Prophet 600 - 8 dead keys

I’ve got a Prophet 600 at a tech’s store right now. It’s been there since November and I’m not getting any good vibes from the phone calls I’m making to them. The tech actually told me to call him if I thought of anything else he could/should check.

Basically there are eight sequential (no pun intended) notes in the middle of the board that will not sound when the keys are pressed. The keybed is good as we’ve tested voltages and we’ve output via MIDI and they eight keys will activate an external MIDI device. The eight dead notes WILL play when activated through MIDI in. The tech is thinking it’s a chip somewhere but the more I’ve talked to him the less confident I feel that he’s going to find it. He does not sound motivated though he went through and cleaned everything and says to the eye everything looks good.

So I know it’s a long shot but has anyone encountered this before, and do you have any thoughts? Thanks!

Typically, groups of dead keys like this point to either a faulty ribbon cable or connector, or a bad trace on a PCB. In looking at the block diagram of the P600, the keybed connects to the left panel board (PCB 1), and since MIDI is being sent fine, you know that everything up until the left panel board is good (keybed, ribbon cable). I’m not sure where MIDI is being generated, but presumably your trouble is after MIDI and before the keyboard data is sent to the voice board (PCB 4). So it could be a bad ribbon cable or connector between boards, or a bad PCB trace, or a bad chip.

I’m not sure if that helps much…

It definitely does help and I appreciate you taking the time to reply. I think my course of action now is to go pick up the keyboard from the tech and let my dad (an electrical engineer) take a crack at it. Seems like the tech has taken it as far as he is willing and/or able.

By the way, I enjoy the Talking Synths show (and your other videos as well) - keep up the great work!

Had the same issue recently except on an Emax II. In the end it was a crack in a PCB trace common to the row of the sequential keys, and the crack was undetectable by visual inspection.

Using a multimeter I was able to ohm it out and narrow down to a small section of the trace which still didn’t have continuity. Had to scratch off some of the green layer to access the trace at various spots to narrow it down. If all the connections from the keys to the inputs are good then it could be the scanning IC or a line pull-up resistor.

These would be almost the guaranteed areas for the defect since these are the circuits which MIDI input does not depend on to trigger sound. Would have to take a look at the 600’s schematic to tell for sure. Good luck, I’m an EE like your dad so he should be able to sort it out!

Thanks for this response! We are going to pick up the board in the next couple of weeks and get to work on it. The info you provided was very helpful and I appreciate it!