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Guidelines to transform Oberheim SEM-1 Modules for external patching.

MOLEX connectors identification

This is a list of the complete I/O connections available on the MOLEX plastic connectors on the back of each SEM-1 Module. (Pin 1 of each Molex connector is the pin at the beveled end of the connector).

Molex Pin number/ Color / Function

A1 (Orange ) Ext.Modulation input VCO1 (*)

A2 Sawtooth output VCO1

A3 Pulse output VCO1

B1 (Red ) CV input #1, VCO1 (*)

B2 Ground

B3 CV input #2, VCO1

C1 (Yellow ) Sync output VCO1

C2 Ground

C3 Sync Input VCO1

D1 (Red ) CV input #1, VCO2 (*)

D2 Ground

D3 CV input #2, VCO2

E1 (Orange) Ext.Mod input VCO2 (*)

E2 Sawtooth output

E3 Pulse output VCO2

F1 (Yellow ) Sync output VCO2

F2 VCA CV input (*)

F3 Sync input VCO2

G1 (Brown )LFO Trigger input (*)

G2 Ground

G3 LFO Output (*)

H1 (Grey) CV input #1, VCF (*)

H2 CV input#2, VCF (*)

H3 Ext. Modulation input VCF (*)

I1 (White) VCA Audio output (*)

I2 Ground

I3 Output Amplifier input

J1 (White) Hight Pass output, VCF

J2 Ground

J3 Bandpass output, VCF

K1 (White) Selected VCF Response output (Notch), VCF

K2 Ground

K3 Lowpass output VCF

L1 (Blue) Ext. Audio input # 1a, VCF (*)

L2 Ground

L3 Ext. Audio input # 1b, VCF

M1 (Blue ) Ext. Audio input #2a, VCF (*)

M2 Ground

M3 Ext. Audio input #2b, VCF

X1 (Green )Output Envelope 1 (*)

X2 Gate input, Envelope 1 (*)

X3 Trigger input, Envelope 1 (*)

Y1 (Green ) Output Envelope 2 (*)

Y2 Gate input, Envelope 2 (*)

Y3 Trigger input, Envelope 2 (*)

Z1 (Black ) +18.5 V Regulated Voltage input

Z2 Ground

Z3 Ground

Z4 -18.5 V Regulated Voltage input

Envelope Generator input Jumpers (near the X and Y connectors)

ENV1, ENV2 Signal input


X>C, Z>H

Gate +



Gate -



Note : all (*) should at least be brought to a patchboard in a minimal configuration. If you have an Oberheim Programmer in your setup, use the alternate pins NOT used by the Programmer.

Calibration procedures

Power suppply adjustments :

Check your power supply : it should be exactly + 18.5 Volts / - 18.5 Volts.(If not, adjust the trimpot near the LM 723 chip for +18.5 volts).

On the SEM board, adjust  the " +15 Volts" trimmer so that the voltage measured at the tip of the big capacitor is exactly +15 Volts.

VCO1 adjustments :

1) Using an oscilloscope, check VCO 1 output (Molex pin A3 ) and adjust INIT PW trimmer for 50% duty cycle pulse wave with the PULSE WIDTH pot set in the center of its range.

2) Set FREQUENCY pot full counter-clockwise and adjust "INIT FREQUENCY" trimmer for 35 Hertz output with NO control voltage input.

3) With a CV input of approximately +3 volts (3 octaves up from the bottom of the keyboard), adjust VCO1  frequency pot for zero beat output (use an external oscillator with an accurate Volt/oct relationship and driven by the same CV-in). Now, change CV in to 0 volt (bottom note of the keyboard and adjust the "VOLT/OCT" trimmer for zero beat (this adjustment can also be done externally  through holes in the patchboard ). If necessary, repeat the procedure to make the two oscillators track perfectly.

4) Now, move FREQUENCY pots on both VCO1 and  external oscillator up several octaves and check for zero beat with 0 volt CV in Finally, apply + 3 volts CV in and check for zero beat by adjusting the "HI FREQ TRACK" trimpot.

5) Repeat  procedure 3) and 4) once.

VCO2 adjustments :

Repeat the above procedure (step 1 through 5 ) using VCO1 rather than an external oscillator.

After all adjustments are completed, set both VCO1 and VCO2 FREQUENCY pots fully counter-clockwise and adjust VCO2 "INIT FREQ" trimmer for zero beat.

VCF adjustments :

1) Using an oscilloscope, check pin Q8 on board -to-board connector (this pin is the nearest to connector I1).

2) Turn "Notch" pot to "HP".

3) Adjust "OFFSET  1" trimmer to zero volt.

4) Turn "Notch" pot to "LP".

5) Adjust "OFFSET 2" trimmer for zero volt.

6) Center VCO1 frequency pot and VCF frequency pot (at 12 o'clock).

7) Apply VCO1 pulse waveform into VCF and rotate resonance (Q) pot fully clockwise.

8) Adjust VCF "INIT FREQ" trimmer until fundamental  (F1) is prominent.

9) Juper CV input to pin H1.

10) Depress key one octave above lowest  keyboard key and adjust VCF "Volt/octave" trimmer for maximum signal.

11) Repeat steps 1 through 5..

VCA adjustments :

Using an oscilloscope, check pin 6 of 741 op amp (nearest Molex connector I).

While repeatedly pressing and releasing Keyboard keys (to generate Gate and/or Trigger pulses), adjust VCA "OFFSET" trimmer for NO voltage change.


This is an example of a Patchboard using banana jacks which blends nicely with the main module layout.

Obviously, the patchboard dimensions will depend on the type of jacks and potemtiometers you are going to use. Hereunder, is another lay-out that you can use : the inputs are on the left and the outputs on the right.(Dimensions 9"x7.5" inches).

Alternative method

If you don't want to go through the hassle of constructing a patchboard, use the space ABOVE each module (long strip of steel frame where the ON/OFF AC Mains switch is located) and drill holes there : use mini-jacks or banana jacks to gain space.

If you never drilled through steel before ,don't do it yourself (the steel frame used by Oberheim was exceptionally thick): have it done by a professional drilling expert and make sure you have a few drill bits in reserve, just in case....


Also, check Guidelines to construct a patchboard for the MS-1A Mini-Sequencer..