Oct 8

Written by:
10/8/2008 8:01 PM 

I've just finished the repair of a Sequential Circuits Pro-One that have been in my workshop for some time. The main problem was that the modulation of oscillator A couldn't be altered by the modweel and that the output of synthesizer distored badly even at moderate levels. The problem with the modweel turned out to be a broken slide-switch in the modulation section of the synthesizer. The switch was probably broken by someone who previously had tried to replace it. Some of the PCB tracks had also been damaged in a way that several functions of the modulation section didn't work. A got some new slide-switches from Vintage Planet and managed to fix the problem rather easily.

The problem of the distorted output bugged me for a while but it eventually turned out to be an opamp that caused the distortion. The strange thing was that the opamp didn't distort at low input levels which first led me to believe that it wasn't broken. Analyzing the circuit around the opamp finally made me replace the opamp which caused the problem to disappear.

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