Expression Pedal – Repair and Mods (M-Audio EX-P)

Expression Pedals: Repair and Mods

(M-Audio EX-P)

modified expression pedal

This article is a followup to a previous blog post about expression pedals, which covered how to buy the right one and get it properly calibrated. If you haven’t read it, be sure to check it out as well.

This article will focus on some mods to improve (or repair) your pedal as well as add some new features.

DISCLAIMER: As always, if you do not feel comfortable modifying and repairing your own gear safely, get a shop to do the work for you. This article should be used as a guide. Blackaddr Audio is not responsible for any damages.

A common failure point in expression pedals is the collar that attaches to the shaft of the pot. They’re usually made of plastic and can easily crack or slide off. Even if the collar hasn’t failed yet, we can easily reinforce it so that it will be far more reliable.

Another common failure point is the pedal cable itself if it’s attached directly to the unit. These cables are usually made of cheap moulded plugs with the cable itself being the skinny-but-rigid-kind that always seems to get tangled or form tripping hazards. I also find they always seem to be an inconvienient length, too long, or too short. If you replace the hardwired cable with a TRS jack, you have complete control over the cable length so that it’s just right for your setup.

One final mod we can do will make our pedal more versatile. Good quality MIDI devices (like effects units and foot controllers) that support an external expression pedal will let you set for each preset whether the EXP (or EXT) jack is being used for expression or as another footswitch. Rather than having to unplug the expression pedal and plug in the footswitch when we need to swap, we can add a TS jack to daisy chain the stomp switch and the expression pedal together. Check with the manufacturer of your MIDI device to determine if they support this feature.

Today we'll make all of these enhancements to a M-Audio EX-P which has a cracked shaft collar. These are very inexpensive units but after reinforcing the collar and replacing the hard-wired cable they become reliable too!

exp pedal and jacks

If you’re not familiar with how an expression pedal circuit works, it’s worth taking a moment to understand it, you’ll see they are quite simple.

All the complexity of an expression pedal is mechanical. That is, translating the linear movement of the pedal to rotational movement of the pot shaft. The electrical part is quite simple as shown in the diagram below. When the expression pedal is in toedown position, the reading from the pot is a high resistance (or high voltage). A pot is a variable resistor that takes the total resistance as measured between the ends, and has a third connection as a tap (also called a wiper) in the middle which slides along the internal resistor. This configuration is called a voltage-divider because if you apply a voltage (VDD) to the top, the wiper will read a voltage between 0% and 100% of VDD as it slides along. VDD is typically 3.3V or 5.0V provided by the effects unit or foot controller.

When the pedal is in the heeldown position, it’s a low voltage (wiper is at bottom of divider) and toedown is the high voltage (wiper at top). Note that some manufacturers choose TIP for the measurement and RING for the reference voltage, and others do the exact opposite. You can measure the resistance with a multi-meter between RING/SLEEVE and TIP/SLEEVE while moving the pedal to figure it out. Only one of those two will measure a resistance that changes as the pedal moves.

If you're going to replace your cable with a jack today, take a moment to figure this out on your pedal and write it down. You'll need the info later.

As a modification, we can add a switch that connects between the TIP and ground. When the switch is open, it has no impact on the expression pedal circuit. However when the switch is closed, it shorts the TIP to ground, make it look like pedal is in the heeldown position, even though it’s not.

The following diagram shows the simple modification. When we want to use the expression pedal, we keep the switch open. When we want to use the switch (as an extra stomp switch for example) we just need to set the expression pedal to the toedown position. This ensures when the switch is open, the measurement reads a high voltage, and a low voltage when it is closed.

Okay, here’s the plan for today:

  • Repair/Reinforce the pot collar.
  • Replace the hardwired TRS cable with a TRS jack. Requires soldering.
  • Add an additional TS jack to add an external stomp switch. Requires soldering.

We’ll need a few materials. A ziptie and hotglue are needed to reinforce the shaft collar. A stereo TRS jack is needed if you want to replace the hard-wired cable. And if you want to add support for daisy-chaining a footswitch you’ll need a TS jack and some spare wire.

Step 1. Open the up the pedal.

For the EX-P, there are 7 screws holding on the baseplate. If you are only doing the collar repair, you do not need to remove the smaller two screws holding the M-AUDIO ↔ OTHER switch. If you will be installing jacks, it’s best to remove these as well to completely detach the baseplate from the pedal housing which makes drilling easier.

Step 2. Inspect the collar.

You can see the plastic collar has slid down the shaft (almost ready to fall off) and if you look very carefully is cracked (circled in red). All we need to do is ensure the collar can’t slide off, or break off of the pot shaft.

Step 3. Install a zip tie.

Slide the collar back to it’s proper position on the shaft and wrap it in a zip tie. Make sure the the buckle on the zip tie is in a position where it won’t interfere with anything as the pedal rotates. In the photo below the pedal is in heeldown position and the buckle is facing up. When the pedal travels to toedown, the buckle will rotate to the right. Trim the zip tie once you are satisfied with the location of the buckle.

trimmed ziptie

Step 4. Hot glue the zip tie.

