The MXR Model 142 was manufactured from approximately 1978 to 1984 when MXR was acquired by Dunlop. Production stopped around 1984 prior to the sale to Dunlop. The 142 was never reissued by Dunlop as other classic MXR pedals were. Although it is the same mustard color as the ubiquitous Distortion +, the circuit design is completely different and although they share the “distortion” moniker they sound nothing alike. The 142 was designed to emulate an over driven amp pushing a 4-12 cabinet. The resonance control provides a signal with a 55hz centered hump or pass-band that could be added to the basic over driven sound. 55hz is the resonant frequency of the old Celestion G12-M speakers loaded in many Marshall cabinets.
Although the 142 is well designed most guitar players find it a bit bass heavy or woofy with certain guitar/amp combinations, particularly when using 4-12 cabinet in their rig. I came up with some basic modifications that alleviate the overbearing bass levels and allow the resonance control to be used with higher settings on the drive control. I also convert these pedals from an AC power cord to a DC power input connector. The power conversion allows the pedal to be powered by an external DC power supply of 18 to 24 volts. You can run this pedal on a 9 volt DC supply but the internal 15 volt regulator drops out at 9 VDC so the 9 volt supply should be regulated. Dont expect the pedal to sound the same on a 9 volt supply as it does from an 18VDC supply. BTW the internal voltage regulator (LM7815) requires an minimum of 18 volts to supply a regulated 15 volts to the circuit.
Here are the mods along with the PCB parts layout to help you locate the components..