Moog MG-1 Multitrigger Mod - Pitch Descending After Release

[facon]

Hi,

This is my first post here and I am hoping to receive some help.

I just installed a mod on my Moog MG-1 that adds retriggering and continuous glide. The retrigger works great and I would love to have this option on my synth, but there is a problem; As soon as a key is released, the pitch immediately starts descending.

The mod information can be found here: http://machines.hyperreal.org/manufacturers/Moog/MG-1/mods/MG-1.multitrig.mod.txt

The schematic says that it needs +/-15V, however the MG-1 supplies +12V and -15V. I don't think this is an issue with the ICs, but it could perhaps play a role in the 100K resistor value used?

I've gone over and over my stripboard layout and it looks good to me. There aren't any bridged connections. One thing to note is when I bridge the KBD CV input and the Glide Pot output together (with the retrigger circuit still in), the pitch ascends instead of descends when a note is released.

I thought that perhaps this was a preexisting issue that the continuous glide is now highlighting. The MG-1 Service Manual mentions this issue.

Symptom: Pitch of both Tone Sources changes after key is released
Cause/Remedy: 1)Defective Keyboard Track and Hold circuit U1007A, C1013. 2) Defective Keyboard Current Source U2009.

I replaced U1007A and C1013 with no change. I did not replace U2009, but I did verify that voltages were correct.

I've drawn up the schematic below so it is a bit easier to read. If you have any advice or input, please let me know. Thank you!




Link main main board schematic - http://jdilisio.com/synth/MG-1Schematics.jpg

Link to keyboard schematic - http://www.sdiy.org/philgallo/mg1_kbd.jpg

[varice]

to the electro-music forum facon

The first op-amp in your mod forms a simple track and hold circuit. The problem you describe would seem to indicate that it is not holding the CV very well after key release. This may be caused by the charge bleeding down at the cap C1. Did you use a low leakage cap for C1 as described in the mod instructions? Also, make sure to clean all of the solder flux off the circuits. Some solder rosins are actually slightly conductive and can cause problems like this!

BTW, there is an error in your schematic with the op-amp input pins. The feedback loop from pin 7 should be connected to the inverting input pin 6, the node of R1 and C1 should be connected to the non-inverting input pin 5.

One of the first mods I made to my MG-1 was to add a better track and hold circuit. I think that the main reason was that the “glide stop on release” issue was unacceptable. It also greatly improved the CV hold from drifting too badly after key release. I forget the exact values of the components that I used, but I think that I used a larger than 1 uF cap. A larger value will hold the voltage longer (slowing pitch drift after key release). You need to be careful not to use too large a value or you will have a permanent minimum glide time, as it will take longer for the cap to charge to new CV voltages.

[facon]

Thank you for your response!

At first I used a 1uF electrolytic (Elna Silmic II) because I had it handy. After hearing the pitch change, I went back through and swapped it in with film (using three of them in parallel). I have plenty higher values handy, I'll try them out to see if it changes anything.

I'll give the bottom another cleaning as well.

Good catch on the schematic. I'll swap that back around so it doesn't get around.

[facon]

Problem solved! I focused on cleaning up around the 1uf cap with rubbing alcohol and an xacto. I went over the entire board twice before, but this gave me a clear direction on where to focus.

[varice]