DigiTech XP400 Reverberator ashbass Set Minimum Level Control Mod

Digitech XP400 User Guide    XP400 Program List

Click on any photo below to see a larger version.
The XP400 Reverberator is one of DigiTech's XP Series pedals from the 1990s. All pedals in the series have the same look and feel and use a built-in expression pedal to control program levels. When the pedal is heel-down the program is off. When the pedal is toe-down the program is on 100%. Zoom G5
I wanted some finer control of the reverb levels and ended up installing a program level control that I mounted under the top of the expression pedal. This way with the pedal in the heel-down (off) position I could turn the knob and dial in some reverb. A side benefit of this mod is that I effectively had a pedal minimum level control. Digitech XP400
I generally keep the set minimum level control (smlc) at about 4 so whatever program I'm using still has a touch of reverb when the pedal is heel-down.

The following steps detail how to do this mod yourself. It is easily reversible and requires no permanent modification to your pedal.

This mod can also be done on the other Digitech XP Series pedals with the same results.
Digitech XP400
Remove the backplate of the pedal and put it and its four screws safely off to the side.

You can print a nice and clean XP400 program list by clicking here.
Digitech XP400
This small piece of circuit board is attached to the end of the expression pedal plunger. Depending on where the expression pedal is set, this bit of board can be distant from the main board (toe-down/on) or close to the main board (heel-down/off). Digitech XP400
On the other side of the little board there is a red LED. On the main board there is a light sensor that controls a program's level based on how much light it receives from the LED. The expression pedal in this photo is toe-down/on and the LED is as far away from the sensor as is possible. The less light that the sensor senses, the higher the program level. Digitech XP400
In this photo the expression pedal is heel-down/off and the LED is as close to the sensor as is possible. The more light that the sensor senses, the lower the program level.

To increase program level without moving the expression pedal we need to make the LED dimmer, thus mimicking the effect of the LED moving away from the sensor.
Digitech XP400
The red wire connected to the small board is the power wire for the LED. If we decrease the power that the LED receives the LED will shine dimmer than when it is getting full power. We know already that the less light the sensor senses, the higher the program level.

We need to insert a dimmer control between the red wire and the LED.
Digitech XP400
Unsolder the red wire from the little board. You will likely need to remove some glue-gun glue from around the red wire to be able to pull it out. The solder point that is closest to where the red wire enters the board is the one you want to put your soldering iron on. Digitech XP400
I cut two 10 inch lengths of wire and stripped and tinned the ends of each.

In this photo I've soldered my new green wire to the point where the red power wire was soldered.
Digitech XP400
In this photo I've soldered my new red wire to the end of the now-loose red power wire.

For the sake of clarity, I did not use any heat-shrink tubing to insulate the connection of the red wires. The connection should of course be insulated.
Digitech XP400
Here is a hole ready-made to pass the new green and red wires to outside the pedal. Digitech XP400
Don't forget that the red wire connection needs to be insulated. Digitech XP400
Here are the two new wires exiting the unit from underneath the expression pedal. Digitech XP400
I mounted a 1k Ohm linear taper potentiometer to a piece of wood and wedged it into the top of the expression pedal's underside. The green wire becomes the dimmer's output when you solder it to the outside lug of the pot. The red wire is the dimmer's input when soldered to the center lug of the pot.

I was limited in on-hand choices for my pot so after some trial and error reached the solution of putting 750 Ohms (two 1.5K Ohm resistors wired in parallel) resistance between the red wire and the pot lug. Doubtless there are more efficient values for the pot and resistors but what I ended up with here works and has been good enough for several years so far.
Digitech XP400
That's that for that. Digitech XP400