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.


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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poly61m deathsmell fix

I’d been working on a track and moved a bunch of stuff around, including my battered Poly61M. After recording I turned everything off for the night. And mistakenly left the Korg on.

I woke up at about 3am thinking “what’s that godawful smell?”

Dead Poly61m safety capacitor on PSU board

Yeah, that looks wrong doesn’t it?

Mmmm, 30-plus years of dust. In the middle we have a bulging Rifa 33nF paper capacitor. It doesn’t smell very nice.

The synth still works, but worryingly the power switch doesn’t actually turn it off any more, so the capacitor has failed short.

I found the recommended replacement according to the Rifa datasheet – here’s the bottom of the old burned-up one on the left and the new one on the right.

Poly61M burned capacitor

…and now it works fine again: boring fix, sorry everyone.

Here are some random photos of the inside. I had to take the keyboard out to get the power supply board out, gahhh

Poly61M PSU board

Here’s the main voice board, socketed SSM2056 and all

Poly61M main board

And this is probably the main processor board – there’s another board wonkily attached for MIDI on the left which I cleverly didn’t get a photo of.

Poly61M processor board

Yes, the 80s:

Poly61M hot pink

The electrolytic capacitors in the power supply could probably do with replacing – as could the tactile switches on the front panel – but that’s a job for another day.

Oh and the keyboard barely works as well, which is standard with these. I’ve been in it before to try and fix it but they always die again, it’s one of those inherently shit designs. Good thing it’s the M version and I can control it externally.

I managed to threaten a few keys into action by banging seven bells out of it – here’s me absent-mindedly stepping through some presets and CLICK-CLICK-CLICKING the half-dead buttons.

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spi4teensy3 and 74hct595 shift registers

Look at this godawful mess. Look at it.

It’s a few surface-mount 74HCT595s chained from a Teensy 3.6, and it took me too long to work out why it wasn’t working.

More to remind myself as much as anything – don’t declare the clock and data pins as output pins. Like this:

void setup() {
  pinMode(10, OUTPUT); // attached to pin 12 of the 595
  pinMode(13, OUTPUT);
  pinMode(11, OUTPUT);

Nooooo. ‘Cos then it doesn’t work.

This’ll do the trick – Teensy pin 10 is acting as our slave select pin in this case:

void setup() {
  pinMode(10, OUTPUT); // attached to pin 12 of the 595

And then the lights come on, kaboom.

Here’s some panicky twiddling of a BCR2000 talking to a DIY monosynth through the above mess (plus a bonus DAC). Pitch and gate currently isn’t MIDI’d up but I’ll get there.

The synth bit sounds way better since I sorted the sawtooth waveform out and tracked down the resistor that was out by a factor of 100 in the filter… everything sounded like RAHHHHHHHMAXVOLUMERAHHHH all the time until I chopped that out. And not in a good way.

With lots of parameters being voltage-controlled, maybe it could have presets and a way to edit those sounds and extra software-generated LFOs and envelopes and MIDI program changes and a pony. It’ll be fun working out how on earth to actually do that.

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