Strange pitch change and noise/vibration...

[x0xjesus]

My x0xb0x is from Mode Machines which I bought about 3 years ago from eBay. I didn't build it by myself.

I have this strange pitch change and noise/vibration problem. Sometimes the pitch goes higher for a while, but usually it goes back to normal level soon afterwards. Sometimes the problem appear more or less constantly after turning the power on, but sometimes it takes many hours, until the problem appears.

I usually have many other connections thru midi, midi in, din sync and I have TR-707 as a master clock. However, I can hear the problem even without any other machines connected to my x0xbox.

I noticed that although I can set the reference voltage to 5.33 volts, I cannot set it much higher than that (not even 5.4 volts), even though build manual says: "You should be able to adjust the trimpot so that it ranges from about 5.25 to 5.75VDC. What is most important is that it can be set to less than 5.3V and higher than 5.4V." Could this be somehow related to my problem and what could cause the low voltage? I have also noticed that my voltmeter shows that the voltage varies between 5.34 and 5.34 (or 5.32, depending on how trim is set), but maybe it mean just that my voltmeter is not that reliable. DC voltage across C3 and C5 is also somewhat lower than the 15 volts that is mentioned on build manual.

[antto]

the 5.33V voltage is very important, as it affects many things including the VCO tuning and the VCF cutoff frequency iirc
so if that voltage "varies" for some reason - it would make the vco and filter change frequency i think

make sure you're using the right adaptor - 9V AC around 300 to 600mA ... you might also want to measure it's output (with multimeter in AC Voltage mode) to see if it's not terribly off or something

i'd also look around the IO-board closely, for anything suspicios

if this issue only affects the VCO but not the VCF tuning - it could be somewhere in the VCO section instead
did you try sweeping the TUNE knob to see if it shifts the vco pitch smoothly?

[MMM]

If the 5.33v supply isn't adjustable over the full range it may have different reasons. For example, on most of the xoxboxes I have seen measuring between pin 5 of IC23 and ground resulted in a lower value than measuring between pin 7 and ground; it seems that the value measured at pin 5 depends on the impedance/resistance of the voltmeter.
So this would be a good start - checking the value at J4.

Otherwise it sounds to me like a possible cold solder joint. You may want to check the CV output when that happens - if that is consistent the problem is probably not with the 5.33v.

[x0xjesus]

I put a sample WAV file here which contains an example of the pitch changing by itself:

https://www.dropbox.com/sh/r12psr9huwu5jwo/PiVRMA-RSM

[x0xjesus]

make sure you're using the right adaptor - 9V AC around 300 to 600mA ... you might also want to measure it's output (with multimeter in AC Voltage mode) to see if it's not terribly off or something

i'd also look around the IO-board closely, for anything suspicios

if this issue only affects the VCO but not the VCF tuning - it could be somewhere in the VCO section instead
did you try sweeping the TUNE knob to see if it shifts the vco pitch smoothly?

The label on adapter says that it is 9V AC 500 mA. When measured without any load the voltage seems to be 11V AC, but with x0xb0x running, the voltage is 10.55V AC.

I think the problem affects both VCO and VCF.

IMHO, the TUNE knob shifts the pitch smoothly.

Btw, I have now an oscilloscope available. Any ideas what I should try to measure with it?

[MMM]

I put a sample WAV file here which contains an example of the pitch changing by itself:

https://www.dropbox.com/sh/r12psr9huwu5jwo/PiVRMA-RSM

My guess would be that this is a thermal problem of one or several components. If the 5.33v supply would fail it probably would also affect the scaling more; and I also would expect at least small changes in VCF and/or VCA. But I may be wrong about this.

[x0xjesus]

Otherwise it sounds to me like a possible cold solder joint. You may want to check the CV output when that happens - if that is consistent the problem is probably not with the 5.33v.

I resoldered everything except the USB chip on IO board. Since then, I haven't had the problem, so it might have been colder solder joint. However, it is a bit too early to say for sure, whether or not the problem has been fixed, because the problem has always been intermittent.