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!

By way of an update - we ordered a 4514 chip and replaced it. Now, none of the keys will play and the sounds cannot be triggered via MIDI. Any thoughts on what we broke? That was a very frustrating day and we’re going to get back in there this evening to see what we can see…

The chip we received was NOS - is it possible that the entire chip is bad and if so, would that cause our new problem?

So we got another solid 2.5 hours of work last night. The good news is that everything we checked has correct voltages running to it. Everything “works”, except no sound (and one new issue outlined below). I can’t remember the name/type of chip but there’s a 14 pin chip that has different frequencies running out of it and all of those checked out ok with the oscilloscope. The last part we tried to check was the actual audio output and had some trouble with the scope. So today dad will be following up with the scope and making sure we were using it correctly. That’s a new item for him (though he used one back in the day).

One thing I noticed was that with the keyboard turned all the way up, and the very small Dell computer speaker we plugged into it also turned all the way up, the “noise” I can hear when the speaker is held up by my ear changes a bit as I switch switches on the keyboard. Also, if I start the arpeggiator I hear “noise” changes at regular intervals. So it seems like something is happening.

The new fault we found last night was that the membrane pad no longer works when we press the 1, 2, 3, and 0 keys, and there are a few of the function buttons that don’t light up also. AND the LCD display is missing the very bottom row of lights. So the number 8 looks like a capital A.

The weekend before we started troubleshooting my friend (who is the actual owner) wanted the Gligli mod installed. Since there is no soldering required I performed that operation on my own and the keyboard worked as expected afterwards. But given my unfamiliarity with the new control scheme with Gligli I’m considering putting the factory chip back in for now while we continue to troubleshoot.

So, there’s the update. We’ve not had any victories yet and keep taking steps back it seems, but we’re not discouraged yet and will get back in there this evening. As always if anything I’ve mentioned sparks an idea please feel free to share.

And I realize this isn’t the most “technical” explanation but it’s the best I can do as I’m in the process of learning more…

Sorry I haven’t posted any updates. The short version (of a long story) is that the keyboard is up and running!

We are guessing that the NOS 4514 we installed was either bad from the start or we damaged it in the handling. At any rate we installed a chip holder, found a brand new 4514 and installed that. When we fired up the keyboard we had a working display again, and all the membrane buttons worked. We also had resolved the problem with the 8 keys not playing. BUT - another row of eight keys wasn’t working now. So we dove back in and found a broken trace. The assumption is that we damaged that trace in the process of installing the chip holder. At any rate we fixed the trace and the keyboard is playing perfectly now! Thanks to everyone who chipped in advice.

Now, on to the next thing - I’m looking at you Polysix…

it is always excellent to hear when a restoration project turns out well and the help from experienced people here especially family. so a quote similar to Depeche mode enjoy the music!

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