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It rises from the grave (Part 3)

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  • Steve Carter
    Changing all the ICs on the KLM 356 board and a 2SA733 transistor that had shorted was the next task and with that done, it was time to test the synth to see
    Message 1 of 9 , Jul 2, 2010
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      Changing all the ICs on the KLM 356 board and a 2SA733 transistor that had shorted was the next task and with that done, it was time to test the synth to see if would work albeit still on life support with the bench power supply.  With everything now wired back together as it ought, I applied power and tried the keyboard to see if anything, sound or triggering wise would happen.  Nothing - it was a bit like Steve Race on 'Face the Music' with his dummy keyboard - a rendition of Bach's French Suite Courante in Cm sounding more like a 1960s typing pool.  I also checked the key assign buttons on the front panel and they didn't change state so it looked like the processor wasn't scanning any switchgear, keyboard or otherwise.  This assumption was only half right as the key assign switch scanning was cured by changing IC2 (74LS174) even though it had previously been changed. It may have been a faulty new part or it was out of spec.  Why is IC2 and IC3 the only ICs that are socketed on that board? Anyway, that only left the inactive keyboard and it didn't take much to realise that the keyboard was on shared data and address lines that other switches worked OK on so could it be the keyboard itself? I've seen a lot of these membrane type key switches give problems but never one where every single note was out - but that is what the problem was - every single note was out.  Removing all the rubber contact strips and then cleaning the gold plated contacts on the printed circuit board and the black conductive rubber 'pips'  using isopropyl alcohol, cured the problem perfectly.

      Although the keyboard now worked, there was something very much amiss. Scaling, tracking, calibrating? If the synth was working perfectly before the previous owner discharged the contents of the National Grid into it, then why was the calibration so far out now?  I was still very suspicious of the SSM2033 ICs and convinced that they were faulty.  However it always pays to completely ignore what previous owners say about a synth - "honest guv', it was working fine before I ran over it in the car"  could really mean "there was a fault with it and so I turned every trim pot I could find and it got worse - it was then that I decided to reverse over it in the SUV in a fit of frustration".  I was to discover later that my Mono/Poly had been a victim of trimpot butchery at some stage.

      Before carrying out a full calibration as given in the service manual and still worrying about those SSM2033s, I thought it was about time to do something about the power supply as I was still operating from the bench supply.  I replaced the 110 volt mains transformer with a 240 volt transformer of similar dimensions - this in itself was a bad move as I will reveal later.  Moving on to the power supply board KLM-376, it was in a bad way with burned-out components as well as all the semis. trashed.  The M5230 IC is difficult to source and I don't like the design of the supply insomuch as that there is no protection whatsoever in the event of overvoltage, short circuit or low flying hang gliders.  The + and - series regulator transistors have to sink around 14 volts at about half an amp (more on the -15V) and run very hot indeed especially with the inadequate heatsinking.  So, the plan was to just get the power supply up and running as quickly as possible and design a new one to be fitted at a later date.  I used the same original transistor arrangement for the + 15 and -15volt rails and used a fixed 78L15 regulator as a stabilised source for the +15v which gave an actual 15.3volts (calibration states + or - 0.5V) and a variable LM337L regulator with a 10 turn trimpot to set the -15v to within the required + or - 0.01v. These new components were easily accommodated on the existing board once the old components were removed.  The existing arrangement to provide the +5v was used using all new components but with a ten turn trimpot used to make the adjustment to within 0.01v easier when setting up. 

      The power supply works fine although the power transistors on the 15 volt lines run hotter than I'd like and there's no real protection.  It will suffice until the new  improved PSU is built. It will be super accurate and have very fast built-in overvolts protection. 

      Next time... tuning, ranging, scaling, loud hum, stiff drink.......
    • Steve Carter
      This is the last part of my tale of woe. I can already hear shouts of who is this guy, where did he crawl out from and why does he think we re interested in
      Message 2 of 9 , Jul 3, 2010
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        This is the last part of my tale of woe. I can already hear shouts of "who is this guy, where did he crawl out from and why does he think we're interested in his trainwreck of a Mono/Poly" Anyway I promise you this is the last!

