep them in a narrow line down the center. The disc is spinning, so if you trailed them across the screen, they would be lighting different spots on the pattern.

    I did have to replace the motor caps now and then. But usually that need resulted in a stopped motor, not a wrong speed motor.

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    Senior Member Euthymia's Avatar
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    What it turned out to be was that whoever it was who worked on it before I did plugged in the Molex connector on the input amplifier PCB one pin off.

    I figured this out by probing voltages. Found that "ground" on that PCB was at +5DC in relation to another part of the unit, then noticed that the connector was plugged in wrong.

    Once I got that right, it calibrated right up. Yahoo! Rust never sleeps!

    The bulbs seem kinda dim, though. Enzo, you say they're not just NE2's, that you get 'em from Peterson? I must phone them, MasterCard in hand.

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    Lifetime Member Enzo's Avatar
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    You could stuff three NE2s in there and see how well they work, won't hurt, but these bulbs are longer and seem to have somewhat different specs, not that I could tell you what those specs might be.

    The bulbs tend to blacken inside over time. And sometimes the glow covers less and less of the anode inside.

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