inevitable SEM clone testing

SEM testing box

Everyone’s done a SEM-clone, haven’t they? Or a SEM-type of thing. There’s a good reason for that, they do sound really nice.

I’ve been plotting this for about five years or so. I was stymied for a while by the designator-less PCB layout in the service manual, but finally got some boards made just before Christmas. I went through the pain of putting it all together in a PCB box just to test, seems like it works quite nicely.

SEM clone board

Maybe too nicely, I need to work up the motivation to make a panel PCB.

I went for multi-turn potentiometers for the presets, 0.1″ headers for the connectors rather than whatever freaky Molex connectors were on the original because I’m cheap, and kept the component spacing and track layout otherwise as far as possible.

I’m using 2N3906 and 2N3904 for the BJTs, and J112s for the FETs just because they seem to be available. The surface-mount adaptors for the LM301s seem to work alright (as long as you actually solder the legs…), and I kept with 741s. I’ve heard that the reissue uses TL062s, for whatever that’s worth.

I’ve bunged the usual CA3046 sub in for the CA3086. I was intending on using a surface-mount version in a DIP adaptor, but then found that the usual adaptor is too wide to fit in the standard socket. In retrospect I should’ve just actually used a surface-mount footprint on the board.

I stuck with the 723 regulator which was probably daft because they’re obsolete, but then I’m probably only going to make a couple of these. I found some probably-way-too-big silver mica caps for the filter capacitors, and some weird massive looking yellow film capacitors on the filter input, mostly chosen because they might be terrible. Maybe in a good way.

Mistakes that I made and discovered so far include swapping the designators for R73 and R75, a couple of transistors (Q7 and Q16) are flipped around the wrong way, and for some reason it seems like I just gave up when it came to the F connector in the middle of the board.

Let’s look at some wires, the horror behind my quick test setup. My wife thinks I eat them or something.

SEM temporary box wiring horror

I’ve deviated from my usual tactic of employing an Oakley PSU run off a Yamaha PA-20 (which works really well), and instead trying out a switching power supply from China off eBay, fronted up by an LM317/LM337 in a vague attempt to filter out any spiky noise. I haven’t put my oscilloscope on the rails, or made any great effort to try and reduce any noise, but the SEM seems to be behaving itself so far.

I made some recordings of the SEM-in-a-box, hopefully it sounds a bit like it should. It’s the usual MC-4-driven frenzied sequencer nonsense.

Low-pass!

High pass!

Band pass!

Also I did a little recording with it along with the 606/Machinedrum for drums and System 100 for bass, and then thought it was boring so I started overdubbing a few more passes by hand. Like I say, this is just a temporary box for testing, it’s not going to have the Oberheim logo on it or anything when it’s done.

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