Items for sale ok?


Andy Birko
 

Hi All,

I had completely forgotten about this group until I saw some messages come across.  I've recently repaired M6r that I'd like to sell since I still have my original M6 keyboard I got in 1985.  Are Oberheim for sale items ok in this group?  If not - so sorry about that!

p.s., repairing the unit was pretty fun actually!  I'm an EE by degree but due to living in Michigan and working for the auto companies for a long time, I never really had to do much electronics stuff as a pro.  However, after first trying to trace the issue using a schematic for a M1k and figuring that it was in the resonator circuits - the main difference between the M6 and M1k, and then finding downloading a copy of the schematics for the M6r that was actually legible and data a load of data sheets it all started coming back to me - turned out to be a bad timer chip!  I'll pop in a socket for the battery and probably upgrade to 2.15 if I can find it before I sell.


Dennis Barrington
 

I like the EE stuff.  Be cool to know more details about tool you used to sleuth it out!


On Jun 14, 2022, at 12:58 PM, Andy Birko <andy.birko@...> wrote:

Hi All,

I had completely forgotten about this group until I saw some messages come across.  I've recently repaired M6r that I'd like to sell since I still have my original M6 keyboard I got in 1985.  Are Oberheim for sale items ok in this group?  If not - so sorry about that!

p.s., repairing the unit was pretty fun actually!  I'm an EE by degree but due to living in Michigan and working for the auto companies for a long time, I never really had to do much electronics stuff as a pro.  However, after first trying to trace the issue using a schematic for a M1k and figuring that it was in the resonator circuits - the main difference between the M6 and M1k, and then finding downloading a copy of the schematics for the M6r that was actually legible and data a load of data sheets it all started coming back to me - turned out to be a bad timer chip!  I'll pop in a socket for the battery and probably upgrade to 2.15 if I can find it before I sell.


Andy Birko
 

Thanks for feeding into today's procrastination efforts.

So, I purchased this M6r a while ago with one bad oscillator in voice 3.  I thought it just needed a master-52 style calibration but I was wrong.  The issue was that the one oscillator did track pitch kind of, but it was the wrong pitch.

I had an old 4 channel Tektronix scope I used at first but I wanted something a bit more modern so I picked up a new Hantek 2-channel scope for like $200 and sold the old CRT scope for like $200 - who knew?

After downloading the CEM 3396 datasheet, I looked around with the scope at the various signals and found the CLKA3 and CLKB3 signals that control the frequency of the DCOs in voice 3.  I played 6 notes on the init patch but I changed the voice assignment to rot so that I would have all oscs playing the same note.  Remember that the oscillators are always oscillating even if the VCA is closed. 

So, with this configuration, CLKA1 - CLKB6 should all be the same.  When probing I found that CLKB3 was not the same frequency as all the others.  I then went down a rabbit hole trying to run the resonator calibration procedure in the service manual but that was useless - why will be apparent in a moment.

CLKB3 is generated by U728 which is a Mitsubishi M5M82C54P-6 programmable interval timer.  The M6/6r have three resonator circuits that run at about 3MHz (IIRC) that clock the the four 8254s - this is the big difference between the 6 and the M1k - only one resonator for the whole instrument and why I couldn’t quite use the schematics for the M1k of which there’s a hi-res digital version available. I did find a high enough quality scan of the original hand drawn M6 schematics from Oberheim where you can actually read the tags.  After checking, all the resonators were running within spec.  

Signal FBU on the schematic is the 3MHz clock from the resonator that runs the 8254s and is assigned to oscillator 2 of voices 3, 4, 5 and 6 (CLKB3, CLKB4, CLKB5, CLKB6).  Since all the other oscillators were in tune, it’s definitely not the resonator.

