MXR M-151 Doubleshot schematic tracing

From the vendor

MXR M-151 Doubleshot schematic tracing

Apr 2, 2023 4 min read

I had the chance to get my hands on the MXR Doubleshot for a little while, and found out there are no available schematics anywhere. It's nice to do some selfless circuit tracing once in a while.

Starting from what was known already, the Doubleshot is usually described as the dual channel version of the Dime, and we'll see this is accurate. That circuit already has something interesting in its own right, but this one builds upon it with the switching to end up with a true monstrosity.

This wasn't an easy trace and it took quite some time. I can see why some people were scared away by the sheer number of components and the SMT board, but even pulling the board out of the enclosure was a challenge in itself: those familiar with EHX pedals will know what it takes to get out a board when the DC jack is stuck in its hole.

With that done, I could start poking around. Knowing about the similarity with the Dime, the schematic on FSB was a great help in giving me a hint as to what to look for to start drawing the backbone of the circuit.

It was a good thing I had a reason to get the board out, because as soon as I did, I found out the two 10Ω resistors on the supply were both burnt. The (second hand) pedal was somehow still working, but it sounded awful, with half functioning EQ and oscillation at certain settings. Replacing the resistors fixed it completely; it's still not my favorite distortion, but it sounds ok and the EQ is very useful.

So here's the result!

Let's get some of things out of the way:

• Since this is a traced schematic, I cannot guarantee that it's fully accurate. Still, having the Dime as reference, I'm pretty confident in what's involved in the signal path at least;

• The schematic might not be complete. Again, I feel good about the signal path, which is the main interest, but the switching scheme was difficult to trace because of traces and joints hidden under the audio jacks. Hopefully, the flip flops and associated logic represent a working circuit, or at least represent something useful both for repairs and for study (I know it does for me).

• Resistor values are easy to read in SMD, but I wasn't going to desolder each ceramic and film capacitor to get a meaningful measurement. The values in [i]italics[/i] are taken from the Dime schematic, but since the rest (topology, resistor values, up to the arrangement of the op amp sections) is exactly the same, they're pretty good guesses.

• IC and electrolytic capacitor numbers reflect the silkscreen annotations, the rest doesn't.

• Since there are many components and it's hard to keep track of all of them, the method I came up with is that all semiconductor components (IC sections, transistors, diodes) are accounted for, and also electrolytic capacitors, switches and pots. The rest follows those.

CMOS, CMOS, on the wall

The first observation about the circuit is that the designer really liked their analog switch