        The final job was to look at the board (KLM-398) that controls the offset voltage to the VCOs and the board (KLM-357) that controls the heating current in the SSM2033s. The two TL072 ICs had blown on the offset board and the two 4558 ICs had bitten the dust but the four 2SD794 power transistors were OK on the heater control board. After changing the ICs I connected the plug to the board and checked that the SSM2033s were getting warm. They certainly were. It was at this stage I thought that I'd better look into the architecture of the Solid State Micro SSM2033 - I wasn't familiar with the chip having cut my teeth on discrete VCOs and then Curtis chips. Looking at the datasheet showed that the negative supply on pin 2 had to be fed via a series resistor of 620R to reduce the current thus allowing the on board 8v zener to maintain a steady 8v. Was this one of the reasons that the ICs hadn't blown? Any series resistance and zener combination has to increase the odds of chip survival in a fry-up. OK so that explains the negative rail but the datasheet shows that pin1 goes straight to the positive rail which is +15v. So maybe the +15v rail didn't go into a massive overvolts situation and only the -15v and +5 volts rail did. All conjecture and maybe faster ICs took the brunt of the surge and shorted out more quickly than the 2033s could respond; either way it was still looking promising for the 2033s.

        It was now time for the calibration as per Korg's service manual. Mmm. Getting the power rails correct was easy enough - they have to be within plus or minus 0.01v for the -15v and +5v rails and within 0.5v for the +15v rail. Next up, the key assigner adjustment. This needs an accurate DVM that can resolve to 3 decimal places. With the 'Tune Point' jumper in place on the KLM-356 board, set VR1 to give 10.583 volts at pin 1 on connector 27 (CV1 Out). This is done with the transpose switch set to 'Up'. Next, set VR2 to give 0.000 volts this time with the transpose switch in the 'Down' position. Finally, adjust VR3 to give 5.250 volts with the transpose switch set to 'Normal'. OK, this went smoothly but I noticed something whilst making the adjustments - all three trim pots were fully clockwise before I adjusted them - a pattern to be repeated later on.

        Next the offset voltages for the four VCOs needed setting - this is done on the piggy back board to the VCO board. The manual doesn't make it clear where this is measured and gives 4 colours for each VCO which I presumed referred to the wires coming from the little board. This wasn't the case, or wasn't with mine anyway.Just measure the voltage on connector 39 on pins 1,3,5 and 7 accordingly and set the respective trimpot for the lowest to zero reading you can.

        OK, with that done it was time to calibrate the VCOs. There have been postings regarding this recently and there's little to add except that I didn't adopt stretch tuning. This is procedure I used: Set VR103 to give 55Hz on bottom A at 16', set VR102 to give 440Hz on top A at 16', set VR101 to give 440Hz on bottom A at 2' and set VR104 to give 3520Hz on top A at 2'. This is repeated with the respective trimpots for all 4 oscillators. Now here is where my earlier comments come into play again - every trimpot for the VCO pitch and tracking had been turned fully clockwise - which shows that you should never take anything on face value or trust the previous owner's story that "it worked perfectly OK until.... " The fact that the trimpots had been set fully clockwise actually causes a headache when trying to calibrate because if you start from that point you can't bring in the top A at 2' - it is always sharp. The four trimpots are relative to each other to some degree and the only way to calibrate successfully is to start with each of the trimpots in the centre position to start with. I wish I'd discovered that earlier - it would have saved a lot of time and having to listen to those tones over and over again drives you crazy! Anyway the really good news is that everything calibrated perfectly and the VCO chips are fine. A testament to the Solid State Micro design of these ICs. A very clever and versatile chip indeed.

        The very last problem only came to light upon reassembly of the Mono/Poly. I screwed everything back together and started to play it and there was significant mains hum present but only when the VCA was 'open' - no notes played everything was fine, play a note and you get a mains hum. It wasn't nasty hum, just an induced kind of hum. Cutting a long story short, it turned out to be the new 240v mains transformer inducing hum into VCA/VCF board simply through proximity. I put the original 110V mains transformer back in its place and used an external stepdown transformer and everything is now fine. The 240v transformer although oriented the same way as the original and in the same place must have a stronger magnetic radiation than the original. As a final precaution I found an American mains lead which I wired in so it's impossible to plug it into a UK supply and only into the adaptor.

