Discussion about the Korg PolySix synthesizer

Your scope looks quite usable.   25 MHz should be OK for what you will find in 
most synths.    You will get a little rounding of corners, but overall it should be 
plenty good.
Op amp circuits that have feedback around the chip, like voltage followers,or inverting or non-inverting amplifiers, are easy to analyze with just two basic rules.
1) The op amp will move its output to try to keep the two inputs at the same voltage.
2) The two inputs will draw essentially no current from the circuity that feeds them.
The second is an approximation, but works fine in most cases.   So for IC9 pin 2,there is feedback around the op amp and this is just an amplifier.  Since pin 3is tied to gnd, we know that the op amp should be trying to set its output so that pin 2is also at ground.   Since that's not what you are seeing, then the op amp is suspect.
   If the op amp is something special, like an OTA (LM13600, CA3080) or an LM3900,then the above rules don't apply.

    Bob
 
      From: "mpe1969@tpg.com.au [PolySix]" <PolySix@yahoogroups.com>
 To: PolySix@yahoogroups.com 
 Sent: Tuesday, May 5, 2015 9:18 AM
 Subject: Re: [PolySix] Re: Key assign mode issue
   
    Thanks (just about to go to bed, but did another quick check). IC9 pin2 is definately  showing the same as pin 1. (problem)The oscilloscope works in reverse:.5 = 10v/ 2cm, 1 = 10v/1cm, 2 = 10v/2.5cm.So in these photo's 2cm =10v approxIt's an old Australian made oscilloscope called a bwd 539d. I'll have another go with it tomorrow night (will turn off auto!) I've mainly just used it for audio in the past. 
Thanks again, I will also change over ic9 tomorrow night. 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