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x0xb0x with srappy VCF
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author x0xb0x with srappy VCF
Hi, i asked the question on the forum ladyada, having no answer, i try here:

It's been several months weeks, i built a x0x which has a problem with the VCF, i left it aside and watch that from time to time ...

Symptoms: the resonance is not going thoroughly, you can not make the vcf go high treeble ... This means that vcf acts bottomless on the accent and the envelope does not affect the VCF, because the resonance is scrappy.

I checked a lot of things, checked all the pcb tracks, checked all VCF resistors and capacitors, replaced all the VCF transistors and checked if they were OK.

By cons i find that at the junction of R111 / Q21, i have about 9.7VDC. Same as the junction of R110 /Q21. It's abnormal, i should have about 6V! And at the junction of Q19 / C14, i have 9VDC, i should have 5.25V! Apparently that's why i have rotten VCF, but i do not understand why these voltages are bad ... d'oh!
it sounds like you compared your voltages to the voltage map on the x0x forum. where any of the other voltages bad? or just the Q21 and Q19 voltages? where did you get your parts from? its possible Q21 is bad. if the voltages on Q20 are good, and R108, R111, and R110 are the right values, then its most likely Q21.
I would like to advise to use a VR4 pot like this to avoid future problems with the filter:
Your problem might lay elsewhere, just a tip.
I do not have time to check this weekend, but i will check next week. The voltages that i compared with other x0xb0x who have the VCF that is OK. I have 5 x0x.
I tried to replace Q21 with another one that comes from an x0xb0x that is OK, no difference. But i noticed that the problem comes from the sector of Q21 / R111 / R112, but for now i do not know to be right.

Out of curiosity i tried to increase the resonance, just to check if it will have an action on the filter, replacing R97 by a lower resistance, i concluded that it did not change anything on this x0x

I use the same potentiometers as RV0, i tried to replace it, no difference. I know these potentiometers i bought 10 at mouser, i removed the central detent washer.
all of the voltages are posted here:

unfortunately you will need to install eagle to look at the brd file, as the png is no longer available. or you could do it the way youve been doing, but the voltage map might be easier.
i put an image file of the voltage map up on my wiki:
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