        Well there you have it - a marathon affair but a great sounding synth at the end of all that and definitely worth the effort. I hope some this is useful to list members and I'll shove the whole document together as a pdf and stick it in the Misc Files folder.


        Steve
      • joe-joe
        these posts are not an issue at all. you have to realize that they are all saved in the yahoo group for future mono-poly users to research and use. never know
        Message 3 of 9 , Jul 3, 2010
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          these posts are not an issue at all. you have to realize that they are all saved in the yahoo group for future mono-poly users to research and use. never know this post could help others in the future. thanks for the info.

          --- In korg_mono-poly@yahoogroups.com, Steve Carter <synth@...> wrote:
          >
          > This is the last part of my tale of woe. I can already hear shouts of "who is this guy, where did he crawl out from and why does he think we're interested in his trainwreck of a Mono/Poly" Anyway I promise you this is the last!
          >
          > The final job was to look at the board (KLM-398) that controls the offset voltage to the VCOs and the board (KLM-357) that controls the heating current in the SSM2033s. The two TL072 ICs had blown on the offset board and the two 4558 ICs had bitten the dust but the four 2SD794 power transistors were OK on the heater control board. After changing the ICs I connected the plug to the board and checked that the SSM2033s were getting warm. They certainly were. It was at this stage I thought that I'd better look into the architecture of the Solid State Micro SSM2033 - I wasn't familiar with the chip having cut my teeth on discrete VCOs and then Curtis chips. Looking at the datasheet showed that the negative supply on pin 2 had to be fed via a series resistor of 620R to reduce the current thus allowing the on board 8v zener to maintain a steady 8v. Was this one of the reasons that the ICs hadn't blown? Any series resistance and zener combination has to increase the odds of chip survival in a fry-up. OK so that explains the negative rail but the datasheet shows that pin1 goes straight to the positive rail which is +15v. So maybe the +15v rail didn't go into a massive overvolts situation and only the -15v and +5 volts rail did. All conjecture and maybe faster ICs took the brunt of the surge and shorted out more quickly than the 2033s could respond; either way it was still looking promising for the 2033s.
          >
          > It was now time for the calibration as per Korg's service manual. Mmm. Getting the power rails correct was easy enough - they have to be within plus or minus 0.01v for the -15v and +5v rails and within 0.5v for the +15v rail. Next up, the key assigner adjustment. This needs an accurate DVM that can resolve to 3 decimal places. With the 'Tune Point' jumper in place on the KLM-356 board, set VR1 to give 10.583 volts at pin 1 on connector 27 (CV1 Out). This is done with the transpose switch set to 'Up'. Next, set VR2 to give 0.000 volts this time with the transpose switch in the 'Down' position. Finally, adjust VR3 to give 5.250 volts with the transpose switch set to 'Normal'. OK, this went smoothly but I noticed something whilst making the adjustments - all three trim pots were fully clockwise before I adjusted them - a pattern to be repeated later on.
          >
          > Next the offset voltages for the four VCOs needed setting - this is done on the piggy back board to the VCO board. The manual doesn't make it clear where this is measured and gives 4 colours for each VCO which I presumed referred to the wires coming from the little board. This wasn't the case, or wasn't with mine anyway.Just measure the voltage on connector 39 on pins 1,3,5 and 7 accordingly and set the respective trimpot for the lowest to zero reading you can.
          >
          > OK, with that done it was time to calibrate the VCOs. There have been postings regarding this recently and there's little to add except that I didn't adopt stretch tuning. This is procedure I used: Set VR103 to give 55Hz on bottom A at 16', set VR102 to give 440Hz on top A at 16', set VR101 to give 440Hz on bottom A at 2' and set VR104 to give 3520Hz on top A at 2'. This is repeated with the respective trimpots for all 4 oscillators. Now here is where my earlier comments come into play again - every trimpot for the VCO pitch and tracking had been turned fully clockwise - which shows that you should never take anything on face value or trust the previous owner's story that "it worked perfectly OK until.... " The fact that the trimpots had been set fully clockwise actually causes a headache when trying to calibrate because if you start from that point you can't bring in the top A at 2' - it is always sharp. The four trimpots are relative to each other to some degree and the only way to calibrate successfully is to start with each of the trimpots in the centre position to start with. I wish I'd discovered that earlier - it would have saved a lot of time and having to listen to those tones over and over again drives you crazy! Anyway the really good news is that everything calibrated perfectly and the VCO chips are fine. A testament to the Solid State Micro design of these ICs. A very clever and versatile chip indeed.
          >
          > The very last problem only came to light upon reassembly of the Mono/Poly. I screwed everything back together and started to play it and there was significant mains hum present but only when the VCA was 'open' - no notes played everything was fine, play a note and you get a mains hum. It wasn't nasty hum, just an induced kind of hum. Cutting a long story short, it turned out to be the new 240v mains transformer inducing hum into VCA/VCF board simply through proximity. I put the original 110V mains transformer back in its place and used an external stepdown transformer and everything is now fine. The 240v transformer although oriented the same way as the original and in the same place must have a stronger magnetic radiation than the original. As a final precaution I found an American mains lead which I wired in so it's impossible to plug it into a UK supply and only into the adaptor.
          >
          > Well there you have it - a marathon affair but a great sounding synth at the end of all that and definitely worth the effort. I hope some this is useful to list members and I'll shove the whole document together as a pdf and stick it in the Misc Files folder.
          >
          >
          > Steve
          >
        • Alan Dicker
          Thanks for sharing Steve very informative posts and good to know people are still ready to put the effort in.   Alan ... From: Steve Carter
          Message 4 of 9 , Jul 3, 2010
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            Thanks for sharing Steve very informative posts and good to know people are still ready to put the effort in.
             
