Roland Juno 6 MIDI Retrofit
A couple of months ago, I ordered the Universal MIDI Retrofit from Mungo Enterprises, planning to install it into my Roland Juno 6. John shipped it out the next business day, and it arrived within about a week or so, if I remember correctly. I had a few other projects in progress at the time, so it sat around for a few weeks until I had a chance to build it.
The kit consists of two PCBs and a preprogrammed Atmel microcontroller – you supply the rest of the components yourself. Since I already had most of the remaining supplies on hand, this worked out well. At $50 AUD (which if ended up as about $46 CAD including shipping), this is much cheaper than Kenton’s Juno 6 MIDI retrofit at $490 CAD or so. Even once you factor the rest of the parts ($30) and the assembly time into the equation, I feel that the kit from Mungo is a much better value. Kenton’s solution does have more features – filter cutoff, volume, sustain, etc. – but for me it really came down to dollars and cents. All I really wanted was basic MIDI control of the my Juno 6 so that I could play notes on it with an external sequencer, and that’s what the Mungo kit delivers.
Most of the assembly was rather straightforward, provided that one is familiar with soldering and general DIY electronics work. I ran into a few snags along the way, which I will share with you here.
Snag 1: many 8-pin headers are too big for the holes. This is a known issue with the boards, in that the headers used for the original prototype fit into a smaller diameter hole than those in the parts list. A few light twists of a scalpel is enough to enlarge each hole, though. If you do scrape off the pads on any of the holes, you can still solder to the adjacent traces easily enough if you’re careful.
Snag 2: The wires connecting the two 8-pin headers from the keyboard to the Juno’s voice board are misleading. Roland used the proper rainbow colouring of wires, running from brown through grey for pins 1-8. The trick is that this is pins 1-8 of the plug (you can see a small 1 on it) – but when you look at the “1..8″ and “9..16″ markings on the voice board, the brown wire (pin 1 of the plug) is actually pin 8 on the voice board, and the grey wire (pin 8 of the plug) is pin 1 on the voice board.
I mistakenly assumed that pin 1 of the voice board would be pin 1 on the retrofit’s Board B – this is not the case. The trick is to connect up pin 1 of each plug to the square pad on the board B layout diagram. Then have your jumper wires match up directly to the voice board of the Juno. The picture gallery for this project explains this much better. In particular, look at the final installation picture. Fortunately, John was both helpful and prompt with his email responses to my inquiries, and this issue was sorted out very quickly. I really appreciated the time that he took to answer my questions so that we could solve the problem.
Snag 3: While I was debugging my original “why isn’t it triggering notes over MIDI?” problem, John suggested that I switch both sets of cables. The Juno then played notes via MIDI, except that high MIDI notes played low on the keyboard, and low MIDI notes played high! This was what pointed out that I had mis-wired the headers on board B for Snag 2. Once I had solved that and hooked everything back up – which involved separating boards A & B, swapping both 8-wire cables and reversing the headers, and putting it all back together – I still wasn’t getting any sound over MIDI.
Following the “something must be backwards” school of thought, reversing the cables again as per John’s suggestion fixed everything up – and it’s now working great. This runs contrary to Step 3 of the installation notes, which suggests that “the connector on the retrofit closest to the diodes connects to the 10-17 pins.” On my Juno 6, the connector on the retrofit closest to the diodes connects to the 1-8 pins.
Snag 4: This one isn’t really a snag, just something to make your life a bit easier when connecting up the arpeggiator clock switch. The Juno 6 has a 3-pin header (brown, red, orange) that carries arp signals to/from the plugs on the back of the synth. The far-left wire (orange) is connected to the hold circuit. The far-right wire (brown) is connected to the tip of the Arpeggio Clock In jack. The middle wire (red) comes from the arpeggiator circuit. When no plug is inserted, the signal is connected to the brown wire; when a plug is inserted, the red wire is disconnected. To ensure that the MIDI clock is also disconnected when a plug is inserted into the jack, the switch that you install will need to switch between that red wire (coming from the panel PCB of the Juno) and the clock signal from the retrofit. The selected signal from the switch should be connected to the middle pin of that plug. Edit: You know, I’m not sure if I actually tested to see if this is working correctly.
This did take me probably about 6 hours or so, spread across a few weekends, but overall this was quite an easy to build and easy to install kit. It’s a fantastic value, and I recommend it for anyone who is confident with their soldering abilities, and who wants to add MIDI capabilities to their old Juno 6.