I was never able to find one of those sequencers that I could afford (a friend had one, and I thought that it was pretty cool) but more recently I started to think about turning that blank panel into a nice patch bay for the unit, making it easier to interface it with some of my other synths, especially the modular system.
It already had a few patch points on the back panel for oscillator CV in and a gate, but one of the truly beautiful things about the SEM is that there are a large number of patch points inside the unit that can be accessed by connecting wires and a Molex connector the the appropriate spot. It's easy now to find a copy of the schematic on the web, and a quick look will show the large number of useful spots that you can take advantage of, in essence turning this already useful module into a mini modular system of sorts. The voltage levels available will likely play very nicely with other components in your setup, and give you lots of cross-patching capabilities.
Although I've seen other patchbays of this sort that employ the smaller 3.5mm jacks for interfacing, my requirements are strictly old school, and I decided to use typical 1/4" jacks for my project. This means that there would be fewer total jacks in the space, but it would make for easier connections to my gear. I found a source for TRS Neutrik jacks with both connections normalled at a very good price, so I purchased a supply of these for my project. This type of jack configuration would give me the most flexibility for any type of connection that might be needed, including normalled inserts, if desired.
Using the front panel washer for these jacks, I started trying different physical layouts for the panel, trying to get the desired connections with most flexibilty without the panel being too crowded.
I had already made a list of what connections I wanted brought out to the panel, and used that as a guide to start with so I wouldn't miss any important ones. I also started to think about what connections should be normalled, so bringing in one oscillator CV would drive all the oscillators on both SEMs, unless I wanted to patch in at other points to separate things. This same idea was also applied to other CVs for the filter and the external CV busses, as well as the gate signals. The final connection of the normalled chain was on the back panel, so I could connect cables here that could be out of sight in my system to reduce the front panel clutter.
Once I found the layout that I was happiest with, I covered the blank panel with masking tape and carefully marked the drill locations for all the holes. When drilling the panel on a drill press, I started with a small drill bit to allow me to more accurately locate the desired position, then cycled through several progressively larger sizes until I reached the 3/8" hole required for mounting the jacks.
I removed the masking tape, cleaned the panel, and tentatively installed the jacks so I could apply the lettering for the various patch points.
For this part of the project, I used an old Brother labelmaker, and printed labels using black letters on clear tape.
Using a pair of sharp scissors, I cut away all the excess tape from around the borders of the actual lettering, which made the finished project look much more like it had actually been silkscreened instead of done by labels. Using an X-acto knife to hold the edge of the labels allowed me to more precisely poistion each one before I applied pressure to bond them to the panel.
The finished panel looked pretty good, so the next step was creating the wiring harness complete with Molex connectors.
I set the case and panel on my bench so I could estimate how long each individual Molex cable had to be, and put a temporary tape label on each one with the name of the connector it would eventually mate with.
This allowed me to create a less cluttered wiring harness that would fit more easily inside once the system was assembled. Once everything was cut and measured, the Molex connectors were attached to one end, and the appropriate jacks were wired to the other. The normalled connections for the jacks were made at this time, and a separate small harness and connectors was created to make the necessary connections to the back panel.
The little mixer panel was originally set up to provide a mono output, but it seemed like it would be really useful to have separate outputs for each SEM module. I added a bit of wiring and a couple of extra jacks to allow for individual outputs which really opened up a lot more possibilities.
Everything was connected, the jacks were all tested for proper operation, the tape markers were removed, and the unit was finally completely assembled and retested.
The final unit worked just like I had hoped, and will be used for several planned projects linking the Oberheim, Moog 55, and my Prophet 5 keyboard together as a system.
A future modification will add an internal noise source and a sample and hold module for each SEM...I added these a few years ago to a standalone single SEM I have, and really liked the added flexibility.
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