            Alan

            --- On Sat, 3/7/10, Steve Carter <synth@...> wrote:

            From: Steve Carter <synth@...>
            Subject: [korg_mono-poly] It rises from the grave (Part 4)
            To: korg_mono-poly@yahoogroups.com
            Date: Saturday, 3 July, 2010, 20:46

             
            This is the last part of my tale of woe. I can already hear shouts of "who is this guy, where did he crawl out from and why does he think we're interested in his trainwreck of a Mono/Poly" Anyway I promise you this is the last!

            The final job was to look at the board (KLM-398) that controls the offset voltage to the VCOs and the board (KLM-357) that controls the heating current in the SSM2033s. The two TL072 ICs had blown on the offset board and the two 4558 ICs had bitten the dust but the four 2SD794 power transistors were OK on the heater control board. After changing the ICs I connected the plug to the board and checked that the SSM2033s were getting warm. They certainly were. It was at this stage I thought that I'd better look into the architecture of the Solid State Micro SSM2033 - I wasn't familiar with the chip having cut my teeth on discrete VCOs and then Curtis chips. Looking at the datasheet showed that the negative supply on pin 2 had to be fed via a series resistor of 620R to reduce the current thus allowing the on board 8v zener to maintain a steady 8v. Was this one of the reasons that the ICs hadn't blown? Any series resistance and zener combination has to increase the odds of chip survival in a fry-up. OK so that explains the negative rail but the datasheet shows that pin1 goes straight to the positive rail which is +15v. So maybe the +15v rail didn't go into a massive overvolts situation and only the -15v and +5 volts rail did. All conjecture and maybe faster ICs took the brunt of the surge and shorted out more quickly than the 2033s could respond; either way it was still looking promising for the 2033s.

            It was now time for the calibration as per Korg's service manual. Mmm. Getting the power rails correct was easy enough - they have to be within plus or minus 0.01v for the -15v and +5v rails and within 0.5v for the +15v rail. Next up, the key assigner adjustment. This needs an accurate DVM that can resolve to 3 decimal places. With the 'Tune Point' jumper in place on the KLM-356 board, set VR1 to give 10.583 volts at pin 1 on connector 27 (CV1 Out). This is done with the transpose switch set to 'Up'. Next, set VR2 to give 0.000 volts this time with the transpose switch in the 'Down' position. Finally, adjust VR3 to give 5.250 volts with the transpose switch set to 'Normal'. OK, this went smoothly but I noticed something whilst making the adjustments - all three trim pots were fully clockwise before I adjusted them - a pattern to be repeated later on.

