Tuning Mods and Tips

Discuss mods, hacks, tweaks, etc.

Moderators: altitude, adafruit_support_bill, adafruit, phono, hamburgers

Please be positive and constructive with your questions and comments.
Locked
msapessoa
 
Posts: 2
Joined: Mon Feb 11, 2008 6:29 am

Tuning Mods and Tips

Post by msapessoa »

One of my favorite mods is to install a switch that defeats the tuning knob intirely.
The tuning knob is so sensitive that even if you use a center detent knob, it never comes back to the same value and it's really easy to knock it out of tune when you're tweaking the other knobs.

It requires two 25K 1% resistors wired in parallel to the Tuning Knob and a DPDT switch that switches between the knob and the fixed resistors. Check the schematics. You need to cut the trace feeding the wiper of the tuning knob and use it as signal source to either the fixed resistors or the knob.

If you never use the tuning knob as an effect you should even consider replacing it with two 25K precision resistors.

In addition to this mod I replace the internal tuning trimpots TM4 and TM5 with multi turn precision trimpots and mount them in the reverse side of the PC board and later drill 2 small holes in the face plate so that I can calibrate the machine without having to open it. I also always mount TM6 in the I/O board in the reverse side of the PC board. It's a lot easier to calibrate it that way since you don't have to remove the I/O board and upset all the jumper connections.

Calibrating the octave width is tricky and often requires some back and forth adjustments between TM4 and TM5. To avoid this make sure they're at the center position before you install them. It's fine to use a digital tuner to calibrate the machine. Turn all the knobs down except for CUTOFF and VOLUME (3 o'clock) .
When adjusting the width I usually program a one note pattern across all the octaves(c1,c2,c3,c4) and slow it down to as much as it will go to give enough time for the tuner to get a reading or just go to keyboard mode and manually use the octave switches.

eil
 
Posts: 440
Joined: Sun Aug 31, 2008 11:09 pm

Re: Tuning Mods and Tips

Post by eil »

Wow, you're really picky. :) Not a bad thing to be, I'm the same way.

It is reeeeally hard to get TM4 and TM5 just right on single-turn trimpots, it's true. It gets to a point, though, where you have to settle for good enough, especially when you take into consideration the fact that all of the components in the x0x are going to change in value slightly with temperature variation and time anyway.

I find that it's easiest to set the octave width first and then the tuning. Far less going back and forth between the trimpots.

Can you recommend a part number for the multi-turn trimpots?

bcbox
 
Posts: 587
Joined: Thu May 25, 2006 1:30 am

Re: Tuning Mods and Tips

Post by bcbox »

there's some good 4-turn trimpots from bourns that I use that fit into the same footprint...
3339P-1-502LF
3339P-1-503LF

alex_dubinsky
 
Posts: 188
Joined: Wed Jul 30, 2008 5:17 pm

Re: Tuning Mods and Tips

Post by alex_dubinsky »

I use this 12-turner:
http://www.mouser.com/Search/ProductDet ... rqzWicQ%3d

Tip for tuning: tune against the highest C, it will save you a lot of trouble. This note doesn't shift as you're changing the width.

Another tip for tuning the trimpots: it's actually not that hard to get good precision out of single-turners, but the way they work is that if you've been turning them one way but overshoot, you can't just start turning them the other way. That gets them to jump your target value and undershoot. And then you try to reverse again, and it jumps, and you start cursing the bloody piece of BANNED. Instead, turn it some distance away from the spot you're trying to hit and then try reaching it in one smooth, single-direction motion.

Finally, don't bother turning it better than about 0.3% (maybe 0.5%), since that's gonna be the imprecision from note to note anyway. (Using 1% resistors.)

Oh, ok, the last tip is you should solder a piece of wire to the waveform switch to connect to your tuner or oscilloscope. This way you don't have to be playing any notes and don't have any filter distortion or decay to worry about.

Locked
Please be positive and constructive with your questions and comments.

Return to “x0xm0dz”