I didn’t get too deep into the how exactly the 8254 chip is used but it’s either going to free run at a particular frequency until told otherwise (which is probably how it’s used) or it has to be reset every time the programmed interval time is reached which would be quite processor intensive.  Doesn’t really matter though.   The 8254 chip only has digital inputs other than the clock input so that lead me to believe that the 8254 had gone bad or possible a trace or pin or something had gone bad on that particular one as all the other timers were working fine.

Just to elaborate a bit on this for folks with less ECE experience as this is the actual moment of diagnosis and how I figured it out- there’s only one data bus from the microcontroller (MCU - 6809 in this case) that is shared and talks to all the chips with digital inputs. There are also chip select lines to make sure the MCU is talking to the right chip - the MCU selects the timer chip - writes a few bytes of data and moves on to the next thing.  If the other timer chips are working, and the only control of the chip is on the data bus, the data bus signals themselves must be good…which means that the timer chip must be bad.

I de-soldered U728, popped in a socket just in case and put in a NOS or possibly OOS replacement chip I found on eBay and boom - synth is fixed.  It was really quite satisfying to fix as this mixed digital/analog stuff was what I was really really good at in school and never really got a chance to use it (school was a LONG time ago).  Shoulda moved to Cali and found a job at a synth place when I graduated I guess.  Instead, I think I’ll look for more M6s to repair as I know it quite well now and maybe expand out a bit as a hobby.


Andrij Birko




On Jun 14, 2022, at 1:30 PM, Dennis Barrington <db9091audio@...> wrote:

I like the EE stuff.  Be cool to know more details about tool you used to sleuth it out!


On Jun 14, 2022, at 12:58 PM, Andy Birko <andy.birko@...> wrote:

Hi All,

I had completely forgotten about this group until I saw some messages come across.  I've recently repaired M6r that I'd like to sell since I still have my original M6 keyboard I got in 1985.  Are Oberheim for sale items ok in this group?  If not - so sorry about that!

p.s., repairing the unit was pretty fun actually!  I'm an EE by degree but due to living in Michigan and working for the auto companies for a long time, I never really had to do much electronics stuff as a pro.  However, after first trying to trace the issue using a schematic for a M1k and figuring that it was in the resonator circuits - the main difference between the M6 and M1k, and then finding downloading a copy of the schematics for the M6r that was actually legible and data a load of data sheets it all started coming back to me - turned out to be a bad timer chip!  I'll pop in a socket for the battery and probably upgrade to 2.15 if I can find it before I sell.


Nicole Massey
 

I understand enough of all that to say, cool.

But this has made me wonder, what does the OS 2.5 bring to the table? I'm not sure what my 6r has in it, and if I power it up with the rest of the rack I get gibberish on the display and no sound. I also will need someone else to do this, as this is way too complex for a blind person to do any repairs. But I can at least whoever I get to fix it what to look for.
Nicole

-----Original Message-----
From: Oberheim@groups.io [mailto:Oberheim@groups.io] On Behalf Of Andy Birko
Sent: Tuesday, June 14, 2022 3:56 PM
To: Oberheim@groups.io
Subject: Re: [Oberheim] Items for sale ok?

Thanks for feeding into today's procrastination efforts.

So, I purchased this M6r a while ago with one bad oscillator in voice 3. I thought it just needed a master-52 style calibration but I was wrong. The issue was that the one oscillator did track pitch kind of, but it was the wrong pitch.

I had an old 4 channel Tektronix scope I used at first but I wanted something a bit more modern so I picked up a new Hantek 2-channel scope for like $200 and sold the old CRT scope for like $200 - who knew?

After downloading the CEM 3396 datasheet, I looked around with the scope at the various signals and found the CLKA3 and CLKB3 signals that control the frequency of the DCOs in voice 3. I played 6 notes on the init patch but I changed the voice assignment to rot so that I would have all oscs playing the same note. Remember that the oscillators are always oscillating even if the VCA is closed.

So, with this configuration, CLKA1 - CLKB6 should all be the same. When probing I found that CLKB3 was not the same frequency as all the others. I then went down a rabbit hole trying to run the resonator calibration procedure in the service manual but that was useless - why will be apparent in a moment.

