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Prophet 600

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Prophet 600

Postby nickle15 » Fri Feb 19, 2021 8:03 pm

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. Anyway, thought I'd cast a net out here to see if anyone has any thoughts. The tech actually told me to call him if I thought of anything else he could/should check. :crazy:

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!
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Re: Prophet 600

Postby Hugh Robjohns » Fri Feb 19, 2021 8:42 pm

Yes, it will be a chip, somewhere. Genius suggestion! :lol:

Slightly odd that the keyboard scanning is feeding the MIDI coding stages correctly, but not the synth-controlling circuitry. Nevertheless, I would have thought a well-defined fault like this would be a fairly easy thing to track down from the schematics.
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Re: Prophet 600

Postby nathanscribe » Fri Feb 19, 2021 9:05 pm

Hate to say it, but the service manual not only gives a complete and clearly labelled schematic, but also explains how everything works. So... maybe point that out to your tech.
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Re: Prophet 600

Postby nickle15 » Wed Mar 03, 2021 3:00 pm

So I had someone pick the Prophet up for me yesterday. I looked at the receipt when I got home and from all indications this outfit either didn't listen to me when I dropped the keyboard off, or just didn't care.

They listed my complaint as being: "no midi out". In fact when I took the board to them I told them there were eight sequential keys that would not sound when using the Prophet keys. BUT - the notes would play when triggered by external MIDI. Further, the Prophet would send a MIDI signal out on those same eight keys. So obviously they did not hear my complaint correctly.

Conclusion on the work order is "problem at this point is probably a bad voice chip(s). These are hard to locate and are probably old/new stock. These may also be questionable."

The one consolation in all this is that they only charged me 30 minutes for a cleaning fee, which by all indications they actually did.

So that ends the saga of my Prophet 600 at the shop for 3 months with no results. Except that Behringer will have a replacement out soon so all is right with the world. Right?? :D
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Re: Prophet 600

Postby Hugh Robjohns » Wed Mar 03, 2021 4:17 pm

Sounds like you need to find a better tech...
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Re: Prophet 600

Postby nickle15 » Wed Mar 03, 2021 4:41 pm

Hugh Robjohns wrote:Sounds like you need to find a better tech...

I agree - I have an electrical engineer lined up who is going to work with me on it. Hoping for better results!
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Re: Prophet 600

Postby Hugh Robjohns » Wed Mar 03, 2021 5:01 pm

From your description, we can rule out a problem with the keyboard contacts and keyboard scanning circuitry.

The fact that it's eight consecutive notes is relevant. The instrument is driven by a Z-80A processor which works with 8-bit data. So somewhere on the side controlling the local voice cards there's going to be an address or data line, or a demux chip, that's not doing what it should.

Careful probing around with a suitable data analyser or scope should be able to track it down...

The manual is here: http://www.synfo.nl/servicemanuals/Sequential/SEQUENTIAL_PROPHET-600_SERVICE_MANUAL.pdf
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Re: Prophet 600

Postby nickle15 » Wed Mar 03, 2021 5:13 pm

Hugh Robjohns wrote:From your description, we can rule out a problem with the keyboard contacts and keyboard scanning circuitry.

The fact that it's eight consecutive notes is relevant. The instrument is driven by a Z-80A processor which works with 8-bit data. So somewhere on the side controlling the local voice cards there's going to be an address or data line, or a demux chip, that's not doing what it should.

Careful probing around with a suitable data analyser or scope should be able to track it down...

The manual is here: http://www.synfo.nl/servicemanuals/Sequential/SEQUENTIAL_PROPHET-600_SERVICE_MANUAL.pdf

Excellent - I appreciate you chiming in on this! I'm hopeful we can at least identify the issue in short order...
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Re: Prophet 600

Postby jjlonbass » Wed Mar 03, 2021 5:46 pm

I've just had a quick look at the manual and I can't work out how the described fault is possible.
My reasoning in summary is that everything is controlled by the Z80 and the software it is running and the same voice output circuitry is used regardless of whether notes come from the keyboard or MIDI. Similarly the same keyboard input circuitry is used regardless of whether the notes are to be sent to the voice circuitry, MIDI or both.

There is no direct path for keyboard data to the voice circuitry - keyboard data passes into the Z80, then out into a set of latches to a DAC and then to the voice circuitry.

