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Discussion about the Korg PolySix synthesizer

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Filter cutoff not working. Thermistor?

Filter cutoff not working. Thermistor?

2018-06-24 by anthony.putzier@gmail.com

I am in the middle of restoring a p6 I aquired recently and have been unable to troubleshoot the cutoff not working. Nothing effects the cutoff (EG, KBD trk, etc) I have traced the +-14v signal from the pot on the face of the synth to pin 2, ic13 on klm366. Up to that point everything works as I would expect (the voltage varies from -14v to +14 as I turn the knob from low to high). After the first opamp stage of ic13 (pin 1) the voltage jumps dramatically when I turn the knob from -13 volts on the lower half of the turn, then jumps to +14 on the upper half of the knob turn and next to nothing in between. I have replaced ic13 and that didn't fix it. Could it be the thermistor th4 after ic13? Any help would be appreciated! Thanks!

Re: [PolySix] Filter cutoff not working. Thermistor?

2018-06-25 by <backshall1@bellsouth.net>

I hope you mean +-5V, not 14. I’m pretty sure there are no pots connected to the 15V supply.
Don B.

From: anthony.putzier@gmail.com [PolySix] 
Sent: Sunday, June 24, 2018 7:46 PM
To: PolySix@yahoogroups.com 
Subject: [PolySix] Filter cutoff not working. Thermistor?

  
I am in the middle of restoring a p6 I aquired recently and have been unable to troubleshoot the cutoff not working. Nothing effects the cutoff (EG, KBD trk, etc) I have traced the +-14v signal from the pot on the face of the synth to pin 2, ic13 on klm366. Up to that point everything works as I would expect (the voltage varies from -14v to +14 as I turn the knob from low to high). After the first opamp stage of ic13 (pin 1) the voltage jumps dramatically when I turn the knob from -13 volts on the lower half of the turn, then jumps to +14 on the upper half of the knob turn and next to nothing in between. I have replaced ic13 and that didn't fix it. Could it be the thermistor th4 after ic13? Any help would be appreciated! Thanks!

Re: [PolySix] Filter cutoff not working. Thermistor?

2018-06-25 by <backshall1@bellsouth.net>

Also, from the way it is acting, it sounds like you have a 393 in the IC slot where it should be a 4558, but IC13 is for PWM speed and MG speed, not Cutoff. Cutoff should be on IC7 pin 7.
Show quoted textHide quoted text
From: backshall1@bellsouth.net 
Sent: Sunday, June 24, 2018 8:43 PM
To: PolySix@yahoogroups.com 
Subject: Re: [PolySix] Filter cutoff not working. Thermistor?

I hope you mean +-5V, not 14. I’m pretty sure there are no pots connected to the 15V supply.
Don B.

From: anthony.putzier@gmail.com [PolySix] 
Sent: Sunday, June 24, 2018 7:46 PM
To: PolySix@yahoogroups.com 
Subject: [PolySix] Filter cutoff not working. Thermistor?

  
I am in the middle of restoring a p6 I aquired recently and have been unable to troubleshoot the cutoff not working. Nothing effects the cutoff (EG, KBD trk, etc) I have traced the +-14v signal from the pot on the face of the synth to pin 2, ic13 on klm366. Up to that point everything works as I would expect (the voltage varies from -14v to +14 as I turn the knob from low to high). After the first opamp stage of ic13 (pin 1) the voltage jumps dramatically when I turn the knob from -13 volts on the lower half of the turn, then jumps to +14 on the upper half of the knob turn and next to nothing in between. I have replaced ic13 and that didn't fix it. Could it be the thermistor th4 after ic13? Any help would be appreciated! Thanks!

Re: [PolySix] Filter cutoff not working. Thermistor?

