The oscillator core is slightly non-linear by design, and the pot compensates for that at higher frequencies (datasheet suggests tuning at 10 kHz).
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ok, and how do you calibrate with that trimmer?
Start with the HF trimmer set to zero (full clockwise on the #1222) do the best tuning you can with the REF and TRK trimmers in the lower octaves. You will notice that the higher octaves tend to be flat (a bit low in frequency) you can then turn the HF up (counterclockwise on the #1222) to bring the upper octaves up in tune, without affecting the lower octaves much.
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So, Iām just getting around to trying to test my VCOās and Iāve run into some issues. Then again, are you surprised? Iām using the tune-o-matic code that Sam linked to on his site. I successfully programmed the arduinoās. I only know this because when I plug in the modules it goes through itās light sequence and then displays the notes in sequence briefly. After that I cannot seem to get anything to show up on the display again. No notes are ever shown on either VCO. I tried moving all of the trimmers, knobs, etcā¦but nothing ever shows up on the display. Iām also not receiving any voltage in the test point. On one VCO I receive the following values on the jacks:
square: 1.69v
ramp: 0v
triangle: 0v
test voltage point: 0v
My other VCO the display cycles through on startup and the LEDās also blink and then the screen goes blank. The values that I am getting on the jacks are:
Square: 1.41v
Ramp: 11.56v
Triangle: 1.97v
Test voltage point: 1.21v
I have reflowed both of these VCOās and then checked and itās still the same. I checked the header for shorts and there are none. I am receiving continuity to the chips as I should be. I cleaned one of the boards with 91% IPA but that made no difference. Iāve tried using both the Tune-O-Matic code and Samās code but itās still the same. One thing I did notice is that the nano on the first VCO I list is I have a solid red power light and a solid red āLā light on the Nano. The second VCO I mentioned, the one I am receiving a small test voltage on, has just a solid red power light on. There isnāt a light on next to the āL.ā
What am I missing? How I could have not one, but both not working just is astonishing to me. 
The tuner and the oscillator are (almost entirely) separate, so concentrate on one at a time when debugging.
How are you measuring the output voltages? Do you see different voltages on the 3340 pins (before the buffers) compared to the output jacks? (see here for expected output voltages).
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Thanks Fredrik for the help!
With the help of the link you gave I was able to calibrate the reference voltage to 4.0v on the 2nd module I mentioned originally. I still do not have anything on the display or any LEDās lit up.
Forgive me, Iām not sure where the ābuffersā are so Iām going to give you the voltage of each pin 
1: -4.44
2: -6.61
3: -7.22
4: 1.18
5: .49
6: -1.27
7: .05
8: 4.20
9: 4.01
10: 1.97
11: 3.85
12: 0
13: 0
14: -.01
15: 0
16: 12.00
Output voltages. I am using the multi-meter on the solder points on the bottom of the jacks. So one probe on the ground of the jack Iām testing and then the other on the 2nd pin to the right of the ground if youāre looking at the back of the module.
Correction, the jacks are:
square: 1.18
ramp: 11.56
triangle: 1.98
Your multimeter might not do a good job on the signals ā many multimeters only give accurate AC readings for sine waves, and even ātrue RMSā meters will mess up on less āorganicā wave forms, due to bandwidth limitations (and even if they do work, theyāll report a weighted average, not the peak to peak values I mentioned in the other post).
The supply voltages (pins 3, 12, and 16) look right, so odds are that your oscillator does something. You may not hear what itās doing, though, so a good next step could be to check that you have a total input CV in an audible range. See this checklist and the other tips in that thread.
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Help please. One of my 3 VCOās has a problem I have been chasing, and in the process I have burned out multiple eyelets and traces. The fine tune knob seems to have the opposite effect. Turning left the pitch gets higher, and right it goes way down to just clicks. I just swapped out that potentiometer ruining several more connections in the process, but no changes.