            Next the offset voltages for the four VCOs needed setting - this is done on the piggy back board to the VCO board. The manual doesn't make it clear where this is measured and gives 4 colours for each VCO which I presumed referred to the wires coming from the little board. This wasn't the case, or wasn't with mine anyway.Just measure the voltage on connector 39 on pins 1,3,5 and 7 accordingly and set the respective trimpot for the lowest to zero reading you can.

            OK, with that done it was time to calibrate the VCOs. There have been postings regarding this recently and there's little to add except that I didn't adopt stretch tuning. This is procedure I used: Set VR103 to give 55Hz on bottom A at 16', set VR102 to give 440Hz on top A at 16', set VR101 to give 440Hz on bottom A at 2' and set VR104 to give 3520Hz on top A at 2'. This is repeated with the respective trimpots for all 4 oscillators. Now here is where my earlier comments come into play again - every trimpot for the VCO pitch and tracking had been turned fully clockwise - which shows that you should never take anything on face value or trust the previous owner's story that "it worked perfectly OK until.... " The fact that the trimpots had been set fully clockwise actually causes a headache when trying to calibrate because if you start from that point you can't bring in the top A at 2' - it is always sharp. The four trimpots are relative to each other to some degree and the only way to calibrate successfully is to start with each of the trimpots in the centre position to start with. I wish I'd discovered that earlier - it would have saved a lot of time and having to listen to those tones over and over again drives you crazy! Anyway the really good news is that everything calibrated perfectly and the VCO chips are fine. A testament to the Solid State Micro design of these ICs. A very clever and versatile chip indeed.

            The very last problem only came to light upon reassembly of the Mono/Poly. I screwed everything back together and started to play it and there was significant mains hum present but only when the VCA was 'open' - no notes played everything was fine, play a note and you get a mains hum. It wasn't nasty hum, just an induced kind of hum. Cutting a long story short, it turned out to be the new 240v mains transformer inducing hum into VCA/VCF board simply through proximity. I put the original 110V mains transformer back in its place and used an external stepdown transformer and everything is now fine. The 240v transformer although oriented the same way as the original and in the same place must have a stronger magnetic radiation than the original. As a final precaution I found an American mains lead which I wired in so it's impossible to plug it into a UK supply and only into the adaptor.

            Well there you have it - a marathon affair but a great sounding synth at the end of all that and definitely worth the effort. I hope some this is useful to list members and I'll shove the whole document together as a pdf and stick it in the Misc Files folder.

            Steve


          • Alain
            Well, Steve, this was a very interesting tale of the resuscitation of your MP4 indeed! Upon reading this fascinating journey, I remembered having had similar
            Message 5 of 9 , Jul 11, 2010
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              Well, Steve, this was a very interesting tale of the resuscitation of your MP4 indeed!

              Upon reading this fascinating journey, I remembered having had similar thoughts about the weak design of the power supply, and the less than perfect calibration procedure from the service manual. Also, that darn Panasonic keyboard misfiring came to mind too.

              Although I performed some definitive modifications on mine that will ensure proper contact for years to come. I simply replaced all the Nylon spacers with thinner ones to close the gap between the rubber contacts and the gold plated pcb. Since these contacts are very similar to the ones in TV remote controls, and having noticed that when those start to be flaky, they WILL still work when pressed down harder, I figured why not for the keyboard of my beloved MP4 also ? Since my modification (about six months ago) not a single key has misfired.

              The transformer hum I had also noticed before (even on my USA model) so I've done another thing there too. I've added a grounded metal shield over the transformer (with some air holes to prevent the already hot thing to get even hotter), taking great care so it doesn't touch the PCB on top of it when the synth is closed up. It's not pretty, but it's inside and it cuts down the hum to almost inaudible levels now.

              Anyway, all this to say that your 4 part story was very well written, absolutely pertinent to this group, and in many ways very helpful. I'm so glad that you could get it back to life so you can enjoy this gem again, and I'll feel a little more confident now that my own four SSM2033s could stand the test of time if yours didn't blow out at twice the power supply input !

              Thanks again!