Use a small amount of hot glue to secure the zip tie to the shaft on both sides. The zip tie will ensure the collar cannot break off (even if it’s already cracked as we have here). The glue will prevent the collar from sliding out of position along the shaft. Do not get hot glue on the very base of the shaft where it enters the pot housing, and do not get glue on the moving gears/levers of the mechanism. The great thing about hot glue is if you screw up, just let it harden then pull it off, and try again.

THAT'S IT!

Your expression pedal is now far more robust and reliable than it was before.

glue applied

If your pedal has a hardwired cable and you’d like to replace it with a jack, let’s carry on! Get out your wirestrippers, solder and soldering iron.

Step 1. Cut the 3 wires entering the external cable, strip the ends and tin them with a bit of solder.

First, figure out what each of the 3 wires in your cable is conntected to with regards to TIP, RING and SLEEVE and write down the color of the wire and what it's connected to. Taking a picture now before cutting is a smart way to troubleshoot later if there is a problem so you have a record of what the original wiring looked like.

If the wires are not long enough to reach where you want to place the TRS jack, you’ll need to solder on extra wire to reach. If you do extend the wire, be sure to wrap the joint in heat shrink tubing or electrical tape to prevent shorts. Since I’ll be putting the jack on the side of the housing, the existing wire is long enough (barely).

trim wires

Step 2. Mark the position of the Jack.

Determine where you are going to place the stereo jack. Check for clearances before you start drilling holes! In my case, I have to make sure the jack doesn’t interfere with the mounting post, or the pot mechanism. Keep in mind when the plug is inserted into the jack, the contacts flex outwards and the plug tip goes in past the TIP contact so leave extra room at the back.

This is the trickiest part of the mod. You also need to get the the jack position such that it does not contact either the pedal housing or the base plate when it’s installed. You can bend the solder contacts inwards a bit to give more room. If your housing or baseplate are metal, and there is any concern that the jack contacts or solder contacts will touch them, put a few layers of electrical tape on the housing (or baseplate) at the areas of concern.

position jack

Step 3. Drill hole for the jacks.

Standard jacks need a 3/8” hole. Double check the size of your jack before you drill to ensure you use the correct size bit. Don’t try to drill immediately with 3/8”, drill a pilot hole with a smaller bit first. Anything around 1/8” is fine to start. If you are drilling a plastic enclosure, the plastic shavings will have a tendency to melt. These can be easily removed by reaming the hole a few times with the drill and breaking them off with your fingers or a utility knife.

Since I’m adding a TS jack for the footswitch, I drilled a second hole on the other side of the pedal as well.

Step 4. Install the jacks.

drilled hole
install jacks

Step 5. Solder the 3 wires for the pot to the jack according to your test results earlier.

Use a multimeter or carefully visually inspect your jack to determine which solder contact correspends to which plug contact. For my EX-P, The mapping was as follows:

TIP - red wire
RING - white wire
SLEEVE - black wire.

solder trs wires

Step 6. Installing a TS jack for a footswitch (optional)

Connect the TIP contact on the TS jack to whichever contact on the TRS jack connects with the wiper on the pot. In my case, this expression pedal lets you select either Tip or Ring as the wiper using the M-AUDIO switch. I always keep it set to “M-AUDIO” which uses the TRS Tip as the wiper. This means I need to connect the TIP on my TS jack to the TIP on my TRS jack.

solder TS

YOU’RE DONE!

I can now use my own stereo TRS cable to connect my expression pedal to my MIDI Foot Controller and use a regular mono TS cable to connect my external footswitch to the expression pedal.

PLEASE NOTE YOU MUST USE A STEREO CABLE FOR EXPRESSION.

These are also called 'balanced' cables as opposed to 'unbalanced' cables like guitar patch cables. It's very simple to figure out what you've got, if the plug ends shows the ring collar, it's a TRS cabale. If the plug only has the tip and sleeve contacts, it's a TS cable. In summary, you need a stereo balanced cable for the expression pedal connection, but you can use a balanced, or unbalanced cable for the footswitch, it doesn't matter.

Remember, if you did the footswitch mod you need to keep in mind how to ensure they don't interference with each other. Failure to do so will not damage anything, but may leave you confused when one of them isn't working as expected. More than once I've caught myself asking, "Why isn't my footswitch working!?!", then I notice I forgot to set the expression pedal to toedown.

  • The footswitch must be ‘open’ when using the expression pedal.
  • The expression pedal must be “Toe-down” when using the footswitch.

The calibration features of the Blackaddr microMIDI display a measurement of the voltage returned by the expression pedal. This is a handy way to test that everything is working correctly, as well as see how the measurement changes as the pedals are used.

In the video, I'm using a momentary switch, but this works equally well for a more common 'latching-type' toggle switch.

If you’re in to digital modelling, or thinking about getting into to it, a MIDI Foot Controller and expression pedal are essential tools to ensure you are getting all the power and versatility your digital models can deliver!

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