Here’s my second go at the potentiometer board for my version of the SEM (previous post here).

I’m fairly happy with it, apart from the fact that my additions have pushed it over the original size a bit, meaning I won’t be able put two together in a 19″ box like the ObieRack.

Extra/different stuff:

• second LFO (hence the wonky LM13700)
• plus/minus one volt octave switches for the VCOs (hence a few references and a quad opamp)
• LED indicators for the LFOs (couple of op-amps)
• LED indicator for the second envelope (which is a bit pointless)
• switches for oscillator shapes and modulation sources (due to the general lack of centre-tapped pots)
• external connections routed one side of the board

There’s loads wrong with my paper and cardboard test front panel (it’s too small, for one), but this is some kind of idea of how it might look. The circles around the pots were just to work out maximum sizes for the pots and spacing while I was arranging it.

The attack pot for the first envelope is something like 1mm too far to the right on the board, aargh. Forget doing the board again though.

Initially I was just going to make two, then I ended up making four of them, because why not.

I mean, Mr Splitradix has five of the things, and Vince has bloody ten of them, so just keeping to four seemed fairly (almost) restrained.

This now means I’ve got to build some sort of panning mixer for the four voices. And also a MIDI interface and maybe some sort of flexible cv/gate assigner thing so I can round-robin play and hold the four voices from one or two channels of the MC-4.

And now I think about it a bit more, it’d be a good idea to have some common VCO pitch and cutoff tuning controls as well. And a common LFO. Let’s cover the world in LFOs.

(I hadn’t really thought this whole “let’s make a load of SEMS” thing through, really.)

I’m definitely not doing common envelope controls though. The envelopes on the boards aren’t voltage controllable as they are; it’s said that the FVS used early versions of the CEM3310 for this, and that just sounds like a massive faff. Even more of a faff.

One thing that I absolutely love about the SEM from a construction point-of-view is that the front panel just plugs into the voice board – here’s a photo of one of them balanced in-between some plants on a windowsill:

OK, so it looks strangely wonky in the photo but everything connects up so easily, and it’s just so compact.

I guess this’ll be no surprise to anyone building Eurorack modules, but after having wired up a System 100 model 102 clone all point-to-point, the relative lack of wiring is glorious.

Fuck wiring! It’s no fun!

There was good discussion on the Analogue Heaven mailing list recently (yeah! mailing lists! from the old days!) about the FVS versus the OB-X, and there was some thought that the OB-X sounded somehow nicer because of the OTA used in the audio summer (and the pre-distortion technique they used to counter-intuitively reduce the distortion on the super dynamic signal), so it’s tempting to try and implement that in the output section.

Continuing with the late-70s Japanese synth recreations that I’ll probably never finish – that thing I was working on back in January has grown into this:

Four voices, “compuphonic”. You know. My version has a sawn-off version of the originals’ motherboard in the vague hope that I might be able to fit it in a 4U (or maybe 3U?) box. Maybe in some alternate universe this actually happened and was released with a Roland model number, maybe MKS-4?

So many problems, some of which have been fixed. The chorus sounds a bit too swooshy. The envelopes leak clock noise into the output. The LFO leaks into the audio path, although that one is at least a known problem with the original.

The oscillators required some resistor-twiddling to get the pulse-width wave square. I fret I might have altered the level of the VCO into the filter slightly, I could do with checking further.

I mean, it can sound quite good. With all four voices in unison and chorus on it’s proper shouty raaaargh full-on “I am a synthesiser”.

We’ll get to the chorus later on but here’s a bit of pulse-width single voice then unison:

Also the LFO randomly goes quite fast on that, quite nice for making a horrible racket.

Pile of (not quite finished) boards!

Initially I made voice boards with single row connectors but they were really wobbly. And the voice board was the wrong way round.

I changed the spacing on the card connectors so they’re on a 0.1″ grid, with the initial thought that I would build the motherboard on stripboard, but eventually decided I was done with such masochism for this one.

Predictably then the module controller board and the voice board don’t quite line up in the same way as the original does. I’d re-do the motherboard and the module controller but… it’s a lot of work. Otherwise the voice board and the module controller should be the same as the original, the track layout should be fairly close.

