Ok. Finally had a chance to pop the lid off this thing and get some probes on it.
1: The keyboard circuit works.
2: The oscillators appear to not work.
3: This could be because I think something flaked out on the power supply while I was trying to get at the oscillator boards. Some of the voltages are wrong now.
I’ve decided to pull the whole thing apart, clean it up and plug-ify each module – it’s way too hard to get at anything, and there’s cruft everywhere.
I’ve been a bit too busy to open the ETI up and keep working on it, but I’ve already started on some plans for the front-end.
The keyboard controller in this thing is primitive digital: shaky, slow (~20ms delay), complicated and probably totally broken. I’m planning on replacing it with a little Arduino rig – I found one in stock at gorillabuilderz.
It runs a 32-bit ARM chip, which is ridiculous overkill for what I want – mainly went with this one because it has lots of digital inputs (enough to just wire up every key on the keyboard) and a couple of 12-bit analog outputs (will use one for CV, maybe the other for an extra LFO?). The nice thing about this is it means I can do other fun stuff later, like build in a chip-tune style arpeggiator, or maybe a MIDI input. It also means I can remove the most complicated board on there – the keyboard controller takes up a lot of space and it’s covered in scary unavailable chips.
The actual keyboard mechanism is really basic: each key activates a switch. At the moment, it’s hooked up to a resistor network: 12 lines for note, 4 lines for octave, which means it can get into a really weird state if you hit multiple keys at once.
After a bit of reading, sorting this out looks dead simple: I need to rewire it so that when a key is down, it pulls a line to ground. That’ll let me rip out a huge amount of horrible wiring, and I doubt the Arduino will be any more delicate than the original digital circuit.
The other problem is that the voltages I need to generate are a bit weird. Trigger is -7V low, +7V high (!?), CV is 0-5V. The outputs I have will probably max out about 3.3V. So there’s probably going to be a little bit of buffer and amp circuitry in there as well.
So, I haven’t written anything here in a couple of years.
I was going to write about how the new go/no-go album is finally finished and will be coming out in early 2013, but I’m way too distracted by this bizarre thing:
It’s an ETI 3600 – a 3 oscillator kit synth released in Australia in the mid 1970’s. Maplin released a synth based on the same circuits in the UK in the late 1970’s (e.g Maplin 3800 – when they’re working, they sound amazing).
This particular one isn’t in working order at the moment – but there’s a ton of resources around (including high-quality schematics), particularly at www.eti4600synthesiser.org.uk, so I’m hopeful I can get this one nursed back to health.
It doesn’t really make any proper noise yet. I’ve verified that the power supply works, and the reverb unit and the last stage of the amp works (I can get signal out of it when I thump the reverb tank) but I can’t really narrow down the fault any further without an oscilloscope… which I’m in the process of tracking down. I have a few hunches as to what it might be, though.
This one is also kinda weird in that it looks like there’s a bunch of stuff has been added by a previous owner. Also, it needs a scrub. I’m going to try and get some real sound out of it first, and then probably restore it back to stock.