SEM – wow…

Recently had a pair of rack mounted Oberheim SEMs to look at (SEM = Synthesiser Expander Module) which came to me a bit unwell. Main fault was that the filter on one of them had turned itself into a volume control.

Anyway powered up faulty module to check if anything else was wrong and was struck by how rich the oscillators were. Dammit this synth sounds really good EVEN WHEN IT IS BROKEN! As I had already studied the circuit diagram and had ordered some spare parts based on a description of the fault from the client it took less than an hour to fix. This was aided by the very neat design that puts pots and switches on one PCB and the electronics on a second PCB that plugs into the first via a number of multiway connectors. A couple of minutes with an oscilloscope confirmed that the fault was as expected, powered down and replaced prime suspect number one (a 3080 OTA in the filter) and bingo, up and running.

What a filter. On paper it should be weedy, only 12dB / octave but coupled with the previously described oscillators it sounded awesome. Reminded me a bit of the power of a set of Moog Taurus pedals.

Neat part of the SEM is the pot that allows you to go from Low pass to High pass filter vis Band pass. There is also a switch on the same pot that allows you to select Notch filter as well.

So the sad bit is that I passed on a beautiful example of an Oberheim 2 voice at a reasonable price which puts two SEMs and a splendid little Minisequencer all into a very neat package along with a 3 octave keyboard.

Oh well one for the shopping list…

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