As IR3109s aren’t falling from the sky (give 2020 a chance, though) I went with the rev D voice board, meaning that I had to make loads of BA662 clones. Praise be to openmusiclabs.

Here’s one of the billions of 662 clones/clowns, missing the two transistors and resistors for the buffer ‘cos they weren’t needed in this case:

Maybe if I get round to doing a Promars I’ll try building it with LM13700s in adaptors, using homemade 662s is an exercise in masochism.

The chorus works and sounds ace but for the none-too subtle swooshing from the LFOs. I bought two sets of MN3007 from eBay – inevitably the cheapest pair made my power supply panic, I guess they were MN3207 or something, re-marked.

Here’s a bit of that – listen out for the (stereo) chorus kicking in second time round the bass loop. Later on (at about 39 seconds) when the filter closes you should be able to hear the zapping on the chorus, and some clock noise from the envelopes.

Poking at the output of the BBD with an oscilloscope suggested that the two outputs are at slightly different levels, so maybe I have to balance them out to cancel the swooshiness. Originally the JP-4 didn’t have anything here for trimming the individual outputs from the BBD, but the System-100M delay/chorus module does.

So much dust, it’s been here for a while:

The original used a pair of MN3005s but I adapted the layout for 3007s using the tactics published by fitzgreyve.

The brain of this mess is a dusty Teensy 3.6, which is massive overkill for such a simple synth, but the inbuilt microSD card is handy for saving and loading patches. And the plentiful I/O will be handy when I come to making a front panel for it (In about five years, at this rate).

The DAC is a DAC8562, which has two 16-bit channels. It’s comically tiny, especially next to the through-hole parts on the weird circuit board I had made.

The output from the DAC for the compuphonic settings and the MIDI-to-CV is 5V, giving five octaves of pitch, which matches the four octave keyboard (plus one octave down via the transpose switch) range on the original, but I’m not sure if it’s a bit limiting when it comes to the keyspan.

It does a few different voice modes as per the original which may or may not work reliably: different flavours of unison, round robin, voice per note held down, and some bonus single voice modes to help me tune the individual voices.

This demo uses the unison mode for the bass drone and the round-robin for the wobbly lead that comes in at about 21 seconds, so the notes (with a long release) smear over each other.

Early on I hadn’t realised how much current this thing was drawing – the wires I was using were too thin and causing the pitch to wander about, so tuning was a regular thing. Beefing up the cables has mostly sorted the drifting pitch, but I still think about incorporating some sort of autotune.

One downside with autotune is that I’d lose a bit of pitch range, so I’d definitely need to amplify the voltage from the DAC. Maybe that’s against the spirit of the wobbly original though.

Portamento is done in software as part of MIDI to CV – the original used a 662 for each voice but I’d had enough of making them by this point.

All the compuphonic settings from the original are MIDI-controllable. How will I get the MC-4 to talk to it? I guess I could do with some sort of assignable CV/gate input thing on the Teensy.

Another sort of demo – I was enjoying playing in Ableton for a change and actually being able to play chords. Loads of reverb on the pad, maybe a bit of chorus on the unison bass bit and a single voice for the squeaky acid bit at the end.

There’s something pleasing about the stepped voltages from the DAC, although it was a struggle to get them right.

It didn’t help that my 90s-era Philips oscilloscope died quite early on into this project, and it turns out now that the same model is a hundred quid or so more expensive on eBay. This picture is from my replacement scope, which is a Tektronix 2445 that I’m not so keen on.

I found I had be quite careful in the sequencing and timing of the DAC and S&H multiplexer to get reliable behaviour, and even then I’m not 100% sure I’ve sorted it. The DAC8562 is posh compared to my usual MCP4922, but it still needs at least 10 microseconds to settle down to a consistent voltage. Boo physics.

I’ve got a screen! It’s in colour! Although I’m not sure that’s a good idea, because I’m having to shift more bits out through the SPI interface than I would otherwise with a monochrome display. Here’s my cheesy splash screen.

There’s nothing much actually happening on the screen yet, but here’s a video to show off the patch loading and how the parameter changes are represented on the screen. When it comes to patch 5 I’m changing multiple parameters through MIDI, so predictably enough the screen goes a bit apeshit. (And the camera is a bit out of focus on the last part, soz about that)

The envelope noise is really obvious on patch one. The voice mode display currently doesn’t get updated when the patch changes so that’s all lies. At least the voice allocation numbering is correct.