              Alain.
            • scituate_mass
              Hi Alain, If you have the info on hand, it would be great if you could supply information about the parts (e.g., what thickness worked, part number or source,
              Message 6 of 9 , Jul 12, 2010
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                Hi Alain,

                If you have the info on hand, it would be great if you could supply information about the parts (e.g., what thickness worked, part number or source, etc.) that you used in modding the contacts, as mentioned below:

                "Although I performed some definitive modifications on mine that will ensure proper contact for years to come. I simply replaced all the Nylon spacers with thinner ones to close the gap between the rubber contacts and the gold plated pcb. Since these contacts are very similar to the ones in TV remote controls, and having noticed that when those start to be flaky, they WILL still work when pressed down harder, I figured why not for the keyboard of my beloved MP4 also ? Since my modification (about six months ago) not a single key has misfired. "

                Thanks!

                Damon




                --- In korg_mono-poly@yahoogroups.com, "Alain" <hubertalain@...> wrote:
                >
                >
                >
                > Well, Steve, this was a very interesting tale of the resuscitation of your MP4 indeed!
                >
                > Upon reading this fascinating journey, I remembered having had similar thoughts about the weak design of the power supply, and the less than perfect calibration procedure from the service manual. Also, that darn Panasonic keyboard misfiring came to mind too.
                >
                > Although I performed some definitive modifications on mine that will ensure proper contact for years to come. I simply replaced all the Nylon spacers with thinner ones to close the gap between the rubber contacts and the gold plated pcb. Since these contacts are very similar to the ones in TV remote controls, and having noticed that when those start to be flaky, they WILL still work when pressed down harder, I figured why not for the keyboard of my beloved MP4 also ? Since my modification (about six months ago) not a single key has misfired.
                >
                > The transformer hum I had also noticed before (even on my USA model) so I've done another thing there too. I've added a grounded metal shield over the transformer (with some air holes to prevent the already hot thing to get even hotter), taking great care so it doesn't touch the PCB on top of it when the synth is closed up. It's not pretty, but it's inside and it cuts down the hum to almost inaudible levels now.
                >
                > Anyway, all this to say that your 4 part story was very well written, absolutely pertinent to this group, and in many ways very helpful. I'm so glad that you could get it back to life so you can enjoy this gem again, and I'll feel a little more confident now that my own four SSM2033s could stand the test of time if yours didn't blow out at twice the power supply input !
                >
                > Thanks again!
                >
                > Alain.
                >
              • Alain
                Hello Damon. Sorry for the long delay in responding, but I ve been busy (sold my house, then moved, then replaced my old PC with a new Mac Mini). As for the
                Message 7 of 9 , Aug 3, 2010
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                  Hello Damon. Sorry for the long delay in responding, but I've been busy (sold my house, then moved, then replaced my old PC with a new Mac Mini).

                  As for the info about those spacers, I replaced them as soon as I bought my MP4 because there were some keys already "flaky" when I got it.

                  If I remember right, I was looking for plastic (nylon) washers, or spacers that I had lying around in my stuff. I found only about 4 of them, but it was enough to see if they were the right thickness to bring the PCB a little closer to the keys. They were about half the thickness of the originals, and the right diameter. So after a successful experimentation, I when to my local hardware store and bought the 8 more needed to replace all of the originals (12 in all).

                  The thickness is not too critical, as long as it's not less than half the thickness of the originals (we don't want the keys to push too hard on the rubber contact so as to not damage them). So, just take one out and compare to what you can find. Don't worry about the two little clips of the originals, they're just there to hold them in place. The new ones will be held by the bolts going through them so it doesn't matter, they'll just be a little harder to install.

                  Like I said before, since doing that mod, never once did any key refuse to play. Only drawback: the keys start playing a little earlier in their travel downward, but I can live with that.

                  Note: it's very important that the new washers/spacers be of non-conducting material (such as plastic) in order to prevent any shorting out of electrical signals on the PCB.

                  Alain.





