This is a really fun synth! EML was a lesser-known American synth company competing with Moog and ARP during the 1970s and made some unique instruments. The EML-101 is a semi-modular duophonic synth similar in structure to the ARP 2600 and very unique and powerful. The bulk of our work on this involved designing and installing a circuit that allows the synth to accept standard 1 volt per octave-scaled CV via its “SEQ” input, as its oscillators have an unusual scale of 1.4 V/octave with a 4.6-volt DC offset.
We made the above video to show off our mod and the cool capabilities of the synth in general. In a lot of the clips, I’m using a Korg SQ-1 sequencer to sequence one of the of the voices.
I also added a switch that bypasses the scaling circuit so it can still be used as it was originally designed. The circuit is a pretty simple op-amp based circuit to convert the scale of the input voltage and also add the offset. The 4.6 volt CV offset must be added to shift the oscillator into audible range, because the oscillators can all function as LFOs and if you were to just send 1 or 2 volts in, you would get an LFO range oscillation. The 4 oscillators and the fact that they can be turned into LFOs are pretty unique and allow you to get some interesting effects. You can also use all of them to cross-modulate each other, including at audio frequencies for FM effects.
One thing about these synthesizers is they use sealed potentiometers. This means they can’t be cleaned and if any of them start to act strangely, it’s best just to replace them since their insides are inaccessible. For example, when we got the synth, Oscillator 3’s pitch would dip dramatically every time the tuning pot was turned – a problem that was caused by the pot being dirty and after it was replaced the oscillator’s seemingly bad behavior went away.
Working on this synth was so much fun, it really made me want to get one for myself!
Work done: replaced power supply capacitors, replaced 4 faulty potentiometers, cleaned key contacts, replaced failed Darlington transistors in oscillators, added 1/v cv scaling circuit to SEQ input, calibrated to factory specs