I’ve completed designs for the Prophet 3000 I-627 rev B memory controller adapter in collaboration with Martin Day in the UK (thanks Martin!). Martin successfully upgraded his 2MB Prophet 3000 to a 4MB unit.
A bit of history: The first revision of the Prophet 3000 (most of which were shipped to Europe) had a first generation custom voice controller chip, the I-627. This chip takes the sound data from RAM and generates 8 voices worth of sample playback. The first generation chip (in a PGA style package) has problems using multiple banks of memory, which is why most Prophet 3000’s only have 2MB. After Yamaha bought Sequential in 1988, they redesigned the I-627 and changed the package to PLCC. The main PCB of the P3000 was redesigned for the PLCC I-627, and these units can be expanded to 4MB.
The corrected I-627 is available from Wine Country Productions; however until now there was no solution to retrofit the first generation Prophet 3000’s. Additionally, the schematics were updated for the new I-627 pinout on one page only, and the rest of the schematics refer to the old PGA I-627. Martin identified the pinouts for the PGA I-627 and cross checked the newer PLCC I-627. He constructed an adapter board for the newer I-627, but unknown to him, there are several errors in the schematics which prevented proper operation. Luckily I have one of each revision of Prophet 3000, and was able to find the errors. It appears as though there was a documenting error with the newly revised I-627. Two power pins were swapped (which Sequential solved by physically removing the PLCC socket contacts for those pins), and the RAM bank select signals were out of order (which they solved by cutting the PCB and using wire jumpers). With the newer units, they reassigned the banks so that banks 0 and 1 were on the PCB and banks 2 and 3 were on the memory expansion connector. I don’t know if Sequential ever made a memory expansion board, but if they did they are very rare.
I’ve completed a design for a memory expansion board which will require the I-627 rev B adapter board for older units. If all goes as planned, this set of boards can be installed without any soldering or modification of the main PCB. Stay tuned!