                  --- In korg_mono-poly@yahoogroups.com, "scituate_mass" <sneakyflute@...> wrote:
                  >
                  > Hi Alain,
                  >
                  > If you have the info on hand, it would be great if you could supply information about the parts (e.g., what thickness worked, part number or source, etc.) that you used in modding the contacts, as mentioned below:
                  >
                  > "Although I performed some definitive modifications on mine that will ensure proper contact for years to come. I simply replaced all the Nylon spacers with thinner ones to close the gap between the rubber contacts and the gold plated pcb. Since these contacts are very similar to the ones in TV remote controls, and having noticed that when those start to be flaky, they WILL still work when pressed down harder, I figured why not for the keyboard of my beloved MP4 also ? Since my modification (about six months ago) not a single key has misfired. "
                  >
                  > Thanks!
                  >
                  > Damon
                  >
                  >
                  >
                  >
                  > --- In korg_mono-poly@yahoogroups.com, "Alain" <hubertalain@> wrote:
                  > >
                  > >
                  > >
                  > > Well, Steve, this was a very interesting tale of the resuscitation of your MP4 indeed!
                  > >
                  > > Upon reading this fascinating journey, I remembered having had similar thoughts about the weak design of the power supply, and the less than perfect calibration procedure from the service manual. Also, that darn Panasonic keyboard misfiring came to mind too.
                  > >
                  > > Although I performed some definitive modifications on mine that will ensure proper contact for years to come. I simply replaced all the Nylon spacers with thinner ones to close the gap between the rubber contacts and the gold plated pcb. Since these contacts are very similar to the ones in TV remote controls, and having noticed that when those start to be flaky, they WILL still work when pressed down harder, I figured why not for the keyboard of my beloved MP4 also ? Since my modification (about six months ago) not a single key has misfired.
                  > >
                  > > The transformer hum I had also noticed before (even on my USA model) so I've done another thing there too. I've added a grounded metal shield over the transformer (with some air holes to prevent the already hot thing to get even hotter), taking great care so it doesn't touch the PCB on top of it when the synth is closed up. It's not pretty, but it's inside and it cuts down the hum to almost inaudible levels now.
                  > >
                  > > Anyway, all this to say that your 4 part story was very well written, absolutely pertinent to this group, and in many ways very helpful. I'm so glad that you could get it back to life so you can enjoy this gem again, and I'll feel a little more confident now that my own four SSM2033s could stand the test of time if yours didn't blow out at twice the power supply input !
                  > >
                  > > Thanks again!
                  > >
                  > > Alain.
                  > >
                  >
                • ktony1313
                  This was a fascinating and informative series. I m not really up on the internal details of the MP4, but I know enough basic audio electronics to follow the
                  Message 8 of 9 , Jan 28, 2011
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                    This was a fascinating and informative series. I'm not really up on the internal details of the MP4, but I know enough basic audio electronics to follow the descriptions. Thanks for sharing!