2018-06-25 by <backshall1@bellsouth.net>

Oops, I was looking at IC7 on the KLM-367, not the 366 board. Where the heck is IC13 on the 366?
Show quoted textHide quoted text
From: backshall1@bellsouth.net 
Sent: Sunday, June 24, 2018 8:49 PM
To: PolySix@yahoogroups.com 
Subject: Re: [PolySix] Filter cutoff not working. Thermistor?

  Also, from the way it is acting, it sounds like you have a 393 in the IC slot where it should be a 4558, but IC13 is for PWM speed and MG speed, not Cutoff. Cutoff should be on IC7 pin 7.

From: backshall1@bellsouth.net 
Sent: Sunday, June 24, 2018 8:43 PM
To: PolySix@yahoogroups.com 
Subject: Re: [PolySix] Filter cutoff not working. Thermistor?

I hope you mean +-5V, not 14. I’m pretty sure there are no pots connected to the 15V supply.
Don B.

From: anthony.putzier@gmail.com [PolySix] 
Sent: Sunday, June 24, 2018 7:46 PM
To: PolySix@yahoogroups.com 
Subject: [PolySix] Filter cutoff not working. Thermistor?

  
I am in the middle of restoring a p6 I aquired recently and have been unable to troubleshoot the cutoff not working. Nothing effects the cutoff (EG, KBD trk, etc) I have traced the +-14v signal from the pot on the face of the synth to pin 2, ic13 on klm366. Up to that point everything works as I would expect (the voltage varies from -14v to +14 as I turn the knob from low to high). After the first opamp stage of ic13 (pin 1) the voltage jumps dramatically when I turn the knob from -13 volts on the lower half of the turn, then jumps to +14 on the upper half of the knob turn and next to nothing in between. I have replaced ic13 and that didn't fix it. Could it be the thermistor th4 after ic13? Any help would be appreciated! Thanks!

Re: [PolySix] Filter cutoff not working. Thermistor?

2018-06-25 by Bob Grieb

The thermistor is between IC13 pin 1 and the other half of IC13.
If pin 1 is misbehaving, I don't think the thermistor is the culprit.
pin 2 is a virtual ground on IC13, so it should be at 0V if the op amp
is working amd has feedback.  If IC13 pin 1 jumps from the negative 
rail to the positive one, that sounds like the op amp does not have feedback, 
which in this case would be R64.   If that's open, then the op amp behaves 
like a comparator with one input at ground.   Check the resistance from pin 1 to pin 2
on IC13 with the power off.

   Bob


--------------------------------------------
On Sun, 6/24/18, anthony.putzier@gmail.com [PolySix] <PolySix@yahoogroups.com> wrote:
Show quoted textHide quoted text
 Subject: [PolySix] Filter cutoff not working. Thermistor?
 To: PolySix@yahoogroups.com
 Date: Sunday, June 24, 2018, 7:46 PM
 
 
  
 
 
 
   
 
 
     
       
       
       I am in the middle of restoring a p6 I aquired
 recently and have been unable to troubleshoot the cutoff not
 working. Nothing effects the cutoff (EG, KBD trk, etc) I
 have traced the +-14v signal from the pot on the face of the
 synth to pin 2, ic13 on klm366. Up to that point everything
 works as I would expect (the voltage varies from -14v to +14
 as I turn the knob from low to high). After the first opamp
 stage of ic13 (pin 1) the voltage jumps dramatically when I
 turn the knob from -13 volts on the lower half of the turn,
 then jumps to +14 on the upper half of the knob turn and
 next to nothing in between. I have replaced ic13 and that
 didn't fix it. Could it be the thermistor th4 after
 ic13? Any help would be appreciated! Thanks!
 
     
      
 
     
     
 
 
 
 #yiv5324291115 #yiv5324291115 --

Re: [PolySix] Filter cutoff not working. Thermistor?

2018-06-25 by anthony.putzier@gmail.com

That totally sounds right. I'm getting .58 meg ohms resistance on pins 1 and 2 of ic13, much more than the 90.9k ohms it should be. I'll order a replacement and see where that gets me. Thanks for the help.

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