There is no direct path for keyboard data to the MIDI port - keyboard data passes into the Z80 using the same route as above, the Z80 then sends MIDI messages out via a UART and line driver. The correct note MIDI messages are generated so there shouldn't be a fault with the keyboard scanning and input circuitry.

There is no direct path for MIDI data to the voice circuitry - MIDI data passes into the line receiver, then UART then into the Z80 which interprets the MIDI data , then out into a set of latches to a DAC and then to the voice circuitry as in the keyboard case. Correct notes sound when sent to the synth over MIDI so there shouldn't be a fault with the note output latching, DAC etc.

I'd be very interested to find out the cause of this fault.


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Re: Prophet 600

Postby nickle15 » Wed Mar 03, 2021 6:01 pm

jjlonbass wrote:I've just had a quick look at the manual and I can't work out how the described fault is possible.
My reasoning in summary is that everything is controlled by the Z80 and the software it is running and the same voice output circuitry is used regardless of whether notes come from the keyboard or MIDI. Similarly the same keyboard input circuitry is used regardless of whether the notes are to be sent to the voice circuitry, MIDI or both.

There is no direct path for keyboard data to the voice circuitry - keyboard data passes into the Z80, then out into a set of latches to a DAC and then to the voice circuitry.

There is no direct path for keyboard data to the MIDI port - keyboard data passes into the Z80 using the same route as above, the Z80 then sends MIDI messages out via a UART and line driver. The correct note MIDI messages are generated so there shouldn't be a fault with the keyboard scanning and input circuitry.

There is no direct path for MIDI data to the voice circuitry - MIDI data passes into the line receiver, then UART then into the Z80 which interprets the MIDI data , then out into a set of latches to a DAC and then to the voice circuitry as in the keyboard case. Correct notes sound when sent to the synth over MIDI so there shouldn't be a fault with the note output latching, DAC etc.

I'd be very interested to find out the cause of this fault.


John

I will certainly keep everyone informed. Also just to be sure I've got all my facts straight I'll run my MIDI tests again this evening to make sure I'm not remembering something incorrectly on the MIDI out side. But last night I was playing all 61 tones on the Prophet via MIDI from my KORG 707. So I know MIDI in will activate the notes. Tonight I'll double check my other statement that all 61 Prophet keys will transmit MIDI. I haven't run that test since we took the keyboard to the tech (November).
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Re: Prophet 600

Postby nickle15 » Wed Mar 03, 2021 11:05 pm

Yeah, I was wrong... :oops:

MIDI out does not work on those eight keys. Works across the rest of the keybed, but not there. Don't know if that helps/makes a difference for anyone pondering this issue but at least you have accurate info.
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Re: Prophet 600

Postby Hugh Robjohns » Wed Mar 03, 2021 11:12 pm

That makes a lot more sense, and in that case I'd suspect that it's either a defective bank of key switches or their wiring/connectors, or one of the multiplexer chips involved in the scanning of those eight switches.

Again, this should be fairly straightforward to track down....
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Re: Prophet 600

Postby nickle15 » Thu Mar 04, 2021 12:05 am

Hugh Robjohns wrote:That makes a lot more sense, and in that case I'd suspect that it's either a defective bank of key switches or their wiring/connectors, or one of the multiplexer chips involved in the scanning of those eight switches.

Again, this should be fairly straightforward to track down....

Sounds good - thanks Hugh. And sorry for the misinformation earlier!
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Re: Prophet 600

Postby jjlonbass » Thu Mar 04, 2021 10:18 am

Please can you name the 8 notes that don't work?
I reckon that there's a good chance of working out where the fault lies if we know this.

My guess would be any, some or all of U104 - a 4514 demultiplexer on board 1, the P103 header on board 1, the J103 connector that mates with P103, the keyboard cable and / or its termination on the keyboard, oh, and any solder joints and IC sockets involved in this chain :D

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Re: Prophet 600

Postby Hugh Robjohns » Thu Mar 04, 2021 11:43 am

I agree -- it certainly seems likely to be in that area.

Given the confusion over whether the MIDI output was working... can I ask if the OP is absolutely sure that all of the other control panel switches are functioning correctly?

If they are, then the problem is likely to be a failed output from the U104 or something associated with it (connectors, PCB, etc), as John has identified above. A decent 'scope or data analyser should quickly reveal if one of the multiplex lines is dead...

But if other switches aren't working, then I'd also check U105 and U106 -- the hex buffers that receive the data from the keyboard switch matrix.
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