CLKB3 is generated by U728 which is a Mitsubishi M5M82C54P-6 programmable interval timer. The M6/6r have three resonator circuits that run at about 3MHz (IIRC) that clock the the four 8254s - this is the big difference between the 6 and the M1k - only one resonator for the whole instrument and why I couldn’t quite use the schematics for the M1k of which there’s a hi-res digital version available. I did find a high enough quality scan of the original hand drawn M6 schematics from Oberheim where you can actually read the tags. After checking, all the resonators were running within spec.

Signal FBU on the schematic is the 3MHz clock from the resonator that runs the 8254s and is assigned to oscillator 2 of voices 3, 4, 5 and 6 (CLKB3, CLKB4, CLKB5, CLKB6). Since all the other oscillators were in tune, it’s definitely not the resonator.

I didn’t get too deep into the how exactly the 8254 chip is used but it’s either going to free run at a particular frequency until told otherwise (which is probably how it’s used) or it has to be reset every time the programmed interval time is reached which would be quite processor intensive. Doesn’t really matter though. The 8254 chip only has digital inputs other than the clock input so that lead me to believe that the 8254 had gone bad or possible a trace or pin or something had gone bad on that particular one as all the other timers were working fine.

Just to elaborate a bit on this for folks with less ECE experience as this is the actual moment of diagnosis and how I figured it out- there’s only one data bus from the microcontroller (MCU - 6809 in this case) that is shared and talks to all the chips with digital inputs. There are also chip select lines to make sure the MCU is talking to the right chip - the MCU selects the timer chip - writes a few bytes of data and moves on to the next thing. If the other timer chips are working, and the only control of the chip is on the data bus, the data bus signals themselves must be good…which means that the timer chip must be bad.

I de-soldered U728, popped in a socket just in case and put in a NOS or possibly OOS replacement chip I found on eBay and boom - synth is fixed. It was really quite satisfying to fix as this mixed digital/analog stuff was what I was really really good at in school and never really got a chance to use it (school was a LONG time ago). Shoulda moved to Cali and found a job at a synth place when I graduated I guess. Instead, I think I’ll look for more M6s to repair as I know it quite well now and maybe expand out a bit as a hobby.


Andrij Birko
andy.birko@... <mailto:andy.birko@...>





On Jun 14, 2022, at 1:30 PM, Dennis Barrington <db9091audio@... <mailto:db9091audio@...> > wrote:

I like the EE stuff. Be cool to know more details about tool you used to sleuth it out!




On Jun 14, 2022, at 12:58 PM, Andy Birko <andy.birko@... <mailto:andy.birko@...> > wrote:



Hi All,

I had completely forgotten about this group until I saw some messages come across. I've recently repaired M6r that I'd like to sell since I still have my original M6 keyboard I got in 1985. Are Oberheim for sale items ok in this group? If not - so sorry about that!

p.s., repairing the unit was pretty fun actually! I'm an EE by degree but due to living in Michigan and working for the auto companies for a long time, I never really had to do much electronics stuff as a pro. However, after first trying to trace the issue using a schematic for a M1k and figuring that it was in the resonator circuits - the main difference between the M6 and M1k, and then finding downloading a copy of the schematics for the M6r that was actually legible and data a load of data sheets it all started coming back to me - turned out to be a bad timer chip! I'll pop in a socket for the battery and probably upgrade to 2.15 if I can find it before I sell.


Andy Birko
 

I’ve seen the gibberish on the display thing before on my original M6 even way back in the day when it was new but the sound still worked if I remember correctly.  Shutting it off for a moment and re-starting usually fixed it.  Without playing around with the unit, don’t really have any guesses as to why it might be silent.  You might try a Master-52 calibration for starters if you haven’t already.

