SCI Prophet-600 Diagnostic Overview 

Keep us posted. I'm in the same boat. Any luck?

Nice! I mean... not nice that the voice is dead, but nice that you're going to re-cap it. Weird that triangle isn't working. Is that across all the voices?

@@mphtower yep no triangle at all! Got this 15 years ago for cheap. Fixed some issues, now its time to finish the job. Got some prophet 5 knob caps from a guy named Lars in Stockholm, looks awesome! techsmechsvintagesynth.com/

@@scottstevenz8732 Oh, and that dude is selling replacement keyboards. That's appealing. I'll try to dig around in the schematic and see if something jumps out at me. But I've not seen that problem before. You know, you might try doing the GliGli upgrade. That fixed a few quirks in mine.

@@mphtower thanks, appreciate it! yeah I put his keybed in my membrane pos pro1 and now it rocks! It has the pro 5 knobs too it is badass!

@@scottstevenz8732 Well, keep me updated!

Well, turns out you might get your wish! Mine died again, and it seems to be the computer board. Hopefully it's just a dead power supply, but since I'll have it open I'll try to show some diagnostics with the oscilloscope. But, who knows when I'll get around to this...

Perhaps the root directory link will work for you: mphtower.net/DiagnosticTools/ If so, lemme know and I'll add that link to the description. That's an odd mix of problems! Glide, volume, and amp are all captured by the same 4067 on Board 2, so it's entirely possible that one's shot. At the least, I'd start there and socket the IC. The pitch and mod wheels not working could be caused by a dead 4067 on Board 1, but it could also just be a loose connection. You might want to check the output on pin 3 of U109 and confirm you're getting +5VDC. I imagine it's fine, but since you're in there it's worth it to check. Regardless, at the moment I'm thinking the problem is in Boards 1 and 2, and probably not on 3 (CPU). I'd love to hear your updates!

Yup! They're available new as reissues from Curtis. The CEM3340 Rev G is a drop-in replacement. You can also use an AS3340 from Alfa but it is slightly different. When you do a replacement it's best to replace all of the chips on the same line. E.g., if you need to replace one 3340 on OSC A, then do all six on OSC A, but you can leave OSC B alone. Same goes for the 3372s. You can find the 3340 Rev G chips by doing a Google search and finding whichever ones are priced best at the moment.

@@mphtower Thank for the info. This will fix the OSC A, Voice 2. If you don't mind me asking, I also noticed that oscillator A voice #6 seems to always be at ~656Hz. The voltage control from U407-7 is either ~0.5V (lower keys) or ~5V (upper keys) I can't find anything wrong is the sample and hold circuit. Looking at the DAC output and the 4051 INH control signal it looks like the CPU is setting it to either ~0.5 or ~5V. Not sure if there is a setting issue or what. Thanks for your time! Jim

@@jimdieckmann2025 Ooo, that's a tricky one. Have you checked the 4049s in U401-404? I tend to be a bit OCD and I would just socket all four so I can test. (I have a 4xxx-/74xxx tester but it doesn't know the difference between buffered and unbuffered, so sometimes you've just gotta swap ICs to see what happens.) The other immediate suspects are the 4013s in U324 and U330. And I'd check the 4174s that are there as well. Finally, if that doesn't do anything, I'd check the 74LS32 quad OR in U312. I doubt that's the problem, but hey. It's not likely that the CPU would be setting just that one channel. I mean, it COULD happen, but a dead 4000 is more likely at the moment. Keep me apprised!

@@mphtower Is there a way to factory reset the unit? If so, will I loose the factory presets or other data?

@@jimdieckmann2025 I don't think there is one since there wouldn't be anything to refresh except clearing out the voices. (And there's no mention of one in the service manual that I can find.) The logic is all hard-coded in the OS EPROM. You could try re-seating that to see if it makes a difference. I do highly recommend installing the GliGli upgrade if you haven't already. But, if your machine is on the fritz then it won't help and might be more confusing.