Adding volume pedal to umx61 midi keyboard - diyAudio
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Old 10th January 2009, 02:48 AM   #1
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Default Adding volume pedal to umx61 midi keyboard

I would like to add a pedal volume control to a Behringer Midi Keyboard umx61. The inbuilt potentiometer is a 10K sliding type. I bought a 20KOhm (actually around 19K) pedal volume controller which does not seem to match the electronics of the keyboard. I wired and soldered a socket that effectively cuts off the sliding potentiometer connection from the circuitry when the pedal jack is inserted so that the rotatory potentiometer in the pedal takes over. However the software on my computer shows the pedals of the virtual instrument vibrating until the pedal is disconnected. The sliding inbuilt potentiometer works well otherwise showing that at least there is nothing amiss with my alteration. I am aware of the difference in resistance value but adding a 27K resistor in parallel to reduce the resistance of the pedal (rotatory) potentiometer does not solve the issue. What am I doing wrong? What can I do to recify the issue and why? Thanks in advance.
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Old 17th January 2009, 11:55 PM   #2
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No electronic geniuses out there?
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Old 19th January 2009, 07:40 PM   #3
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You can take a wire from the breaks on your bicycle and glue it to the potentiometer, put a spring on the other side and then glue the wire to the pedal ;-)

Anyway, what kind of cable do you use? Make sure itīs all properly shielded and grounded. Is the potentiometer connected to ground somewhere? Then use that for shield. If not, use a four pole connector so you can connect ground to the shield, make sure itīs shielded also in the pedal. Perhaps you need separately shielded cables??

I think the reason to the "vibration" is 50 Hz noise.

Hard to know when i donīt see the circuit.
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Old 20th January 2009, 03:06 AM   #4
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Hi. Thanks for your comments (though I take it that your first comment was a pull of the leg and not a fancy way of earthing [grounding] my keyboard).

All I did was to use a commercial expression (foot) pedal (with its own original wire) instead of the inbuilt sliding potentiometer in the keyboard.

The midi keyboard is connected to the computer via a USB cable. There is no direct power supply to the unit )other than through the USB) although it can take batteries or a transformer.

The modification I did was to disconnect the inbuilt sliding potentiometer in the keyboard from the circuit board so that the circuit is re-routed to a jack-plug socket and the wires from this were soldered back to the sliding potentiometer restoring the circuitry.

Thus, when I plug the expression pedal into the socket I installed, the keyboard potentiometer is effectively disconnected so that the pedal becomes the variable resistor. Although I would have thought that this would be a simple and straight forward alteration, it proved not to be so and I cannot fathom why!

I can take photos and send them to you but I would have to dismantle the whole keyboard and circuit board to do so!
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Old 20th January 2009, 09:26 AM   #5
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You can try to measure between ground on usb and the pins on the potentiomere, if you have connection there somewhere, use that one for shield.

Is it two- or three pole connector to the pedal and how is it connected to the keyboard?


Iīm sure the strange effects is because of 50 hz hum in a sensitive circuit.
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Old 20th January 2009, 11:57 PM   #6
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The on-board potentiometer is connected via two poles. The pedal however has 3 wires i.e. the jack plug is identical to that of a pair of stereo headphones.
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Old 21st January 2009, 07:29 AM   #7
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In that case, i should check the pedals configuration. Is the shield only shield, or is it carrying signal as well? I suppose you have a three pole connector in the keyboard after modification. Connect sleeve to ground in the keyboard to get proper shielding. Then, make sure the cable to the pedal is shielded, and the shield is not carrying signal. Use tip and ring for the potentiometer.
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Old 22nd January 2009, 03:39 AM   #8
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I took some photos of the keyboard, my rewiring and the foot-pedal (external view with its jack plug) that I hope I will upload or send to you in the coming days. In my re-wiring, the socket will connect the pedal via the tip and ring as you recommended. I figured this out using a digital meter to see which terminals would make the pedal function as needed ie to increase the volume on pressing down (least resistance) and to decrease volume on lifting up (max resistance).

May I please ask what might be a stupid question (remember I have very limited electronics knowledge other than what I learnt at school 30 odd years ago in Physics!): how does one "shield" or "ground" a component? I am not sure I quite understand the difference in the terms. Are these the equivalent of "earthing"? Inside the keyboard, everything is plastic except for the electronic bits. If earthing is what's needed, how can I go about doing that?
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Old 22nd January 2009, 09:41 PM   #9
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No problem!

ground can be earth, chassie, and so on.. In this case you should be able to put a ohm meter at the usb plug "cover" (shield, the big shiny part) and find a grounded point near the potentiometer inside the keyboard. Connect this point to sleeve on the connector. This should connect the shield in the cable to the pedal to ground inside the keyboard and remove the noise that is disturbing the circuit.

Shield can be the "cover" of the cable to the pedal, it can also be something inside a circuit that protects a circuit from hf noise and so on..

Grounding, earthing, can be the same, but not always, sometimes it's separate points. In this case i suppose itīs the same.
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Old 23rd January 2009, 01:24 AM   #10
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I am truly greatful for that information. Thank you very much indeed. I shall get to work on that over the weekend (I hope). I shall keep you posted with my progress.
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