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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SH-5 helper box

SH-5 with helper box in the near darkness

Instead of putting the Juno-6 DCB/MIDI converter thing in a box, trying to fix the envelope and chorus noises in the Jupiter-4 clone, or actually trying to make music, I made this helper box for my SH-5, which doesn’t get as much love as it should.

Basically there’s a switch on the front to allow me to flip between playing the synth from the keyboard or the CV/gate inputs, which saves all the faffing about around the back. Also there’s an attenuator for the VCF mod CV input, and as a bonus prize, the switchable portamento nicked off the MC-202.

Here’s a demo! Only I don’t do very much demo-ing of the actual box, oh well.

Bass is System 100, strings bit is JX3P, drums are TR-606 (and I need to start using something else, because I think I’ve worn the 606 out for now). The MC-4 is varying the VCF CV a bit, and I’m switching the portamento on and off using the MPX output on the channel.

Here’s the portamento stripboard:

SH-5 box portamento stripboard

I’d publish the layout but it’s all wrong, the Kicad symbol for the LM317 that I happened to pick had a couple of the pins swapped – luckily I noticed before I soldered it up. In the end the simplest thing to do was to insulate one leg and just bend them round into place.

It’s run off 12V DC, regulated down to 9V by the mini LM317, so it won’t cover the full 10.67V of the MC-4 but pfft, I can live without that octave. The LM358 won’t quite go down to 0V either, but the SH-5 goes low enough anyway from the octave switches, it shouldn’t be a problem.

Here’s the mess inside: just ram it all in, it’ll be fine:

SH-5 box innards

…although I did insulate the base and the back of the circuit board.

I cut two holes in the front by mistake, so I bunged a power light on there (always forget to do those usually, they’re handy) and just a LED lens to cover up the other one. I wished I’d included a LED driver to show when gate or MPX was on, but there wasn’t a lot of space. I got lucky that the board fitted vertically in the pedal box.

Speaking of which, I love these Hammond pedal enclosures. Even though they’re a bit boring looking. Maybe I should’ve bought one of the bright orange ones instead.

Anyway it’s good to integrate the SH-5 a bit more. Also I could do with some sort of mega-switch-box as well so I could flip between triggering synths from the MC-4 without having to unplug things. One for another day. Probably I should actually try and make some music as well.

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Juno-6 DCB retrofit

Onwards with the questionable Juno-6 modification – what I thought would be some Christmas fun turned into January work, and then February annoyance.

It works now, although cleverly I’d swapped the +V and ground on the 74LS14, so it needed a bit of a bodge with a track cut and some dodgy wiring.

I couldn’t get the 4MHz clock to start with the substitute I’d perhaps naïvely used, a 7402. In the end I wimped out and bought a Toshiba TC40H002 (with a date code of 1988!) and it started up.

Once I’d got the board populated and the clock running , the next thing was to see if I got any output on the connector to the Juno. I used a program from this thread on the Arduino forums as a starter-for-ten – and it seemed like I was getting some activity on the socket, and it was within safe limits, so I felt like it was safe to connect it to the Juno connector, which is direct into the Juno’s CPU.

In the meantime, I’d heard a thing about there being a hole for a connector in the back of the Juno behind the serial number plate – and so there was:

Warning! Risk of electric chocolate

It’s a rectangular hole, looks like it was meant for a DCB-shaped connector – maybe a never-released upgrade? If you look to the left of the warning plate, you can see a couple of holes that are exactly sized for the serial number plate to move to. It all seems planned by Roland, but I’ve never seen any mention of an upgrade in literature of the time.

The DCB connector is an Amphenol DDK (D-shaped) connector, and would likely have been a pain to buy and fit. Also I found out that Kenton’s pre-made one-way DCB connector wire is £44, which put me off the whole thing. I was interested to see if a DIN socket would fit, let’s try it…

Checking that a DIN socket will fit in the DCB cutout in the Juno back panel

…success – so some cutting up metal in the garage led to this, which I’m strangely proud of:

DIN socket fitted to the back of the Juno-6 in a metal plate

It’s a little bit wonky but it’s probably the best bit of metalwork I’ve ever done. (You should see my previous crimes). The edges could’ve done with a bit more filing, but it’s near enough.

Here’s my forest-of-wires test set-up, with an old spare Arduino Duemilanove as the controller at top left – the intention is that this will live in an external box. I wanted to leave the connector on the outside of the Juno as (electrically) DCB in case I ever come across a non-stupidly-expensive MSQ-700, although chances seems slim now.

Juno-6 MIDI-to-DCB test set-up

The code from the Arduino forum sort-of worked, but I was getting some weird triggering, where the next key actually played the last note pressed. Eventually I realised that the end code (&FF) did need to be sent, and then everything fell into place.

Well, almost. I found that when the DCB board was powered off the Arduino board, everything was fine, but then when run off the 5V connector on the Juno board it wouldn’t respond to MIDI input.

I did some head-scratching, and didn’t get very far until I had a look at the DCB out on the MSQ-700:

MSQ-700 schematic DCB out

…and it turns out they’ve used a bunch of 4.5nF capacitors to ground on each output line, so I replicated this on the lines from the Arduino to my DCB connector on my breadboard, and then everything was alright.

Initially I had it going with MIDI from my MPC1000, but really I want to use it from the MC-4, so I altered the code to run off CV/gate.

Here’s a demo of it playing a fairly random tune. The first couple of bars are just one voice of the Juno, and then when the bassline comes in (on the 100M clone) there are additional wonky harmony notes played from CV2, triggered when MPX goes high.

Anyway, you get the idea.

I need to put the Arduino and supporting circuit in a box, with a switch for MIDI or CV/gate operation, and possibly a switch for mono operation to let it go as fast as possible, and also perhaps for passing CV2 through to the filter CV input.

I could do with cleaning up the code so I can bung it on here, it seems fairly reliable, but I’d like to check. I realised when playing the Juno through the MC-4 from the System 100 keyboard (which was a bit weird), that if you play legato it doesn’t pick up the change in pitch, so that needs tweaking as well.

In terms of timing, from looking at audio output only (alright, not very scientific) I’ve managed to get it down to triggering between a half and one millisecond later than a note from the 100M clone, which seems acceptable enough.

I was thinking of how that might compare to the OP-8, but I can’t find any specs or service manual for it. There’s some suggestion it uses the same Intel 8048 microprocessor as the CSQ-100 and 600, which runs at a terrifying 365kHz, give or take 10kHz. Not sure it’ll be too sharp, in that case.

Actually, thinking about it I’ve got a couple of CSQ-100s, although only one (sort of) works, and the other is dead, and bit me severely once when I tried to poke about inside with the power on.

It’ll be nice to be able to make a bit more use of the Juno – over the years I’ve mostly used it for drones, triggered it off the arpeggiator clock input, or – horror of horrors, actually had to physically play it. Shudder.

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