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 lead 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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