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Prophet 600

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Re: Prophet 600

Postby nickle15 » Thu Mar 04, 2021 2:24 pm

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?
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Re: Prophet 600

Postby Martin Walker » Thu Mar 04, 2021 2:41 pm

Wow - I never expected that our resident experts would be up for remote diagnosis of a 'non-repairable P600 as declared by an industry professional'

:clap: :clap: :clap:

Carry on chaps :thumbup:


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Re: Prophet 600

Postby nickle15 » Thu Mar 04, 2021 2:51 pm

This is definitely the place to hang out - so many geniuses in one place!!
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Re: Prophet 600

Postby Hugh Robjohns » Thu Mar 04, 2021 3:17 pm

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.
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Re: Prophet 600

Postby nickle15 » Thu Mar 04, 2021 3:48 pm

Hugh Robjohns wrote:
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.
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Re: Prophet 600

Postby Tomás Mulcahy » Thu Mar 04, 2021 3:51 pm

nickle15 wrote: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.
:D
That says to me they did not clean the keyboard contacts, because that would take an hour for someone skilled.

Those would be my first port of call. I was recently fooled by an apparent pattern to failing keys in my MX-1000. Went bananas diagnosing the matrix.

Turns out it was dust.

Do this first:
https://www.synthtopia.com/content/2010 ... ophet-600/
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Re: Prophet 600

Postby nickle15 » Thu Mar 04, 2021 3:58 pm

Tomás Mulcahy wrote:
nickle15 wrote: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.
:D
That says to me they did not clean the keyboard contacts, because that would take an hour for someone skilled.

Those would be my first port of call. I was recently fooled by an apparent pattern to failing keys in my MX-1000. Went bananas diagnosing the matrix.

Turns out it was dust.

Do this first:
https://www.synthtopia.com/content/2010 ... ophet-600/

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.
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Re: Prophet 600

Postby Tomás Mulcahy » Thu Mar 04, 2021 4:04 pm

Ah OK, I was not aware you'd already done that.
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Re: Prophet 600

Postby Hugh Robjohns » Thu Mar 04, 2021 4:18 pm

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.
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Re: Prophet 600

Postby jjlonbass » Thu Mar 04, 2021 4:27 pm

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.

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Re: Prophet 600

Postby Tomás Mulcahy » Thu Mar 04, 2021 4:49 pm

Hugh Robjohns wrote: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.
Agreed yes, but given that this keyboard has been in the hands of at least one "genius"and applying the KISS principle ;) it's possible the rubber strip was not replaced correctly. It's easy to do when you don't know how!
jjlonbass wrote: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.
John
Yes I've seen this too, it seems to be happening a lot in the last few years with vintage gear. After all, it's effectively corroding metal so it's feasible that only a part of it would fail.
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Re: Prophet 600

Postby nickle15 » Sun Mar 14, 2021 12:50 am

So we spent a little time in the Prophet 600 this afternoon. Appears that there is a problem with the SCL4514 chip. I have no idea what it does but everything was good up until that point. A new one has been ordered and we'll put it in when it arrives. Fingers crossed!
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Re: Prophet 600

Postby jjlonbass » Sun Mar 14, 2021 12:41 pm

The 4514 is a combined 4 bit latch and 4 to 16 demultiplexer often called a 1 of 16 decoder. It takes a 4 bit number on its D0 to D3 inputs and drives the corresponding "S" output high with all other outputs low. For example, if the input code is 0 (0000 binary) output S0 will be high and all others low.

In the Prophet 600, U104 - the 4514, is one of the important elements of the keyboard and front panel switch scanning circuitry driving the "rows" of the switch matrix. The Z80 microprocessor will output a row code to the 4514 then read the resulting "column" code that indicates which if any switches on the selected row are closed.

Incidentally the "SCL" part of the device code that you state is just a manufacturer's prefix and is unimportant for this application. Many IC manufacturers would have made 4514 devices - the Texas Instruments version would be a CD4514, the National Semiconductors version MM4514. Any of these would work.

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Re: Prophet 600

Postby Hugh Robjohns » Sun Mar 14, 2021 12:50 pm

Good work on tracking downthe failed chip, and nice to know our collective hypothesising identified the likely culprit. Fingers crossed the replacement process goes smoothly.
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Re: Prophet 600

Postby nickle15 » Sun Mar 14, 2021 8:02 pm

jjlonbass wrote:The 4514 is a combined 4 bit latch and 4 to 16 demultiplexer often called a 1 of 16 decoder. It takes a 4 bit number on its D0 to D3 inputs and drives the corresponding "S" output high with all other outputs low. For example, if the input code is 0 (0000 binary) output S0 will be high and all others low.

In the Prophet 600, U104 - the 4514, is one of the important elements of the keyboard and front panel switch scanning circuitry driving the "rows" of the switch matrix. The Z80 microprocessor will output a row code to the 4514 then read the resulting "column" code that indicates which if any switches on the selected row are closed.

Incidentally the "SCL" part of the device code that you state is just a manufacturer's prefix and is unimportant for this application. Many IC manufacturers would have made 4514 devices - the Texas Instruments version would be a CD4514, the National Semiconductors version MM4514. Any of these would work.

John

Thanks for this great explanation! My dad walked me through the functionality as we were checking things out yesterday but I didn't retain a whole lot of it. It's nice to see it in writing here to help me better get a handle on things. Thanks again!
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