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Modcan Dual VDO Doesn't stay in tune
MUFF WIGGLER Forum Index -> 5U Format Modules  
Author Modcan Dual VDO Doesn't stay in tune
Hi everyone !

I have a DVDO 62A that can't stay in tune, it is the first time I have such a problem with digital oscillators...

Anyone know where that could be from and how to solve the problem ?

The 62A (like my FMVDO) employs a crystal to deliver stable tuning, so stability should be guaranteed.

Is there any modulation source plugged into your 62A when the pitch is drifting? Perhaps some CV is bleeding through from a modulation input?

<edit> Also, does the module's tuning drift when it is installed in different cases, employing different power supplies? Does the presence or absence of other modules make a difference?
Thank you for your reply Blairio !

There is nothing plug into the module, I think you are right about the power supply, my 4 cases are alimented by the same Random Source power.

It is not full but I removed one module (not from the same case) and it seems to be better, I also put it in Quantize mode, maybe that helped to, I will do other tests as soon as I can !
The quantiser will mask the problem, rather than solve it. From your investigations the cause does seem power related. I have little knowledge of electronics, and I would like to understand how power issues can affect the pitch stability of a digital sound source.
Hmm hmm... Well it seems that it wasn't a power problem, I started using the dual vdo again, it still doesn't stay in tune, it is really not usable right now, all my other oscillators (analog) stays perfectly in tune.

Any one been thru this kind of problem ?

Modcan runs on +/- 15V.

Does your Random*Source power supply supply +/- 15V? Generally they are +/- 12V.
I think it is 12V, but if I remember right, I've read that it works for 15V aswell if you don't use all the xlr outputs, is that true ?

I have 2 Random Source PSU with 5 xlr output each, and I use only 2 of them on each.

I don't have any other problem with other modules (pretty much all Modcan) I will try to clean it well that might be the problem...
No, it looks like the random*source psu is +-12v only, so that may well be your problem. Quite a few modules will "work" at lower voltages than expected, but some won't. Looks like you found one.
That's weird, it used to work before when I had not a lot of modules

Anyway thank you for your answers !
sduck wrote:
No, it looks like the random*source psu is +-12v only, so that may well be your problem. Quite a few modules will "work" at lower voltages than expected, but some won't. Looks like you found one.
I beg to differ! cool From
BOM - Parts needed:
Power One Linear Power Supply HBB15-1.5-AG

16. Make sure the Schurter power inlet is turned OFF. Connect a power cord to the PSU, power on and calibrate +V and -V as closely to +12V and -12V as possible. (Alternatively the HBB15-1.5 can be set to 15V by just using the PSUs trimmers.)

So while typically setup for +-12v, +-15v is possible. A good idea that the OP should double check though! thumbs up
Yes, I wasn't sure about that - the stock ones are +-12v, but can be adjusted. Maybe it's something else...
What’s is the nature of the pitch instability? Does it drift, or is it wildly erratic?
The only stability issues that I ever had with Modcan modules were due to failed potentiometers. Those controls were wildly erratic.
I will look on my Random Source, the pitch instability is quite light somethimes, a bit less other times.. But it is not wild.

Yesterday I made a patch with 3 voices, one of them was with the dual vdo, and after 5 minutes I clearly heard that this voice was not in tune with the other voices anymore (I checked with a reference tone, and that was the dual vdo that wasn't in tune anymore, not the other analog vco).

And then it was mooving a little bit all the time, I retuned it on the fly but after 10 secondes it was drifting again, just enough so I realised that it was not in tune anymore.... not really musical !
Plattform wrote:
That's weird, it used to work before when I had not a lot of modules


Sounds like you've loaded that power supply too much. Check your voltages with everything on.
Can you confirm whether this r*s psu you're using is set up for 12v or 15v? Do you have a DMM or something you can check the voltage output with? Do you by any chance have another psu that outputs +-15v that you can test it with?
Yep it is 15v !
I had a similar issue with my VCOs a while back. I checked the voltage on both the -15v rail and +15v rail over an extended period of time and noticed that the +15v rail would sometimes dither just a little bit, but it was enough to noticeably throw off the pitch of the oscillators. In my case one of my diy modules had a mistake that caused it to draw more current then it should, and the PSU couldn't quite keep up. I found the module, fixed it, and haven't had any problems since. It'd be a good idea for you to add up the current draw of all your modules and see how it compares to the maximum current output that the PSU is capable of.
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