I would like to source the original schematics from #Generalmusic, though. There are a few things that I found strange and would like to check.

For instance, I am quite puzzled by the connections of U1C and U6D with R5. There's no reason for that, apart using a couple of spare gates for introducing appropriate delays. I am more an analog electronics guy and I do not have extensive experience on such things. The 47Ω resistors are there for the taming the risetime and reduce noise? Similar solutions are everywhere in the S2 schematics (that I own).

I had the opportunity to check with the original GEM Turbokit schematics and sure enough the quirky solutions are well present there as well. BTW, they used a 74AC00, 74HC139 and 74AC138. To me it would make sense to stick to a single logic family and not mix them.

I think the real problem to build a complete Turbokit replica for my S2 is to source a NEC µPD72064 floppy disk controller. It seems to be quite unobtanium today, unfortunately. I see some sources on Ebay, but they list it as "SRAM" that is clearly wrong and the datecode is 1989 where the earliest datasheet I could source is from September 1990. Maybe older datasheets existed, but buying the chip would cost me about 30 euros, so I don't want to risk too much.

I found this, https://www.utsource.net/it/itm/p/558694.html anyone has experience with them?

I tried to place an order and at the same time get the EPROM and the SRAM chips. Let's hope I got lucky. The price was fair for components that you would expect to work.

I received the order and the first impression is very good. Everything was properly packaged in antistatic bags. The most difficult to source chip seems OK, but we will know only when installed in the keyboard. It would be amazing to build a #Turbokit replica for my #Generalmusic #S2. It is a massively underrated synth from 1992, a project started by #Elka!

In the last few weeks I did a first tentative design for the Turbokit replica to be installed in my #Generalmusic S2 keyboard. I had to adjust the position of a few connectors as I originally planned them under the keybed and there is no place there. This second iteration is much better, but still does not work because it interferes with the connector on the right. #S2 #Turbokit. #DIYelectronics #Elka

After a few iterations, I could slightly shrink the #Turbokit card. Now it should fit. Or at least I hope so, there are a few capacitors below the upper part of the board that can create problems. I may try to print a test on paper to see how it would fit on the real thing. #DIYelectronics #Generalmusic #S2

I could remove some really stuck screws, lift the keybed and expose the whole sound and digital board. The battery is still charged after 33 years 😳 #Generalmusic #Gem #S2

Yesterday I could successfully test a Gotek USB drive in place of the original disk drive. It is still slow, but much more practical than using 3.5 inch disks, that tend to be unreliable when formatted at 1.6MB. #Generalmusic #GEM #S2 Now I only have to install it in a more definitive way.

I assembled back the #Generalmusic #S2 with the Gotek disk emulator in place the result is very good and I love it. I now have a few hundred disk images to test 😅

I resumed working on the #Turbokit replica for the #Generalmusic #S2. I tweaked a little the 4-layer PCB and got a quote from a well-known company: the estimated cost is US $81.87 for 5 PCB's. 2-layer boards would have been less expensive, but the circuit was too complex and I couldn't route it. #electronics #diy #synth

I realized I swapped the position of the left and right connectors that will be connected in the original EPROMs sockets in the #GEM #S2. I'm glad I checked, that would have been an expensive mistake to fix... #electronics #DIY #PCB #Turbokit

I created a GitHub project for the #Turbokit replica for the #Generalmusic #S2. It is not a project for the faint of heart, however, at it will require to modify the original boards of the #synth. It is still pretty much an ongoing project: https://github.com/DarwinNE/GEM_turbokit #openhardware #electronics #diy

I'm not very experienced with the routing of a 4-layer PCB but it is fun. The different copper layers are starting to look nice! #Turbokit #Generalmusic #Electronics #Diy

I corrected some errors and I think I reached what I think it will be the v1.0 design of the #PCB. I hope it will work! #Generalmusic #Turbokit #electronics #DIY #Synth

Btw I received today this book that has been written by one of the members of the group who developed the #Generalmusic #S2. It is very interesting and full of details and interesting anecdotes about this Italian #synth. M. Colò, R. Gerbi “GEM S2, Quello che non sai… raccontato dai protagonisti”. It does not cost much and it id really interesting.

In the meanwhile, I sent the order both for the components that remain to be sourced and the PCB. Now, I just have to wait. #Turbokit #Generalmusic