The random noise burst when it turns on is alarming, but I quite like it. It reminds me of Robotron starting up, or a robot angrily being woken from its slumber. It does need sorting out though, it’s on the list.

I spent way too long on implementing the arpeggiator and got tied up in how it should respond to MIDI clock, so that’s in a half-broken state.

On the arp front I saw a rumour (…in YouTube comments) that the original didn’t do a real random, but that it was actually based on a bunch of numbers stored in ROM. So if you hold the right keys down, eventually you get the “Rio” arp line, god help you.

Presumably that would be in the key assigner board mask ROM, which unfortunately hasn’t been dumped out as far as I can tell. Not that I’m sure I’d be able to do anything with it if I had it.

Next move is to experiment with temporarily taking the voice outputs through a different route to see if we can make the induced clock noise from the envelopes disappear. I suspect it’ll mean cutting tracks on the motherboard – there’s already a suggested fix for the LFO bleed on the originals mentioned elsewhere that I’ll look at as well.

I’d already tried to balance out the chorus swooshiness with a pot and promptly fried one of the BA662s on the output, so that’s next after the envelope noise. Still a long way to go.

I bought a Midiverb II a while back, and when it arrived, it was dead.

It was advertised as working so I should’ve sent it back, but it was thirty quid and I thought I’m a super–DIY-electronics-pillock who totally knows what he’s doing – I’ll have this fixed before bedtime.

Four years later, and the Midiverb had fallen into a tiny gap between the guitar amp I got when I was fifteen and an Ikea shelving unit, still waiting for me to resurrect it.

Here it is in its dead state:

The dust! You’d think I’d have given it a bit of a wipe before taking a photo.

And inside – the board takes up about three-quarters of the rack.

The big square chip with the sticker on it is the magic Alesis reverb chip, and the chip to the left is the DAC.

The ICs above are for memory, and on the left-hand side there are some old-fashioned silver-topped TL084 quad op-amps, some of which might be used for output.

I’m guessing because there’s no schematic available, which makes it more exciting/fruitless, although the Midiverb III service manual might be similar in some ways. Anyway, for the Midiverb II this page on wolzow.com gives a far better impression of the insides than I ever could.

Here’s a random picture of the power regulation bit – the Alesis external PSU is a 9V AC adaptor, which gets rectified into +12V, -12V and +5V.

I optimistically replaced the capacitors in this section a while back, plugged it in, and when it still didn’t work I thought, oh well, that’s me fresh out of ideas.

Looking at the date codes on the ICs this seems like it was probably a good thing to do anyway.

I rechecked the +5V power supply after reading this post on anlage-e – seems like some of the decoupling capacitors go bad after a while – no shorts, seemed fine.

Recently I plugged it in again and boringly got the same result, all LEDs on, no LED display. Maybe something to do with the microprocessor? I wondered if the reset line on the 80C31 wasn’t going high – measured it on the oscilloscope, it seemed to be blipping high when plugging the MVII in.

Alright, next thing – clock on the 80C31, it should be ticking along at 12MHz, what have we got?

FLAT LINE. (I should’ve taken a photo, although it wouldn’t have been very interesting). At this point I dragged the probe slightly, LEDs flickered, and the front panel 7-segment display burst into life.

Tears of joy.

I worked out that if I flicked the resonator next to the 80C31 hard enough with my finger it would jump-start the clock – restarting the Midiverb would produce the reset signal and get it going.

This wasn’t much of a fix, so I replaced the resonator with a brand new 12MHz crystal.

And it’s been working happily ever since. Here’s a picture of it under my Quadraverb, on the infinitely large patch 29.

Here’s an edit of the second thing I did with it, which might be an actual track eventually. Soz about the quality, it was onto cassette tape…

The main high-ish sound is the System 100 102 expander through patch 29 on the Midiverb, the bonky bass is the 100M clone, drums are a 606, and the pad-ish synth at the end is the Poly-61M. All running off the MC-4.

Anyway, what have I learned from this? The main thing is that digital things aren’t totally unfixable. Also, the lack of a service manual isn’t the end of the world.