Fatman, pitch problems
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Author:  axtelius [ Mon Feb 17, 2014 2:47 pm ]
Post subject:  Fatman, pitch problems

If I just slam one single note on the keyboard the pitch of the sound produced varies in a random way. Sometime it's just spot on and then jumps up or down about a note or two. Sometimes it goes totally crazy and jumps all over the place. It's usually worse if I make a large jump on the keyboard. I can't really find a pattern, it's just crazy. The problem is the same on both VCOs.

The voltage from the DAC it fine and super stable, so is the voltage out of 10a.

I have poked around in the VCOs but can't really find anything obviously wrong.

Anyone have any ideas what to check?


Author:  PAiA-Scott [ Tue Feb 18, 2014 11:47 am ]
Post subject:  Re: Fatman, pitch problems

You might be hearing a mix of the Pitch and Velocity CVs. The C7 and C8 capacitors which are part of the cv demultiplexing s/h circuit-sections get a ground through a short jumper wire near R86. This jumper wire connects the capacitors with ground. An open-circuit here would result in the two cvs working to be the reference for one another.

Reference the kit manual schematic fig. 5a to see the connections in this circuit-section: The uC port lines o11 and o12 go to IC8 and IC8 goes to IC11 to select the cvs at points in time from IC9. If something isn't connecting along the way here, it could result in interaction between the pitch and velocity control voltages.

Author:  axtelius [ Sun Feb 23, 2014 2:56 am ]
Post subject:  Re: Fatman, pitch problems

Ah, yes, of course. I was only looking at the other half of the schematics :(
You where 100% correct. The jumper for C7,C8 was the problem.


Author:  axtelius [ Sun Feb 23, 2014 11:55 am ]
Post subject:  Re: Fatman, pitch problems

Wait a moment, something's not working, hmm... Ok, it's not better, but different in a better way (eh).

Author:  axtelius [ Sun Mar 30, 2014 3:19 pm ]
Post subject:  Re: Fatman, pitch problems


Tuning VCO1 from the front panel made it go crazy in pitch, turned out to be just a dirty pot.

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