Polysix - Modifying the MG Delay Circuit

As I've been discussing in the last few posts, the MG Delay circuit on my Korg Polysix doesn't work quite right -- even with an MG Delay of zero, it still suppresses my MG signal at the start of each new note.  In my most recent post, I found that my MG Delay control voltage (CV) from the Polysix DAC is still a tiny bit too high, even when set to zero.  Since I can't do much about the DAC, I chose instead to modify the keypress signal that is compared to the MG Delay. The result is that my MG Delay now works as I think that it should -- when set to zero, the MG signal is applied smoothly across all notes, with no MG transients at the start of the notes.  Below is an audio demo with the MG Delay modified (top) compared to how it sounded on my Polysix prior to modification (bottom).  In addition to the MG Delay being set to zero in both cases, all of the other settings are the same, too.



Find the MG Delay Circuit:  A schematic of the relevant part of the Polysix circuitry is shown below.  The MG Delay functionality is effected by IC14, which compares the MG Delay CV produced by the DAC sample-and-hold (arriving at Pin 2 of IC14) to a voltage signal that pulses which each new keypress (arriving at Pin 3 of IC 14).  Whenever Pin 3 goes lower than Pin 2, the MG is suppressed.  I'd like modify the circuit so that, when the MG Delay is set to zero, the voltage at Pin 3 does not drop below whatever voltage is being delivered by the DAC to Pin 2.

Swap R98 to Control the Voltage Drop at Pin 3:  The voltage at Pin 3 drops with each new keypress because a keypress causes Q5 to conduct, which allows the charge stored in C33 to drain out via R98. Because Q5 only conducts for a short amount of time, we can limit how low C33 gets by constricting the flow of charge out of C33.  The easiest way to do that is to simply swap R98 from its default 4.7K value to a higher value.  At first, I tried 20K, but decided that 10K was better.

Removing R98.

New R98.  I first tried 20K.  I settled on 10K.

Viewing the Impact of R98:  To visually confirming that I correctly affected the voltage at Pin 3, I used my oscilloscope to view the voltage at Pin 3 (as altered by my modification of R98) and the voltage at Pin 2 (the MG Delay CV).  As shown in the picture below, changing R98 now keeps the voltage at Pin 3 from dropping below the MG Delay CV that is being applied to Pin 2.  As a result, the output of IC14 now stays high, which means that the MG signal is not suppressed by the start of the note.  Success!

Ensuring I Did Not Over-Correct:  It would be very easy to use too large a value for R98.  How do you know that you have an OK value?  If the resistor were too large, then the MG Delay functionality would be defeated for values other than zero.  I'm not trying to completely defeat the MG Delay, I just wanted zero to be zero.  So, to make sure that my MG Delay still worked well, I turned the MG Delay knob a little bit until the DAC put out a higher MG Delay CV.  It turns out that there is a pretty big dead zone on my MG Delay knob (that is unrelated at all to changes to R98)...I had to turn my knob all the way up to "1" in order to see the DAC output tick up one notch.  Once I saw it tick up, I retested the voltage at Pin 3 to ensure that it dropped low enough to trigger the MG suppression.  With my initial change to an R98 of 20K, Pin 3 did not drop low enough.  By swapping R98 to 10K, I got the result shown in the picture below.  As you can see, Pin 3 does indeed drop briefly below Pin 2, as desired.  Success!

So, with this modification, I have achieved my goal of making the MG Delay work as I want.  When turned to zero, the MG Delay is defeated.  When turned slightly above zero, the MG Delay works as before.  I'm pleased.