As far as I can remember, OS 2.15 improves the real time sysex response for folks looking to have smoother changes when using knobby editors and so forth.


Andrij Birko




On Jun 14, 2022, at 5:15 PM, Nicole Massey <nyyki@...> wrote:

I understand enough of all that to say, cool.

But this has made me wonder, what does the OS 2.5 bring to the table? I'm not sure what my 6r has in it, and if I power it up with the rest of the rack I get gibberish on the display and no sound. I also will need someone else to do this, as this is way too complex for a blind person to do any repairs. But I can at least whoever I get to fix it what to look for.
Nicole

-----Original Message-----
From: Oberheim@groups.io [mailto:Oberheim@groups.io] On Behalf Of Andy Birko
Sent: Tuesday, June 14, 2022 3:56 PM
To: Oberheim@groups.io
Subject: Re: [Oberheim] Items for sale ok?

Thanks for feeding into today's procrastination efforts.

So, I purchased this M6r a while ago with one bad oscillator in voice 3.  I thought it just needed a master-52 style calibration but I was wrong.  The issue was that the one oscillator did track pitch kind of, but it was the wrong pitch.

I had an old 4 channel Tektronix scope I used at first but I wanted something a bit more modern so I picked up a new Hantek 2-channel scope for like $200 and sold the old CRT scope for like $200 - who knew?

After downloading the CEM 3396 datasheet, I looked around with the scope at the various signals and found the CLKA3 and CLKB3 signals that control the frequency of the DCOs in voice 3.  I played 6 notes on the init patch but I changed the voice assignment to rot so that I would have all oscs playing the same note.  Remember that the oscillators are always oscillating even if the VCA is closed.

So, with this configuration, CLKA1 - CLKB6 should all be the same.  When probing I found that CLKB3 was not the same frequency as all the others.  I then went down a rabbit hole trying to run the resonator calibration procedure in the service manual but that was useless - why will be apparent in a moment.

CLKB3 is generated by U728 which is a Mitsubishi M5M82C54P-6 programmable interval timer.  The M6/6r have three resonator circuits that run at about 3MHz (IIRC) that clock the the four 8254s - this is the big difference between the 6 and the M1k - only one resonator for the whole instrument and why I couldn’t quite use the schematics for the M1k of which there’s a hi-res digital version available. I did find a high enough quality scan of the original hand drawn M6 schematics from Oberheim where you can actually read the tags.  After checking, all the resonators were running within spec.  

Signal FBU on the schematic is the 3MHz clock from the resonator that runs the 8254s and is assigned to oscillator 2 of voices 3, 4, 5 and 6 (CLKB3, CLKB4, CLKB5, CLKB6).  Since all the other oscillators were in tune, it’s definitely not the resonator.

I didn’t get too deep into the how exactly the 8254 chip is used but it’s either going to free run at a particular frequency until told otherwise (which is probably how it’s used) or it has to be reset every time the programmed interval time is reached which would be quite processor intensive.  Doesn’t really matter though.   The 8254 chip only has digital inputs other than the clock input so that lead me to believe that the 8254 had gone bad or possible a trace or pin or something had gone bad on that particular one as all the other timers were working fine.

Just to elaborate a bit on this for folks with less ECE experience as this is the actual moment of diagnosis and how I figured it out- there’s only one data bus from the microcontroller (MCU - 6809 in this case) that is shared and talks to all the chips with digital inputs. There are also chip select lines to make sure the MCU is talking to the right chip - the MCU selects the timer chip - writes a few bytes of data and moves on to the next thing.  If the other timer chips are working, and the only control of the chip is on the data bus, the data bus signals themselves must be good…which means that the timer chip must be bad.

I de-soldered U728, popped in a socket just in case and put in a NOS or possibly OOS replacement chip I found on eBay and boom - synth is fixed.  It was really quite satisfying to fix as this mixed digital/analog stuff was what I was really really good at in school and never really got a chance to use it (school was a LONG time ago).  Shoulda moved to Cali and found a job at a synth place when I graduated I guess.  Instead, I think I’ll look for more M6s to repair as I know it quite well now and maybe expand out a bit as a hobby.


