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Bent Fishbowl

MXR M-151 Doubleshot schematic tracing

Based on the Dime

Category
Distortion
Price
see vendor
Tags
distortion, circuit tracing, derivative circuits, original circuits
Read the post at Bent Fishbowl Schematic image (CC BY-NC-SA)

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

Schematic

The schematic is in the post. Open it at Bent Fishbowl. No KiCad fragment has been drawn for this circuit yet.

Parts list 113 rows

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RefValueNormalizedTypeNotes
Resistors 80
R11.8M1.8Mfrom schematic
R22.7M2.7Mfrom schematic
R45.9M5.9Mfrom schematic
R510k10kfrom schematic
R6100k100kfrom schematic
R74.7k4.7kfrom schematic
R810k10kfrom schematic
R101k1kfrom schematic
R121.8k1.8kfrom schematic
R13470k470kfrom schematic
R1410k10kfrom schematic
R153.3k3.3kfrom schematic
R1647k47kfrom schematic
R171M1Mfrom schematic
R181M1Mfrom schematic
R19470k470kfrom schematic
R2011k11kfrom schematic
R2411k11kfrom schematic
R2539k39kfrom schematic
R261k1kfrom schematic
R2739k39kfrom schematic
R2811k11kfrom schematic
R292.2k2.2kfrom schematic
R3020k20kfrom schematic
R3111k11kfrom schematic
R3211k11kfrom schematic
R336.8k6.8kfrom schematic
R344.7k4.7kfrom schematic
R3511k11kfrom schematic
R3611k11kfrom schematic
R376.8k6.8kfrom schematic
R384.7k4.7kfrom schematic
R3911k11kfrom schematic
R40141k141kfrom schematic
R43100100from schematic
R4413k13kfrom schematic
R452.2k2.2kfrom schematic
R4611k11kfrom schematic
R471M1Mfrom schematic
R4811k11kfrom schematic
R4920k20kfrom schematic
R5011k11kfrom schematic
R5111k11kfrom schematic
R52680680from schematic
R5311k11kfrom schematic
R5411k11kfrom schematic
R5512k12kfrom schematic
R5622k22kfrom schematic
R57910910from schematic
R5822k22kfrom schematic
R593.3k3.3kfrom schematic
R604.3k4.3kfrom schematic
R6111k11kfrom schematic
R6211k11kfrom schematic
R6318k18kfrom schematic
R641010from schematic
R65100100from schematic
R66270270from schematic
R67270270from schematic
R681k1kfrom schematic
R69470k470kfrom schematic
R704.7k4.7kfrom schematic
R7110k10kfrom schematic
R721010from schematic
R731010from schematic
R74100k100kfrom schematic
R77470k470kfrom schematic
R78470k470kfrom schematic
R79470k470kfrom schematic
R80470k470kfrom schematic
R8291k91kfrom schematic
R83470k470kfrom schematic
R84470k470kfrom schematic
R85470k470kfrom schematic
R86330k330kfrom schematic
R87330k330kfrom schematic
R88470k470kfrom schematic
R89110110from schematic
R9047k47kfrom schematic
R914.7k4.7kfrom schematic
Capacitors 18
C122n22nfrom schematic
C34.7n4.7nfrom schematic
C6100n100nfrom schematic
C74.7n4.7nfrom schematic
C922u22ufrom schematic
C1022u22ufrom schematic
C12100n100nfrom schematic
C1310n10nfrom schematic
C144.7u4.7ufrom schematic
C15220n220nfrom schematic
C21470p470pfrom schematic
C2310n10nfrom schematic
C277n7nfrom schematic
C294.7u4.7ufrom schematic
C304.7u4.7ufrom schematic
C324.7u4.7ufrom schematic
C394.7u4.7ufrom schematic
C4422u22ufrom schematic
Diodes 7
D4BAS28BAS28from schematic
D5BAS28BAS28from schematic
D6BAS28BAS28from schematic
D8BAS28BAS28from schematic
D9BAS28BAS28from schematic
D13BAS28BAS28from schematic
D18BAS28BAS28from schematic
ICs 7
U1MC33179MC33179from schematic
U2MC33179MC33179from schematic
U3MC33179MC33179from schematic
U4MC33178MC33178from schematic
U5MC33178MC33178from schematic
U6MC33178MC33178from schematic
U8MC33178MC33178from schematic
Potentiometers 1
Pkk10KAA10kPotentiometerfrom schematic