Juno-6 DCB retrofit, part two

Juno-6 DCB circuit board wired up and in-place inside a Juno-6

Anyone with any sense would just buy Tubbutec’s apparently excellent Juno-66 mod and be happy, but just for the freaks I’ve finally fixed my Juno-6 DCB interface layout so it’s now ready for sharing.

Here are the Gerbers, zipped up ready for the likes of JLCPCB:


…and here’s the BOM.

Basically it’s this bit of the Juno-60 schematic broken out into a separate board.

The original Juno-60 DCB interface schematic, excerpted from the service manual

Now it’s the 21st century all this could probably be replaced with some sort of microcontroller rather than having to go through all this faff, but here we are anyway. And this keeps it authentically 1982, if that matters to anyone.

Here’s the layout, featuring my usual drunken wobbly routing:

Juno-6 DIY DCB circuitboard layout in its wonky glory

Note the annoying mixed orientation of the ICs (U1 and U4 pointing downwards, U2 and U2 upwards), yeah, I know.

The connectors on the left are all labelled and they’re in the right order to connect to the pins on the right of the Juno’s main board, which are also all labelled up. DB1 is just labelled “1”, as I wasn’t able to squeeze the rest of the letters into that bit of the PCB.

It’s powered off 5V, and there’s a header on the Juno main board for the gate outputs which includes a ground and 5V line, which is where I ended up taking the power for this board.

Just for completeness, here’s my schematic as a screenshot (and here’s the strangely massive PDF):

Juno-6 DCB interface schematic, drawn out in Kicad by me

Given all that and you’re daft enough to still want to give a go, when I bought my boards from JLCPCB in early April, they came out as £4.89 (including postage to the UK, as an example) for 5 boards on the cheapest settings.

This is probably one of those mini-projects where you have to be very confident about opening up synths and messing around. Juno-6s are now worth a fair amount, and I’d hate anyone to kill theirs as a result of trying this board out.

I would really recommend testing the PCB outside of the Juno first; check to make sure 5V and ground aren’t shorted, and power it up from an external test power supply, and check that the outputs aren’t putting out strange voltage levels.

The original connectors were Amphenol DDK types, and are obsolete as far as I can tell. As I don’t have any original Roland DCB gear to play with (and my urge to buy an MSQ-700 has faded now the prices have gone crazy), I used a DIN connector, which is a special kind of pain in itself – I’ve never liked wiring them up.

Eventually I’ll get round to writing up the CV/gate/MIDI to DCB box as well, here’s the mildly shoddy (but practical!) box for that:

Homemade CV/gate/MIDI to DCB interface box

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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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made a cake

Fairly successful Yorkshire tea loaf, made by me

Something like a Yorkshire tea loaf – about half a kilogram of fruit soaked over-night in tea, one egg, brown sugar, some bicarb, a bit of butter, plain flour, and some brandy that I randomly threw in ‘cos it’s Christmas.

And it turned out alright! Usually everything I make turns out flat, but this one wasn’t too bad. I suspect because it has a vast amount of fruit propping it up.

Apart from that, mostly I’ve been frenziedly drawing circuit boards to order from China to try and beat the UK customs changes come January 1st.

To explain a bit – at the moment there’s an arrangement called Low Value Consignment Relief, which means that any goods imported under the value of £15 don’t attract any VAT charges, which most of my PCB orders squeeze into.

Come January 1st, HMRC are binning this arrangement and have stated that the seller will be responsible for collecting the VAT, so that makes things less fun.

There were loads of things I was going to make (a Kobol clone with 3310s subbing in for the rare SSM2050s! an MS10! the DAC/gate output board for my imagined future MC-4 ripoff that I’ve been going on about for years) but mostly I’ve been churning out boring old clones of 100m boards. Yep, even the overly-complicated looking portamento module.

Other than that, the silliest is probably this board intended for my lovely Juno-6, which is basically a copy of the DCB interface in the Juno-60. Yeah! DCB!

DIY Juno-6 DCB circuit board

At this point you’re understandably wondering why would anyone do this in this day and age, and I’d say, fair enough, it’s a bit daft.

Mostly it’s because I was too cheap to invest in a MIDI interface for this thing, but also because it’s more fun trying to work out how to get it going, even though it’ll be a bit more convoluted.

I’ve long missed the boat on buying an MSQ-700 for super-cheap, and OP-8/OP-8m are super-rare, so I guess I’ll end up writing a MIDI-to-DCB and CV/gate-to-DCB thing for an Arduino.

I’m sure it’d do-able to go from the Arduino to the Juno-6 direct, but I’m slightly nervy about knackering the MPU inside the Juno.

I opened the Juno up before I ordered the board just to see how much room I’ve got. Turns out I could easily fit some sandwiches and a banana in there as well as the board.

Inside my Juno-6, showing that there's plenty of space for an additional circuit board

Loads of room. Probably not going to do the sandwich-box mod this time round.

Also there’s a suspiciously DCB-port-shaped hole covered by a plate at the back that will hopefully mean I can avoid any drilling, because you know that’s a bad idea. Potential Christmas fun to be had anyway.

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