Andrij Birko
andy.birko@... <mailto:andy.birko@...>





On Jun 14, 2022, at 1:30 PM, Dennis Barrington <db9091audio@... <mailto:db9091audio@...> > wrote:

I like the EE stuff.  Be cool to know more details about tool you used to sleuth it out!




On Jun 14, 2022, at 12:58 PM, Andy Birko <andy.birko@... <mailto:andy.birko@...> > wrote:



Hi All,

I had completely forgotten about this group until I saw some messages come across.  I've recently repaired M6r that I'd like to sell since I still have my original M6 keyboard I got in 1985.  Are Oberheim for sale items ok in this group?  If not - so sorry about that!

p.s., repairing the unit was pretty fun actually!  I'm an EE by degree but due to living in Michigan and working for the auto companies for a long time, I never really had to do much electronics stuff as a pro.  However, after first trying to trace the issue using a schematic for a M1k and figuring that it was in the resonator circuits - the main difference between the M6 and M1k, and then finding downloading a copy of the schematics for the M6r that was actually legible and data a load of data sheets it all started coming back to me - turned out to be a bad timer chip!  I'll pop in a socket for the battery and probably upgrade to 2.15 if I can find it before I sell.











Nicole Massey
 

Yeah, that tends to fix the problem. To be fair, there's a lot pulling from that power strip -- it's a 44 space rack.

I wish someone would do an NRPN OS upgrade for the 6 and 6r like they did for the 1000.

-----Original Message-----
From: Oberheim@groups.io [mailto:Oberheim@groups.io] On Behalf Of Andy Birko
Sent: Tuesday, June 14, 2022 4:34 PM
To: Oberheim@groups.io
Subject: Re: [Oberheim] Items for sale ok?

I’ve seen the gibberish on the display thing before on my original M6 even way back in the day when it was new but the sound still worked if I remember correctly. Shutting it off for a moment and re-starting usually fixed it. Without playing around with the unit, don’t really have any guesses as to why it might be silent. You might try a Master-52 calibration for starters if you haven’t already.

As far as I can remember, OS 2.15 improves the real time sysex response for folks looking to have smoother changes when using knobby editors and so forth.



Andrij Birko
andy.birko@... <mailto:andy.birko@...>





On Jun 14, 2022, at 5:15 PM, Nicole Massey <nyyki@... <mailto:nyyki@...> > wrote:

I understand enough of all that to say, cool.

But this has made me wonder, what does the OS 2.5 bring to the table? I'm not sure what my 6r has in it, and if I power it up with the rest of the rack I get gibberish on the display and no sound. I also will need someone else to do this, as this is way too complex for a blind person to do any repairs. But I can at least whoever I get to fix it what to look for.
Nicole

-----Original Message-----
From: Oberheim@groups.io <mailto:Oberheim@groups.io> [mailto:Oberheim@groups.io] On Behalf Of Andy Birko
Sent: Tuesday, June 14, 2022 3:56 PM
To: Oberheim@groups.io <mailto:Oberheim@groups.io>
Subject: Re: [Oberheim] Items for sale ok?

Thanks for feeding into today's procrastination efforts.

So, I purchased this M6r a while ago with one bad oscillator in voice 3. I thought it just needed a master-52 style calibration but I was wrong. The issue was that the one oscillator did track pitch kind of, but it was the wrong pitch.

I had an old 4 channel Tektronix scope I used at first but I wanted something a bit more modern so I picked up a new Hantek 2-channel scope for like $200 and sold the old CRT scope for like $200 - who knew?

After downloading the CEM 3396 datasheet, I looked around with the scope at the various signals and found the CLKA3 and CLKB3 signals that control the frequency of the DCOs in voice 3. I played 6 notes on the init patch but I changed the voice assignment to rot so that I would have all oscs playing the same note. Remember that the oscillators are always oscillating even if the VCA is closed.