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If you get a decent range out of the potentiometer it doesnāt really matter which direction you need to turn the knob. This kind of thing happens to me sometimes. For instance I once (with the mains power turned off) replaced a light switch in my house and only found out I had it upside down after I closed the case and turned the mains power back on. I weighed up the risk of opening it up again against the inconvenience of having to learn to flip it up for on and down for off, and decided to leave it alone.
If you want to fix this pot, though, the thing to do is reverse the connections. Leave the wiper connection alone but swap the other two. Youāre not dealing with potentially lethal 230VAC mains as I would have been with the light switch, so itās okay to go ahead and try it.
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that is truly odd I havenāt heard that one before! do the other two work fine?
im trying to think how it could be that the knob has managed to switch polarity. and im really not sure. especially if the other two are working fine. is there any shorts on any other the pins on the tl074??? the only thing I can think is somehow that has been somehow shorted into an inverting amplifier.
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Weāre talking about RV7, right? (the one right under the 7-segment display marked āfine tuneā on the PCB). That goes directly to the 3340 (via a 100k resistor) so no opamps involved (except for the one inside the 3340). Itās also only has a 1 V range, so it ranging from clicks to a clear tone sounds a bit odd. Can you measure the voltage at the wiper, with the pot at min and max positions? (measure at the middle pin on the pot, or at the R24 summing resistor, i.e. the 100k next to the big K).
(reverse tapers are a thing (but usually log-ish, e.g. C and E here) but theyāre not very common, so I doubt thatās what happening here, unless you got your pots from some obscure surplus store)
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The other two work fine. Connections look good on the tl074. I guess I will look at the other connections associated with that tl074 and trace it back further. Thanks for the suggestions.
Yes fredrick RV7. The pots are 2 different brands.
I guess they could in theory have different pinouts, but that doesnāt seem too likely. The voltages will tell you whatās going on.
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So I went through the checklist you provided. First, I canāt tell which is the input and which is the output on anything so Iām going to give you all values. When the power is off I am measuring the following:
āWith power off, make sure that you have 100k resistance from the centre note trimmer output to pin 15. Check the same for fine tune, the octave selector, and CV in.ā
Center Note Trimmer and pin 15:
Top pin (if youāre looking at the back of the module): 56.7
Middle Pin: 56.3
Bottom Pin: 60.3
Fine Tune and pin 15:
Left small pin (if youāre looking at the back of the module): 103.3
Middle small Pin: 58.2
Right small Pin: 56.7
Octave Selector and pin 15:
Center pin in middle of selector: 60.4
Pin v4: 60.5
Pin v3: 65.4
Pin v2: 66.5
Pin v1: 56.7
Last pin: 63.6
CV In and pin 15:
(going clockwise around the jack from the ground pin)
ground: 0
0
0
137.7
0
Iāll start with this first item on the checklist as I believe there are issues here since I do not have 100k resistance. 
Oh, thatās a lot of measurements, but assuming the values are kiloohm it seems you have around 60k on the CV inputs. Iād expect a bit higher, my builds have the expected ~100k but I donāt have a complete #1222 build so maybe Iām missing something. Iād suggest moving on to measuring the voltages.
(if I find the time this weekend, Iāll list some specific voltage test points that you can check, i.e. supply and CV points.)
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Thanks. Any help is appreciated. 
I did plug it into my Fender Bronco practice amp and on this module I heard the square wave working for a moment then I unplugged it and tried the ramp and there was a power surge and now thereās nothing on any output. I switched the nano and programmed it again but that made no difference.
The other module when I plug in just gives a high pitch hum that never changes. I was going to go through and test each resistor to make sure they are the correct value and reflow everything again. I donāt understand because I was so careful when I built the second one.
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Just took a deep breath and plugged in the 1222vco, no smoke!
Trying to upload the code but the error comes up as not in sync. Any ideas?
Arduino new to me.
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Check if the correct usb connection is selected in the Arduino ide and also, with the nanos there are older and newer bootloaders around, so try if you can set to old bootloader and try again. Somewhere in the menu:
Processor > "ATmega328P (Old Bootloader)
This took me 2 hours to find out and I thought I got a bunch of broken nanos!
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just an image in case it helps.
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