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.
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.