Minimoog Pitch Wheel Mod

Goom · Post by **Goom** » Mon Jun 06, 2011 9:08 pm

Hello all,
I installed the pitch wheel mod (AKA dead band mod) on my Minimoog last night, and now my Mini is playing a octave or two lower than it was before. I believe this is because the mod entails removing the ground connection of the pitch wheel pot, and routing -10 volts there instead.

What would be the best way to restore the correct pitch?

Thanks for any help!

Kevin Lightner · Post by **Kevin Lightner** » Tue Jun 07, 2011 2:42 am

If you did everything correctly, you will just have to loosen the wheel's setscrew, set the pot to the correct pitch (use needle-nose pliers) and then tighten the setscrew again with the wheel set at the center.
It may take a few tries to get it exactly at the deadzone, but you'll likely do it fine once you get the hang of it.
Good luck!

Goom · Post by **Goom** » Tue Jun 07, 2011 8:03 am

Hi Kevin!
The last part of the bulletin's mod entailed loosening the pitch wheel to set it for .1V or less across the diodes. I did that and got it under .05V.

I think it's a 1 (or possibly 2) full octave shift that took place. Would the pitch wheel pot allow me to shift the freq. that far? Or should I leave it to stay within the .1V spec? Maybe I should just redo the oscillator cal. The only other thing i can think of is removing the -10V wire to the pot, and replacing the ground wire on that pin. Would that work?

One thing that did happen was when I pulled the left hand controller out - two wires came loose from the 12 pin cinch jones connector.I resoldered a violet wire to pin 7 (that made two violets at pin 7), and I resoldered a red wire to pin 4. I believe that is where they belong (according to the schematics), but can you verify that this is correct, please?

Goom · Post by **Goom** » Tue Jun 07, 2011 8:30 am

I forgot to mention - The factory bulletin on this mod specs a 27K Ohm resistor. I installed a 25K Ohm (It's all I had on hand). This wouldn't cause the octave shift problem, would it?

Kevin Lightner · Post by **Kevin Lightner** » Tue Jun 07, 2011 9:58 am

I forgot to ask.

If you have the later VCO board, did you add the single 619K resistor?
If the earlier board, did you add the three 330K-332K resistors?

With either VCO board, one must add the resistors above to +10V in order to offset the pitch back to normal.

Can you show me the mod you followed?
A deadband mod requires both the pitchwheel modded as you did, but also the VCO board.
Did you do both?

Goom · Post by **Goom** » Tue Jun 07, 2011 11:59 am

Thanks Kevin. I have the later/last VCO board. I can post the Factory Bulletin (#830) when I get home tonight. I haven't been able to find it on the net for some reason. I did see the 619K resistor mod on the same bulletin, but it said that it was for temp stability, so I didn't do it. Maybe I should...

Technician Larry · Post by **Technician Larry** » Tue Jun 07, 2011 1:50 pm

Goom wrote:Thanks Kevin. I have the later/last VCO board. I can post the Factory Bulletin (#830) when I get home tonight. I haven't been able to find it on the net for some reason. I did see the 619K resistor mod on the same bulletin, but it said that it was for temp stability, so I didn't do it. Maybe I should...

Whoever wrote #830 was not paying attention. I just looked at my copy and there is no question about the statement except that it is wrong. The 619K resistor added from +10V to the summing node of the VCO common CV has nothing to do with temperature stability and clearly resets the VCO range back to where it should be after dropping the output of the pitch bend control to 0 volts.

I'm sure that they specified a 619K resistor INSTEAD OF a 620K resistor for temperature stability reasons since 619K is only available in a precision package and 620K is a 5% or 10% tolerance part. A higher quality resistor is for temp stability, but the resistor mod itself is to reset the proper range after the dead band mod.

Goom · Post by **Goom** » Tue Jun 07, 2011 2:14 pm

Thanks for the info Larry. I thought it was strange that this resistor was for temp stability. I have a few 620Ks here. Do you suppose a 620K would be fine to use?

Technician Larry · Post by **Technician Larry** » Tue Jun 07, 2011 2:58 pm

Goom wrote:Thanks for the info Larry. I thought it was strange that this resistor was for temp stability. I have a few 620Ks here. Do you suppose a 620K would be fine to use?

Yes, it will be fine, though you will have to recalibrate of course. I honestly do not know how much drift might be caused by the use of a 5 or 10% tolerance part vs a precision resistor, but the fact is that whatever the error, it will effect all 3 VCOs the same. Any drift can be compensated for with a minor adjustment of the front panel tuning control and will certainly be WAY less aggravating than the old pitch bender voltage not returning to "0" reliably.

Kevin Lightner · Post by **Kevin Lightner** » Tue Jun 07, 2011 3:28 pm

The 619K is precision because that value is actually needed.
A 620K will usually cause the range trims to be maxed and ideally one wants them to be centered.
It certainly doesn't hurt that 619K's only come in 1%, low PPM drift, but in this case that value is actually required.

Same for the older boards- a 332K is requested because a 330K won't allow trims and/or the front panel tuning pot to be centered.
I wrote 330K-332K above because I didn't recall the exact value in the middle of the night, but I've since checked the diagrams.

Good luck with the mod.

Goom · Post by **Goom** » Tue Jun 07, 2011 5:47 pm

Thank you both for all the helpful info. I went through a stash of 620K resistors (carbon), and found 1 that is 619K. I'll install it tonight, and will post again to let everyone know how it turned out. Thanks again!

RL · Post by RL » Tue Jun 07, 2011 7:29 pm

Is it like that?

Goom · Post by **Goom** » Tue Jun 07, 2011 9:31 pm

Wow, very nice! Is that yours? Can you tell us about it?

The good news here is that I installed the 619K resistor, and this Mini is now officially rockin'! The pitch wheel is great to use now. The osc shift cal pots are sitting pretty about 11 o'clock.

Thanks once again to Kevin and Larry. You guys are great!!!

Kevin Lightner · Post by **Kevin Lightner** » Wed Jun 08, 2011 3:02 am

Goom wrote: The good news here is that I installed the 619K resistor, and this Mini is now officially rockin'! The pitch wheel is great to use now. The osc shift cal pots are sitting pretty about 11 o'clock.

Kewl! Glad you were able to make it happen.
I wouldn't worry about it being a 5% type.
The main tuning pot probably has more error than that and you just eliminated a lot more error by doing the mod.

RL · Post by RL » Wed Jun 08, 2011 5:32 am

Goom wrote:Wow, very nice! Is that yours? Can you tell us about it?

I'm using this board for the Minimoog upgrades. The good thing is - there's a small noise reduction for the pitch wheel pot, a buffer and golden pins of the connector. The bad thing is - you have to replace the panel connector too.
Have fun,
Rudi

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