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.
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!
Yes, it will be a chip, somewhere. Genius suggestion!
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.
Last edited by Eddy Deegan on Thu Mar 04, 2021 2:26 pm, edited 1 time in total.
Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
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.
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??
Last edited by nickle15 on Wed Mar 03, 2021 2:02 pm, edited 2 times in total.
Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
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...
Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
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...
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.
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).
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.
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....
Last edited by Hugh Robjohns on Wed Mar 03, 2021 10:14 pm, edited 1 time in total.
Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
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!
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
John
Last edited by jjlonbass on Thu Mar 04, 2021 9:29 am, edited 1 time in total.
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.
Last edited by Hugh Robjohns on Thu Mar 04, 2021 10:45 am, edited 4 times in total.
Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
The keys that aren't working are Ab through Eb (left to right). Just right of the middle of the keyboard.
All other switches appear to be working correctly. I have played with all the buttons, switches and knobs and as far as I can tell they're all behaving as expected.
If one of the multiplex lines is dead would that mean replacing the multiplexer itself?
nickle15 wrote:The keys that aren't working are Ab through Eb (left to right). Just right of the middle of the keyboard.
Looks like that would tie it into the line from pin 14 (output 12) of the 4514 multiplexer chip, or the wiring between it and the keyboard (connector p103), or links within the keyboard itself to that particular range of key switches.
If one of the multiplex lines is dead would that mean replacing the multiplexer itself?
Yes. It's a 24-pin IC which is readily available. It would be a fiddly and delicate thing to replace, but perfectly doable given the appropriate tools, experience, and expertise.
But it would be unusual to only have one dead output on a chip like that, so a wiring/connector PCB problem is probably more likely.
Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
nickle15 wrote:The keys that aren't working are Ab through Eb (left to right). Just right of the middle of the keyboard.
Looks like that would tie it into the line from pin 14 (output 12) of the 4514 multiplexer chip, or the wiring between it and the keyboard (connector p103), or links within the keyboard itself to that particular range of key switches.
If one of the multiplex lines is dead would that mean replacing the multiplexer itself?
Yes. It's a 24-pin IC which is readily available. It would be a fiddly and delicate thing to replace, but perfectly doable given the appropriate tools, experience, and expertise.
But it would be unusual to only have one dead output on a chip like that, so a wiring/connector PCB problem is probably more likely.
As I've already demonstrated here my memory is a little fuzzy from the troubleshooting that we did in October/November but I do know that we tested the keybed, starting at individual keys, through the contacts and the wiring harness and we had good connections/continuity up to the multiplexer - if that makes sense. That's the point at which we decided to let the tech have a shot. As long as our schedules match up we're going to sit down with the keyboard on Saturday to crack it back open. The info that has been shared here will help a lot.
Last edited by nickle15 on Thu Mar 04, 2021 2:49 pm, edited 2 times in total.
Thanks for that - I'll be sure to go over it again but I did clean all the contacts myself as the first step of trying to fix the problem. Never hurts to try again though, especially since we'll already have it opened up!
I'm not sure how much time they actually spent on the cleaning but I was glad they only charged me for 30 minutes. I was afraid they would want to charge me a bundle just for spending time trying to figure it out.
I wouldn't have expected a contiguous group of 8 failed keys if it was contact issues, but with a synth of this age a good clean wouldn't be a bad idea anyway. But check those links between boards -- a dry joint or broken wire there could easily cause the identified problem.
Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
It would have to be all 8 commonned contacts on the "bus bar" or all 8 individual key contacts to be similarly dirty for those keys to be inoperable if the cause was a dirty key contact problem. This is possible, but it seems a bit unlikely.
If it's not that, the happy outcome would be a dry solder joint somewhere along the line; that would be easy to remake. The less happy outcome would be that U104 is faulty and needs to be replaced.
It's not unheard of for individual outputs of logic chips to fail and this seems to be more common with CMOS devices like this.
