The MXR Bypass

    MXR has a particular way of making bypass for pedals. It's still "True Bypass", but it doesn't use a 3PDT switch or relays. Instead it uses a DPDT switch and a few p-type JFETs. It is closer to the "Millennium" Bypass" but with a few modifications. Unfortunately, it is not uncommon that one or several JFETs fails and prevent the pedal to function properly.

    So, in this article, we will see how the MXR bypass works, and how to fix it quickly.

The Circuit

  First of all, here's the schematic:

 

 

 

 

 

 

 

 

As you can see, MXR can claim "True-Bypass" pedals, because the dry signal is completely isolated from the effect when the pedal is bypassed.

Secondly, the JFETs are used as switches to pull the source to ground when the pedal is bypassed. This way you can use a DPDT (instead of a 3PDT) to switch the audio signal and control the Bypass LED. Q1 and Q2 pull the effect input and output to ground when the pedal is bypassed, this helps reduce crosstalk noise.

More recently, MXR introduced varicap diodes (ZMV832ATA) between the drain and source of each JFET. Their role is to protect the JFETs from big spikes from the source.

Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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Repair Before MXR introduced those diodes, pedals often died because of a broken J177. In fact, I have a cardboard box full of dead MXR pedals from the Dunlop factory and more than half could be repaired by changing a J177. The symptoms are usually, no signal when the pedal is engaged, sometimes the bypass LED stops working. Unfortunately, for most MXR pedals, J177 are SMD parts (SOT-23) that are a bit harder to replace than the through hole version. You can recognize the J177 by their "6Y" marking (followed by a date marker), they are often placed near the bypass footswitch. If you see a "6Y" on a MXR board you can almost be sure that it is a bypass JFET since J177 are not commonly used in the audio part of pedal circuits. The protection diodes are marked "AD". ﷯ Two J177 and a protection diode ( CustomBadass Modified O.D.) One very quick fix is to simply remove the J177. As you can see in the schematic, the J177 are non-essentials to the pedal sound. You can easily break them with pliers or you can desolder them properly. Howerver, you will lose control of the bypass LED if you remove Q3. If you're not afraid of SMD, the best option is to replace them. It will avoid crosstalk for high gain pedals and the bypass LED will work as new. The SMD version of the J177 are listed as MMBFJ177. Adding a protection diode can be a good idea, if none are already in place. However, the original varicaps are hard to come by and I never tried to replace them by another model. This problem apparently happened a lot in the first runs of the Carbon Copy Analog Delay (my cardboard box is packed with these). The Carbon Copy actually use four J177, one is added for the delay signal return. Here's a picture of where each of the are in case you want to repair a Carbon Copy. ﷯
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