Home > SDIY, Synths > SEM Modifications – VCO Section

SEM Modifications – VCO Section

SEM VCO PCB
SEM VCO Section – Top of PCB

A quick look at my proposed panel should make you realize that there are more connections than the SEM accomidates on the molex connectors and features available than are not  on the SEM.  There are a total of 3 frequency CV inputs on the panel and only two on the SEM (not counting the EXT input) and there is no pulse width modulation input on the SEM.  The addition of sine and triangle waveform outputs will be handled in a later post, this post concentrates on how and where to add the additional cv inputs – the frequency and pw mod.

The SEM has provision for two frequency mod inputs and an input that routes to the EXT of both VCO1 and VCO2 on its molex connectors.  The frequency mod inputs for VCO1 are B1 and B2 with the VCO1 EXT source on A1.  For VCO2, the frequency inputs are D1 and D3 with EXT on E1.  A quick look at the SEM VCO schematic(taken form Kevin Lightner’s Synthfool site) clearly shows where the external modulation points enter the VCO.  If you look at the inputs B1,B2 on VCO1 and D1,D3 on VCO2, you see that they just run through a 100k resistor summing node before getting mixed together the 741 opamps A2 and A7.  All we need to do to add additional frequency inputs is attach additional 100k resistors in parallel to those already there.  You can add as many as you need – I needed only one additional.  I could have used the EXT input without having to add any more inputs, but I find that I generally want to have the mods active without having to engage the EXT.  In order to maximize the usability of the SEM though, the second CV input on the panel is routed through a pot which has an integrated push pull switch.  When the pot is pushed in the mod runs through my additional input.  When the pot is pulled out, it routes to the EXT mod location on the SEM front panel.  This gives me the best of both worlds.  Make sure you are attaching your additional resistors on the correct part of the PCB so that they are before the 741 input and in parallel with the other 100k input resistors.  Oberheim hand matched the 100k resistors on the SEM but I am not going to bother as I do not need my extra modulation input to track precisely as I intend to use the existing 100k matched set for v/oct inputs and my additional input with an attenuator for LFO modulation.  the original SEM needed to be able to precisely track incoming pitch CV as in a 2 voice there was a sequencer for one of the inputs and in the 4 and 8 voice there was a programmer that needed to have predictable inputs.  If you need this kind of precision on your additional inputs, you will need to hand match your 100k resistors to the ones on the board or just replace all of them with hand matched ones.

Mods all attached and completed for the VCOs

Resistors mid-wire with heat shrink

The pulse width modulation is a similar add-on.  If you follow the schematic to where the PW mod internally to the SEM comes in, you will see another summing opamp (A5 on VCO1,  A10 for VCO2).  This time, use a 47k resistor attached as part of the summing resistor nodes.  You can where all the mods need to attach in the picture.  I’ve shown the top of the board to give you some landmarks.  You can attach to the components on top of the board but I tend to instead make all my modifications to the bottom surface of the PCB for several reasons.  First, it is really easy to melt a nearby wire or worse a near-by component.  The underside of the board simply gives you more room to work.  Secondly, these types of mods are easily reversible, putting your SEM back to its stock factory state.  Having these on the bottom not only makes reversal easy, but also repairs and undoing them in the future if you should choose to do so for some reason.

There are a couple of ways to go about attaching the resistors.  First, locate where you are going to attach the resistors on the bottom of the board.  This is best done with a multimeter using the continuity function (where it beeps when you have a connection).  Put one probe on the top of the board in the locations shown in the picture and find a corresponding solder pad on the bottom.  Once you do this you will need to solder the new resistors to these locations.  I used to make a small loop in each end of the resistor and trim them close.  Then I would solder to the appropraite pad, then attach the wire.  I have found over the years that any movement in the wires tends to fatigue the resistor lead rapidly and you will end up with it breaking off.  Now, i still form the loops but instead I attach a short length of wire to one and and then solder it to the board.  This simple extra step provides some flexibility and will not break off the way soldering a resistor directly to the board will.  Remember, solder is a physical connection, not a mechanical connection and these sorts of mods are relatively fragile.  After you have made your wires with the appropriate resistors, make sure you use some shrink wrap to cover the resistor.  You don’t want these shorting out on you once you are finished.  I use clear shrink wrap so I can see the values of the components.  I’ve included a close-up picture of a resistor attached to the SEM with the wires attached so you can see.  I’ve never had one of these fail.

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