Adding volume pedal to umx61 midi keyboard

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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.
 
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 it´s 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 it´s 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 don´t see the circuit.
 
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!
 
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?


I´m sure the strange effects is because of 50 hz hum in a sensitive circuit.
 
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.
 
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?
 
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 it´s the same.
 
I added a ground / earth as you recommended. The vibration is now much much less; however, the pedal (remember this is about 18KOhm compared to the inbuilt potentiometer which is 10KOhm) still does not work at all. I am sure I used the correct terminals. In fact I checked again with a digital meter the tip and ring of the plug: the meter shows 18.4K when pedal is up and 0.00K when completely down. It should work at least for half the excursion of the pedal! I am baffled... :cannotbe:
 
It should just work.

The arrangement in the keyboard (only 2 connections to the pot) is ugly although not uncommon in audio. Are you sure that this is really the case? I always take the wiper to one end of the track if I want a simple variable resistor rather than a potential divider.

Disregarding that, put a DVM across the wires from the pedal and pick 2 that give you a variable resistance in the sense (rising or falling) that you want. Arrange the connections to switch in this pair instead of the pot. As you say it should work for half the excursion at least, so check, once you have it connected, that you can see the variation in resistance at the point that you broke into the circuit.

w
 
Sleeve - Ring = constant 18K
Sleeve - Tip = variable resistance in the reverse as for ring-tip ie max when down; 0 when up.

But the sleeve is just connected to earth. Should it have any effect on the variable resistance between ring and tip?

Should I disconnect the sleeve connection within the pedal? If yes, should I leave the sleeve grounded? If yes, why?
 
Thanks Wakibaki for your contribution. I did all that before I ventured to do the alterations to make sure I understand how the inbuilt keyboard potentiometer works ie which leads to disconnect and the same for the pedal. That's why I am baffled by the result of what should have been a pretty straight forward simple alteration!
 
Upon opening the pedal, I notice that the "sleeve" is grounded to the metal base of the pedal armature.

Anyhow, I disconnected that from one terminal of the potentiometer. I checked again and confirmed that there is a variation in resistance between the other two terminals when the pedal is swung down and up. There is no resistance at all now between sleeve and ring and sleeve and tip (ofcourse).

The virtual organ still shows some vibration in the virtual pedal despite all these alterations. :confused:

Amazing isn't it!
 
Ok, the sleeve is connected to one end of the pot's track, the ring to the other. The tip is connected to the wiper.

The connections you want to switch in are the tip+the ring or the tip+the sleeve. Just pick one or the other. Forget about hum until you get the basic function. If the pedal works upside-down you can swap the sleeve for the ring.

Connections on a switching stereo socket can be confusing. Plug a (preferrably unwired) jack into the socket and use a DVM to make sure the connections work the way you think they do, by checking that there is continuity between the bare connection on the jack plug and the point where you broke into the circuit.

Finally plug the pedal in and again make sure that there is a variable resistance connected to the circuit.

It should just work. If it doesn't it's because the connections are wrong, even if you think that they are right. It's common for people to look at incorrect wiring they have done themselves dozens of times and not SEE the fault. It's just a human nature thing.

w
 
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