[sdiy] Understanding 80s Synth Architectures

[Benjamin Tremblay]

Honestly it’s tempting to buy a junk old synth and see if I can bring it back to life. But I’m not a craftsman and I can’t fix blemishes, and sourcing old parts is expensive. 

Interesting, yes! Do I want to recreate the old 8 bit computer designs? No.
My Six Track was lovely, but it was easy to overload its buffer, and the chunky quantization on the parameters was frustrating. 
I would much rather attach a bank of AS3394 chips to a multiplexer array connected to a modern 32 bit MCU than try to reproduce the weird old memory-mapped architecture running at 1MHz. 

I spent some time trying to understand chiptune and old video game sound chips. I came to the conclusion that anything cool in there could be reproduced in my Teensy (Just a bunch of square waves, and PWM/phase tricks only an assembly language coder would care about). The best part of the 80s synths were the filters. So I tend to use analog filters with Teensy-based waveforms. I got enough cheap i2s audio decoder boards I could make an 8-voice polysynth feeding 8 individual Teensy voices into 8 real analog filters (etc). I think that covers the DW-KORGS, the DCO-based Rolands, the ESQ-1, and other weird hybrids. And I won’t miss the analog oscillators. 

That’s just my weird old point of view. If you’re having fun working on the old keyboards, enjoy! Fortunately the multiplexers and analog switches used the 80s polysynths are pretty easy to replace. 
I had a good experience purchasing a replacement CPU for my Roland CR-5000 from this joint in Italy. https://reverb.com/shop/vintagesynthparts <https://reverb.com/shop/vintagesynthparts>
If the CPU or ROM is truly dead, this may be your best option.

Best wishes,
Ben




> On Feb 4, 2022, at 5:28 PM, Nathan Trites <nathan at idmclassics.net> wrote:
> 
> Focusing on polysynth/polyphonic sampler architecture was how I spent my winter so far. To echo previous recommendations: Musical Applications of Microprocessors is excellent, as well as the SCI Prophet 5 service manual, Rhodes Chroma site and service manuals.Other SCI service manuals are good too. 
> 
> For the computer side, I don't have many specific recommendations beyond those, but various intro to Z80 and 86k processor sites and courses  <https://www.cs.mcgill.ca/~cs573/fall2002/notes/lec273/lecture8/>I've stumbled on through searching have been helpful to get a better understanding of memory mapped computers in general. There is also a huge retro computer scene now, and some Youtube channels like Adrians Digital Basement, Noel's Retro Lab and similar have in-depth videos on debugging computer problems that can help fill in the gaps missing from synth repair info online. I struggled to find good resources specifically for fixing embedded computers from the 70s/80s that would have been targeted towards service techs but would love something like that. 
> 
> Nathan
> 
> On Fri, Feb 4, 2022 at 5:22 AM ackolonges fds via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>> wrote:
> Hi Everyone,
> 
> As the local SDIYer I sometimes get asked to try and fix synths from the 80s like various Rolands, Korgs, Oberheims etc. and I generally just try to pinpoint the rough area of the issue and replace logic chips until the issue is resolved...
> 
> Most of these synths have a CPU connected to everything via a parallel address bus and a parallel data bus, with all sorts of glue logic chips doing various things. They also usually just have 1 DAC, time-domain multiplexed to all of the different parameters via more logic chips. 
> 
> Obviously this is very different to the modern ways that microcontrollers and DACs are used in synths, and since I wasn't around in the 80s, these older architectures are very foreign to me. To aid in my troubleshooting efforts, I would love to better understand the details of how these architectures work, so I was wondering if anyone on here would be able to point me to any resources that could explain these types of systems to me, be they websites, articles, or books?
> 
> Thanks a lot for any advice you might have.
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