Modcan B Series VCDO issue

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Ramases
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Modcan B Series VCDO issue

Post by Ramases » Sat Jul 15, 2017 11:56 am

I have a problem with my B series VCDO.
The FM ratio is working but makes a crackling sound at various points when either the pot is turned or a CV is applied to the relevant input.

Has anybody experienced this with a VCDO or have any ideas where to start looking for a solution? I did wonder whether changing the pot would help but the fact that it's happening with the CV input too makes me unsure.

Edit: It's FM Ratio that is the problem. I got it and FM Index muddled up.
Last edited by Ramases on Sun Jul 16, 2017 12:31 pm, edited 1 time in total.

JohnLRice

Post by JohnLRice » Sat Jul 15, 2017 1:01 pm

I haven't noticed a problem like that with my 58Bs. Can you post a short video clip or at least an audio clip and maybe a picture of the knob settings?

The first thing I would check is your power supply rails to make sure they are at the correct voltage and also check to make sure the +5v regulator on-board the module is still working correctly too.

Ramases
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Post by Ramases » Sat Jul 15, 2017 1:08 pm

Thanks for your reply John.

I will post a piece of audio tomorrow.

I don't think it's a power supply issue (will check it and the 5V regulator though) as I have a second VCDO which is working just fine in the same case. I also removed all the other modules from the case just to check it wasn't something to do with them for some reason - this made no difference.

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The Disquiet
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Post by The Disquiet » Sun Jul 16, 2017 3:06 am

when did you buy the module Ramasas? About three years ago Bruce received a batch of faulty Alpha pots that started acting up after some use. I had a problem with two modules and had the pots replaced.

thermionicjunky
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Post by thermionicjunky » Sun Jul 16, 2017 9:25 am

The Disquiet wrote:when did you buy the module Ramasas? About three years ago Bruce received a batch of faulty Alpha pots that started acting up after some use. I had a problem with two modules and had the pots replaced.
But if the CV input causes the same noise, I wouldn't blame the pot. Both CV sources go to the same analog VCA, so I would poke around that circuit.

Ramases
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Post by Ramases » Sun Jul 16, 2017 12:51 pm

Thanks The Disquiet and thermionicjunky for your input.

Some more details and a sound clip attached.

Note: It's the FM Ratio that's the problem. I said FM Index initially - have edited my first post to make this clear.

The first two sweeps of the clip are me turning the FM Ratio pot manually. The second two are using a sine wave from a Modcan Quad LFO into the FM Ratio CV input.

Details:

The PCB of the faulty module says "DVCO 2013." For reference my other VCDO (which is working fine) says "DVCO Rev 3.0 2008."

Here are the settings I used for the sound clip.

Sine wave
Range: 5
Fine: 0
IN PW/AMP: +5
Quantize: Off
FM Index: 5
FM Ratio (initial point): 0

When I first heard the noise I was not looking at my computer screen and presumed I was overdriving the sound card input. That makes the comment about the VCA interesting as that might make a similar sort of noise if not working correctly for some reason.

I did wonder about the pot but the fact that the CV input has the same effect makes me question that. Although is the pot read digitally by the software? If so, could a bad pot potentially introduce some noise into the system, even if not being turned manually?

I am 99.9% certain this is an issue with the module and not something else that I have missed. I cannot replicate this with my other VCDO using exactly the same module and recording settings.
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