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Persussion problems in RT3

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  • Persussion problems in RT3

    I have an early '60s RT3 and within the last two days, the percussion has been making an odd static sound when I press and depress the keys. The percussion is working fine otherwise. It pings (2nd/3rd), changes volume, and decays (fast/slow). It almost sounds like a whoosh. The same sound is made when the key is pressed and when the key is depressed. I get the same sound when I toggle the percussion decay switch. I get the same sound when I toggle the percussion delay switch when the percussion is off. None of the other percussion switches cause this sound.

    I noticed it one day. The next day it was gone. Now it is back again.

    Any thoughts?

  • #2
    Hmm. Someone here can probably tell you exactly which capacitor might cause this, as that's generally the component in the preamps that will fail, especially in an early 60s unit that will have the wax-and-paper capacitors.
    My first suggestion is always re-seating the tubes and trying different ones or at least switching the compatible ones around in the preamp. Maybe use a little bit of deoxit on the pins and work the tubes in and out of the preamp a couple times. On an AO-28 preamp, I believe the tubes in the percussion part of the amplifier are V4, V6, and V7 - looking at the preamp as mounted in the back of the organ, this should be the tube immediately to the right of the volume control housing (V4), the tube on the far right (V7) and the tube two to its left (V6). The tubes, in order from the volume control housing left to right, are V4, V1, V2, V6, V5, V7. This is with the tubes pointing up - I haven't been inside an RT console, but I'm pretty sure they're mounted the same as other full-depth consoles. The A100 has the preamp mounted upside-down on the other side of the board from the tone generator.
    I probably own too many keyboards
    https://bensnacksturner.com/the-fleet/

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    • #3
      I played with the tubes, but no change. I got 26.5v on the K terminal (on slow), went down to zero (and then up to about 6-7v) when key pressed on upper manual. I believe this is correct from what I have read.

      Other observations:

      With Fast Decay selected, When I turn the percussion cut off all the way counter-clockwise, the static/low whoosh (weird sound) goes away, but no decay. Percussion cut-off all the way clockwise, background noise is there when percussion engaged.

      With Slow Decay selected: Percussion works as it should

      I only get the weird noise with percussion on fast decay and when I hold key for more than a second before depressing key. If I move fast through the keys, percussion sounds normal (except for the first key pressed and the last key depressed).

      I also get weird nose when toggling between fast/slow decay whether percussion on or off.

      Pre-Amp (H-AO-28-1) problem? Percussion switch problem?

      Not sure if this sheds any more light on the situation.

      Can live with slow decay, but would rather not.

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      • #4
        Interesting. Have you swapped the 2 6C4?

        Jim

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        • #5
          Yes. Twice. If I just strike key and don't hold it, percussion works, and no deep whoosh sound. If I hold key, percussion works and weird sound kicks in. When I depress the key, whoosh sound again. Only lasts a second when key is pressed and then another second when key is released. Again, this is only on fast decay. However, it will make the same sound, though not as pronounced, on slow decay when percussion cut off turned up too high.

          Same sound when rocking percussion delay switch from fast to slow, whether percussion switch on or off.

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          • #6
            This is a good one.
            1) Appears C31 is OK as the prec decays. It seems like there is a transient when the 12AU7 cuts off. Could C31 be leaky?

            2) Try with the draw bars all in. Perc should still work. Same result?

            Same sound when rocking percussion delay switch from fast to slow, whether percussion switch on or off.
            3) Try disconnecting terminals L and M (fast / slow select) . Perc should still work. If that fixes it I would expect dendrites in the switches.

            4) Try grounding perc input Terminal H. Same result?

            5)Pull perc tubes and check voltages as per schematic.

            6) I have an AO29 on the bench being burned in to replace one with perc static. Between that and the A105 I can take comparative readings if need be.

            Jim

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            • #7
              Haven't had a chance to play with the insides of the organ yet, but with drawbars in, changes nothing. Also, noticed that sound goes away when expression pedal is pushed all the way in.

              I will open up the back at some point this weekend.

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              • #8
                I tested the perc on the AO29 (perc circuit same as the AO28) I have on the bench. Nothing connected to terminals H, J, K, L, M & P. I don't think it matters if you leave the connections intact as they are normally open. If you inject a tone on terminal H with a tone generator or a jumper from a TWG terminal, you should hear the tone. Grounding terminal K will cause a "plink" and the tone will decay. Removing ground will restart the sequence.

                Jim

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                • #9
                  Originally posted by 32Pedals View Post
                  Haven't had a chance to play with the insides of the organ yet, but with drawbars in, changes nothing. Also, noticed that sound goes away when expression pedal is pushed all the way in.

                  I will open up the back at some point this weekend.
                  Do you mean that it dies at "full throttle"? The issue here is that the expression capacitor is over-rotating and shorting against the box in which it lives. Adjust where the expression rod attaches to the actuator arm (that rotates the expression capacitor) and that should fix it.

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                  • #10
                    Originally posted by Tim_at_Jonas View Post
                    Do you mean that it dies at "full throttle"? The issue here is that the expression capacitor is over-rotating and shorting against the box in which it lives. Adjust where the expression rod attaches to the actuator arm (that rotates the expression capacitor) and that should fix it.
                    'Wrapped around the backstop' 'Hit the wall' yep,that happens when you don't make a note of where the shaft came off the collett,or the fitting fastener is loose,or both.....
                    Still working on the mall organ coffee table book for cork sniffers

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                    • #11
                      Originally posted by Sweet Pete View Post
                      'Wrapped around the backstop' 'Hit the wall' yep,that happens when you don't make a note of where the shaft came off the collett,or the fitting fastener is loose,or both.....
                      The shaft has a flat, so the position of the arm on the shaft can't be 'a little wrong', only 'very wrong'. But yes, if the set screw is loose or the collet is loose on the arm, it can happen.

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