Strange detuning after not using my x0xb0x for a while

by Apekooi on Sun Apr 08, 2012 7:48 am

Hello Everyone,

I have a problem with my x0xb0x that I was hoping someone might be able to help me with. I completed my x0xb0x (xoxshop.de kit) about 4 months ago, and it used to work perfectly, but when I tried using it yesterday the tuning of my VCO's seemed way too low. I tested the oscillators with an external CV and checked if there was anything wrong with the trimpots, but the VCO circuitry all worked like you'd expect. There are some other glitches, like my E being about a minor third higher than my F, and notes pitching down after releasing a key in keyboard mode, that lead me to suspect that the problem probably lies elsewhere. Is there anyone who knows what might cause this? Or where I could start looking for the problem?
Thanks in advance!
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Re: Strange detuning after not using my x0xb0x for a while

by izze on Sun Apr 08, 2012 8:10 am

Hello Apekooi

Have you been trying recalibrating your x0x with the as_x0x_tune VST ? See if you are able to calibrate it
viewtopic.php?f=12&t=24759

Maybe reflowing solder joints around the vco section helps, never heard of a problem like that, you'd better wait till guest makes his statement :)

Greets and good luck
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Re: Strange detuning after not using my x0xb0x for a while

by antto on Sun Apr 08, 2012 11:49 am

if it's tuned correctly from one octave to the other, but the notes inbetween are not equally spaced - then the problem might be in the resistor network..

afaik those resistors there should be low tolerance and matched with a multimeter for equal values..
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)
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Re: Strange detuning after not using my x0xb0x for a while

by Apekooi on Mon Apr 09, 2012 12:41 pm

Thank you for your replies.
If the problem lies with the resistor network wouldn't it be strange that it used to work perfectly before? As for the tuning, I did it with an oscilloscope. I guess reheating the solder could help, I'll try that and report results.
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Re: Strange detuning after not using my x0xb0x for a while

by guest on Mon Apr 09, 2012 6:27 pm

the note pitching down after keypress is strange
that sounds like a problem with the slide circuitry

although shouldnt the note stop playing
after the key is released in keyboard mode
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Re: Strange detuning after not using my x0xb0x for a while

by Apekooi on Fri Apr 13, 2012 4:30 am

Thank you for your response and sorry for my late response, I got caught up in some work this week.
The note has a release time of about half a second when you let go in keyboard mode and in that time it is pitched down. I can't remember if this is normal or not but it does not seem out of place for a synthesizer. Would it help if I posted an audio/video sample? Is the slide circuitry something that can be repaired or resoldered?
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Re: Strange detuning after not using my x0xb0x for a while

by guest on Fri Apr 13, 2012 8:56 am

it sounds like something is shorted out

can you measure the cv out in keyboard mode
and see what the value is for when a key is pressed
and then what it is when a key is released
try this for a few notes throughout the scale
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Re: Strange detuning after not using my x0xb0x for a while

by guest on Fri Apr 13, 2012 8:59 am

also
if you have an osciliscope
you can watch the cv out as you play up the scale
it should make sharp jumps for each note
and then hold that value till a new note is played
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Re: Strange detuning after not using my x0xb0x for a while

by Apekooi on Mon Apr 16, 2012 10:16 am

I measured values from the CV out for different notes, C and F# in every octave:
note on off
c0 1.49 1.55
f#0 2.24 2.35
c1 2.98 3.08
f#1 3.70 3.82
c2 4.50 4.68
f#2 5.24 5.44
c3 5.92 6.07
f#3 6.69 6.90

and the full scale from C1 to C2:
C 2.98 3.09
C# 1.4 3.36
D 1.5 1.45
D# 1.54 1.52
E 1.65 1.63
F 1.67 1.64
F# 1.77 1.75
G 1.81 1.79
G# 1.92 1.90
A 4.09 4.35
A# 4.25 4.45
B 4.35 4.56


the on values is the CV value when the key is held on the keyboard, the off value is the voltage it assumes after the corresponding note is released. What's strange is that the voltage goes up, but the sound sounds like it is pitched down. Once I get my audio interface set up I'll see if I can provide some audio samples.

What's very strange is that I seemed to get different values for a note occasionally, with differences of up to a couple of volts. It seems to be dependent on what note I play before it, which confirms the possibility that the problem lies with the slide circuitry.
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Re: Strange detuning after not using my x0xb0x for a while

by guest on Mon Apr 16, 2012 3:04 pm

there is so much wrong there
i hardly know where to begin

can you measure the 5.333v
for both note on and note off
and then turn off the power
and measure the resistance of all the resistors
r74 through r90

from the readings you gave
im guessing those resistors are all bad
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Re: Strange detuning after not using my x0xb0x for a while

by Apekooi on Tue Apr 17, 2012 3:30 pm

Thank you for your response. Here are the 5.333V values for every note. As I said in my previous post, the voltages seemed to vary depending on what note was played before it. This was the case here as well, and the value represents the value the voltage settles into after repeating the note a few times. The scale is from C1 to C2.
Code: Select all | TOGGLE FULL SIZE
C    7.89  8.22
C#   3.93  3.83
D    3.98  3.92
D#   3.95  3.88
E    3.98  3.94
F    3.96  3.90
F#   4.00  3.96
G    3.99  3.95
G#   4.01  3.98
A    8.25  8.77
A#   8.15  8.54
B    8.16  8.56
C    8.12  8.46


These are the resistor values for R74 through R90. I measured every resistor leaving it in the PCB, I hope that's OK. All values are in Kohms.
Code: Select all | TOGGLE FULL SIZE
R74 97.6
R75 97.6
R76 97
R77 97
R78 196
R79 196
R80 196
R81 195
R82 195
R83 97
R84 97
R85 98
R86 96
R87 195
R88 97
R89 97
R90 196


You suggest the transistors are bad, could they have somehow 'gone bad' without anyone opening the machine? Because it used to function properly.
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Re: Strange detuning after not using my x0xb0x for a while

by guest on Tue Apr 17, 2012 3:47 pm

what point on the x0x
did you take the 5.333v measurements from

what are all the voltages on ic23
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Re: Strange detuning after not using my x0xb0x for a while

by Apekooi on Tue Apr 17, 2012 4:17 pm

I took my measurements off what in the instruction manual is the green wire in J4. The voltages on IC23 are:
Code: Select all | TOGGLE FULL SIZE
1 17.4
2 7.82
3 8.09
4 5.98
5 5.23
6 8.09
7 8.08
8 19.0
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Re: Strange detuning after not using my x0xb0x for a while

by guest on Tue Apr 17, 2012 5:30 pm

disconnect j4 before turning your x0x on again
the powersupply is very broken
and can damage the mainboard

measure the resistance to ground
from pin4 of ic23
it should be 0ohms

then look for bad solder joints or shorts on ic23
something is not right with it
it might be busted
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Re: Strange detuning after not using my x0xb0x for a while

by Apekooi on Thu Apr 19, 2012 12:22 pm

Again, thank you very much for your response. I tested the resistance, and it WAS broken. I resoldered all of IC23, and afterwards it was 0 ohms, so there WAS in fact a bad solder joint. However, the problem seems to persist. Do you think it might be worthwile to remeasure the CV voltages? Could they have changed? I will now check the board for other bad solder joints and report back if I find anything.
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