So, with this configuration, CLKA1 - CLKB6 should all be the same. When probing I found that CLKB3 was not the same frequency as all the others. I then went down a rabbit hole trying to run the resonator calibration procedure in the service manual but that was useless - why will be apparent in a moment.

CLKB3 is generated by U728 which is a Mitsubishi M5M82C54P-6 programmable interval timer. The M6/6r have three resonator circuits that run at about 3MHz (IIRC) that clock the the four 8254s - this is the big difference between the 6 and the M1k - only one resonator for the whole instrument and why I couldn’t quite use the schematics for the M1k of which there’s a hi-res digital version available. I did find a high enough quality scan of the original hand drawn M6 schematics from Oberheim where you can actually read the tags. After checking, all the resonators were running within spec.

Signal FBU on the schematic is the 3MHz clock from the resonator that runs the 8254s and is assigned to oscillator 2 of voices 3, 4, 5 and 6 (CLKB3, CLKB4, CLKB5, CLKB6). Since all the other oscillators were in tune, it’s definitely not the resonator.

I didn’t get too deep into the how exactly the 8254 chip is used but it’s either going to free run at a particular frequency until told otherwise (which is probably how it’s used) or it has to be reset every time the programmed interval time is reached which would be quite processor intensive. Doesn’t really matter though. The 8254 chip only has digital inputs other than the clock input so that lead me to believe that the 8254 had gone bad or possible a trace or pin or something had gone bad on that particular one as all the other timers were working fine.

Just to elaborate a bit on this for folks with less ECE experience as this is the actual moment of diagnosis and how I figured it out- there’s only one data bus from the microcontroller (MCU - 6809 in this case) that is shared and talks to all the chips with digital inputs. There are also chip select lines to make sure the MCU is talking to the right chip - the MCU selects the timer chip - writes a few bytes of data and moves on to the next thing. If the other timer chips are working, and the only control of the chip is on the data bus, the data bus signals themselves must be good…which means that the timer chip must be bad.

I de-soldered U728, popped in a socket just in case and put in a NOS or possibly OOS replacement chip I found on eBay and boom - synth is fixed. It was really quite satisfying to fix as this mixed digital/analog stuff was what I was really really good at in school and never really got a chance to use it (school was a LONG time ago). Shoulda moved to Cali and found a job at a synth place when I graduated I guess. Instead, I think I’ll look for more M6s to repair as I know it quite well now and maybe expand out a bit as a hobby.


Andrij Birko
andy.birko@... <mailto:andy.birko@...> <mailto:andy.birko@...>





On Jun 14, 2022, at 1:30 PM, Dennis Barrington <db9091audio@... <mailto:db9091audio@...> <mailto:db9091audio@...> > wrote:

I like the EE stuff. Be cool to know more details about tool you used to sleuth it out!




On Jun 14, 2022, at 12:58 PM, Andy Birko <andy.birko@... <mailto:andy.birko@...> <mailto:andy.birko@...> > wrote:



Hi All,

I had completely forgotten about this group until I saw some messages come across. I've recently repaired M6r that I'd like to sell since I still have my original M6 keyboard I got in 1985. Are Oberheim for sale items ok in this group? If not - so sorry about that!

p.s., repairing the unit was pretty fun actually! I'm an EE by degree but due to living in Michigan and working for the auto companies for a long time, I never really had to do much electronics stuff as a pro. However, after first trying to trace the issue using a schematic for a M1k and figuring that it was in the resonator circuits - the main difference between the M6 and M1k, and then finding downloading a copy of the schematics for the M6r that was actually legible and data a load of data sheets it all started coming back to me - turned out to be a bad timer chip! I'll pop in a socket for the battery and probably upgrade to 2.15 if I can find it before I sell.