                    --- In korg_mono-poly@yahoogroups.com, Steve Carter <synth@...> wrote:
                    >
                    > This is the last part of my tale of woe. I can already hear shouts of "who is this guy, where did he crawl out from and why does he think we're interested in his trainwreck of a Mono/Poly" Anyway I promise you this is the last!
                    >
                    > The final job was to look at the board (KLM-398) that controls the offset voltage to the VCOs and the board (KLM-357) that controls the heating current in the SSM2033s. The two TL072 ICs had blown on the offset board and the two 4558 ICs had bitten the dust but the four 2SD794 power transistors were OK on the heater control board. After changing the ICs I connected the plug to the board and checked that the SSM2033s were getting warm. They certainly were. It was at this stage I thought that I'd better look into the architecture of the Solid State Micro SSM2033 - I wasn't familiar with the chip having cut my teeth on discrete VCOs and then Curtis chips. Looking at the datasheet showed that the negative supply on pin 2 had to be fed via a series resistor of 620R to reduce the current thus allowing the on board 8v zener to maintain a steady 8v. Was this one of the reasons that the ICs hadn't blown? Any series resistance and zener combination has to increase the odds of chip survival in a fry-up. OK so that explains the negative rail but the datasheet shows that pin1 goes straight to the positive rail which is +15v. So maybe the +15v rail didn't go into a massive overvolts situation and only the -15v and +5 volts rail did. All conjecture and maybe faster ICs took the brunt of the surge and shorted out more quickly than the 2033s could respond; either way it was still looking promising for the 2033s.
                    >
                    > It was now time for the calibration as per Korg's service manual. Mmm. Getting the power rails correct was easy enough - they have to be within plus or minus 0.01v for the -15v and +5v rails and within 0.5v for the +15v rail. Next up, the key assigner adjustment. This needs an accurate DVM that can resolve to 3 decimal places. With the 'Tune Point' jumper in place on the KLM-356 board, set VR1 to give 10.583 volts at pin 1 on connector 27 (CV1 Out). This is done with the transpose switch set to 'Up'. Next, set VR2 to give 0.000 volts this time with the transpose switch in the 'Down' position. Finally, adjust VR3 to give 5.250 volts with the transpose switch set to 'Normal'. OK, this went smoothly but I noticed something whilst making the adjustments - all three trim pots were fully clockwise before I adjusted them - a pattern to be repeated later on.
                    >
                    > Next the offset voltages for the four VCOs needed setting - this is done on the piggy back board to the VCO board. The manual doesn't make it clear where this is measured and gives 4 colours for each VCO which I presumed referred to the wires coming from the little board. This wasn't the case, or wasn't with mine anyway.Just measure the voltage on connector 39 on pins 1,3,5 and 7 accordingly and set the respective trimpot for the lowest to zero reading you can.
                    >
                    > OK, with that done it was time to calibrate the VCOs. There have been postings regarding this recently and there's little to add except that I didn't adopt stretch tuning. This is procedure I used: Set VR103 to give 55Hz on bottom A at 16', set VR102 to give 440Hz on top A at 16', set VR101 to give 440Hz on bottom A at 2' and set VR104 to give 3520Hz on top A at 2'. This is repeated with the respective trimpots for all 4 oscillators. Now here is where my earlier comments come into play again - every trimpot for the VCO pitch and tracking had been turned fully clockwise - which shows that you should never take anything on face value or trust the previous owner's story that "it worked perfectly OK until.... " The fact that the trimpots had been set fully clockwise actually causes a headache when trying to calibrate because if you start from that point you can't bring in the top A at 2' - it is always sharp. The four trimpots are relative to each other to some degree and the only way to calibrate successfully is to start with each of the trimpots in the centre position to start with. I wish I'd discovered that earlier - it would have saved a lot of time and having to listen to those tones over and over again drives you crazy! Anyway the really good news is that everything calibrated perfectly and the VCO chips are fine. A testament to the Solid State Micro design of these ICs. A very clever and versatile chip indeed.
                    >
                    > The very last problem only came to light upon reassembly of the Mono/Poly. I screwed everything back together and started to play it and there was significant mains hum present but only when the VCA was 'open' - no notes played everything was fine, play a note and you get a mains hum. It wasn't nasty hum, just an induced kind of hum. Cutting a long story short, it turned out to be the new 240v mains transformer inducing hum into VCA/VCF board simply through proximity. I put the original 110V mains transformer back in its place and used an external stepdown transformer and everything is now fine. The 240v transformer although oriented the same way as the original and in the same place must have a stronger magnetic radiation than the original. As a final precaution I found an American mains lead which I wired in so it's impossible to plug it into a UK supply and only into the adaptor.
                    >
                    > Well there you have it - a marathon affair but a great sounding synth at the end of all that and definitely worth the effort. I hope some this is useful to list members and I'll shove the whole document together as a pdf and stick it in the Misc Files folder.
                    >
                    >
                    > Steve
                    >
                  • Florian Anwander
                    Hello ... As first: welcome back to the MP comunity. And thanks for the hint. I can only recommend to the new members of the last weeks to poke around (right
                    Message 9 of 9 , Jan 28, 2011
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                      Hello

                      ktony1313 schrieb:
                      > This was a fascinating and informative series. I'm not really up on the internal details of the MP4, but I know enough basic audio electronics to follow the descriptions. Thanks for sharing!
                      As first: welcome back to the MP comunity.
                      And thanks for the hint. I can only recommend to the new members of the
                      last weeks to "poke around" (right word?) in the archives.

                      And yes: Steves articles contain great information!

                      Florian
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