Home > SDIY, Synths > SEM Modifications – LFO Section

SEM Modifications – LFO Section

Location of LFO Signals

As part of some design decision ages ago, Oberheim opted to only provide a sine wave output for the LFO section even though the triangle and square waves are available in the circuitry. Bringing both of these out is easy and well worth doing. The square wave is a little more work as it is present as a timing element in the circuit and its voltage is well beyond usable ranges as it swings pretty much completely between the +/18.5v power. For this, we need to construct a simple divider. Take a look at the picture of the component side of the SEM where I’ve marked the locations you need to bring out. In both cases, they are on pin 6 of the 741 ICs.

Attached Triangle and Square Wave Output Mods

The triangle is very straight forward, all you need to do is attach a 1k resistor to pin 6. The square requires you to make a small voltage divider between pin 6 and ground to bring it down to standard modular voltage levels. If you are unfamiliar with a voltage divider, you should make sure conversant now. You see these all over schematics and you also will regularly need these in your mods. Wikipedia has a good entry on the topic and I use this handy calculator for determining the correct resistor values. There are many such pages out there. In this case, you need a 1k8 resistor attached to pin 6 of the IC and a 1k resistor attached to ground. You then need to attach a wire to both of these resistors. I started by twisting the resistor leads together with the wire and then soldered and applied heat shrink to the join. I then used wire to connect the two resistors and used heat shrink to make sure they would be no shorts. This is the same method I’ve used throughout on all the breakout mods. It should be clear from the image.

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