Great Lakes Modular
158D Dual Sine-Saw Generator Module

I was sent a Great Lakes Modular 158D for repairs.  There is a BOM, parts reference, and PCB modifications document on the site.


The available 158 schematics are quite poor.  I cleaned this one up as much as I could.

Buchla 158 schematic


The PCB is a simple singled-sided and mirrored construction.


There are a few cuts and jumpers to add.  I found two additional jumpers needed in addition to the mod documentation.  I also added two jumpers at the bottom for the trimmers.


The BOM lists a MPF102 as a replacement for the U147 jFET, but the U147 is a P-Channel device and the MPF102 is a N-Channel device.  I had some 2N2608 P-Channel devices so I substituted them.  There was also significant ringing on the saw output and I decreased the value of C11 to correct this.



Trimmer R17 adjusts the lower voltage level for the ramp and this adjusts the symmetry of the triangle that is used to generate the sine wave.  Adjust R17 until you get the best looking sine.  These waveshapes are pretty good.  I could adjust the bump out but then the symmetry wasn't as good.


This scope image shows the significant ringing on the rising edge of the saw.  I changed the peaking capacitor C11 from 47 nF to 1.5 nF for a nicely formed saw.


This scope image shows the lowest frequency at just below 5Hz.  Note also the elimination of ringing on the rising edge of the saw.


This scope image shows the waveshape just above 7KHz.  The rising edge of the saw is a bit rounded at the high end.  I was able to calibrate the highest frequency to just over 12 KHz.