Siel Opera 6

~ A Synth Re-Engineered ~

Several years ago I was given a non-working Siel Opera 6 by a very generous guy off the synth-diy mailing list. As befalls many of these particular synths the backup battery had leaked sometime in the past and attacked the CPU board resulting in much damage to tracks and components. Considerable work would be needed to bring this synth back to its original condition.

So I have decided to re-engineer it.

This page documents the various changes and modifications I have, am, or planning to make to this synth. I have made a conscious decision not to modify the analogue voice board - this will be kept as-is.

So, to begin with, here is my Opera 6 right at the start of this project:

(Note the presence of Management)

Cosmetic condition is not too bad, so nothing much needed there. The most egregious feature are the programmer buttons - they will be coloured or replaced with some that are a little more in keeping with the colour scheme.

Front Panel

LEDs

Twenty or so years after this synth was released, we now have blue LEDs. One of this first things to do is replace all of the red LEDs with blue ones. As well as replacing the red LEDs I have also had to change the current limiting resistors from the original 270R up to 33k, otherwise the blue LEDs are just too bright. It is even possible to change the LEDs in the switches - you just have to carefully prise off the switch cap to get to the LED underneath.

Pots

The pots are standard "commercial" 16mm splined-shaft 10k linear pots. Rapid sell a TruOhm part that is compatible, order code 65-0715.

Knobs

The existing knobs looked a bit, well, sad. So I've found a nice replacement, in the form of the MultiComp CR-MS-5 soft-touch knob, Farnell part 144-0013.

The result of these changes looks more in keeping with the grey/blue colour scheme:

Display

I also hope to replace the 7-segment LED displays with blue ones, but it is proving a little tricky to find drop-in replacements. I hope to avoid making up little adaptor boards.

In the meantime I have found some green 7-segment displays in the form of Kingbright SC04-11GWA. They are stocked by Rapid Electronics, order code 57-0442. The color is a better match with blue than the old red ones, and the effect is a brighter display.

Buttons

After much hunting around, and a stroke of luck, I have found the manufacturer of the round push buttons used in the programming section. They are made by Camden Electronics Ltd, type CKS05R4. I have fitted a new set of black buttons, and I think the result looks much cleaner than the original cream-and-red scheme.

In the UK they are stocked by Rapid, part no. 78-2525.

The photo below shows the new green display, new buttons (all black) and the blue LED on:

Pitch Bend Wheel

I have added a return spring to the pitch bend wheel, so now it springs back to the centre.

Keyboard

The Opera 6 has the same keyboard mechanics as the other Italian synth I am familiar with - the Jen SX1000. It was made in Italy by "ITAR".

As expected, the keyboard had seen all sorts of liquids spilt on it (coffee, cola, beer are my guess - stinky after all these years!). So time for a complete strip and clean, with plenty of warm soapy water.

The contacts themselves are mounted on a long and thin (i.e., fragile) PCB. The two bars are held in place by plastic mounting blocks, with the springs soldered to the PCB pads. These looked OK, so only needed a light clean.

The new keyboard scanner board is now complete. It scans the full keyboard at about 1kHz rate, measures both play and release velocity, and also scans the pitchbend and modulation wheels, and the expression and sustain pedal inputs. It sends all this data over an SPI link to the master CPU (yet to be designed).

I am making the schematic, PCB overlay and source available:

• Schematic
• PCB overlay
• Source files (zip archive) REVISED

To build the source you will need the AVR GCC tools installed (binutils + compiler). I use WinAVR. The source code should build in the default directory just by running make. The only wrinkle to be aware of is that you need to set the fuses to use an external crystal oscillator or ceramic resonator in the 3.0-8.0MHz range.

Documentation on the communications protocol used is in the source code.

Errors/Omissions: I think I've fixed all the bugs from the previous version. The version online is now release 1.1.

CPU Logic Board

The hardware is almost complete - just waiting on some transistors to complete the MIDI interface.

Voice Architecture

Thanks to the most excellent and very generous Gino Wong I now have a second analogue voice board. And yes, it will be joining the existing voice board, making this a 12-voice Opera6. That's 24 oscillators. Or two layered voice banks. Or true bitimbral operation. Or one uber-ffaaatt monosynth.

Exciting times indeed.

In the meantime I have sketched out the major blocks of the Opera 6's voice architecture (PDF, 31kB). The orange block represents the global controls on the original CPU board, and the stack of green blocks representing the six voices.

Documents

Manuals / Schematics

• Users Manual (English, 0.5MB)
• Schematics (9MB)

Datasheets

SSM2031

High Frequency Oscillator

SSM2024

Quad Current Controlled Amplifier

SSM2044

4-Pole Voltage Controlled Filter

SSM2056

Voltage Controlled Transient Generator

ROM Dumps

• Download EPROM images

Links

Siel Synthesizers Website

A very useful online resource for all Siel synthesizers. Well worth checking out.

Siel Users Group

A low-traffic users group for these interesting Italian synths.

Suppliers

• Camden Electronics Ltd
• Farnell
• Rapid Electronics