resonance pot not working well

[hellmo]

hello everybody. here i come again. my second xoxbox is now finished. my first xoxbox, from a TT kit finally works, very well. the second and last one is also working. good, not really, but works. however, i have a problem with my resonance pot. it has a small effect on the resonance of my xoxbox. my xoxbox doesn't scream, as the first. every other pots working good , enveloppe, accent, cutoff, decay, but not the resonnance. don't know whats is wrong, may be a transistor, or enything else. does anyone have some similar trouble, or something for me to explain what to test. thank you all, Manu

[antto]

is it not resonant enough or severly lacking any resonance?

maybe check the resistors in the VCF section

[hellmo]

hi. i can hear a soft resonance variation...resistors look good, but i'll check them again ....

[MMM]

hi. i can hear a soft resonance variation...resistors look good, but i'll check them again ....

Does "soft resonance variation" mean that the signal actually sounds different or is it getting just a bit louder?

Perhaps it's a good idea to start with the general principle how the resonance is generated.
If you look at the schematic you'll see the "filter ladder" starting with Q12, the continuing through Q13/15, Q14/17, Q16/23 and then ending in Q21 (Q22 is also part of the circuit). The signal then goes through Q19 and C14. The filter frequency control is happening at Q12.
The unfiltered signal enters the circuit at Q12 on the "left" side (when looking at the schematic); the filtered signal leaves at C14.

The obtain resonance the filtered signal is fed back into the filter. What happens here is not unlike the typical "feedback" problem then most people will have witnessed when a micrphone gets too close to a loudspeaker. In fact with sufficiently high amounts of "resonance" the filter will generate a sound itself (like a microphone held directly in front of a speaker).

This part of the circuit is coming from the resonance pot through C15 and finally enters the ladder though R97 on the "right" side of Q12.

If you unsolder R97 you can easily measure (with an oscilloscope) if enough signal is making it through the resonance circuit back to the ladder. There also is the possibility that the ladder itself is faulty (I've had a xoxbox on my desk yesterday where the resonance was way too high - turned out to be a bad Q22).

Do you have an oscilloscope?

[hellmo]

hi...sorry for the delay, but, i'm father now, for a second time ) .

ok, don't know how, don't know why, but, during the check of every components, i do a new test of the resonnance, and for the moment, everything working well. thank you for your help, i ask you if i need anything else....

[MMM]

hi...sorry for the delay, but, i'm father now, for a second time ) .

It's always good to have the right priorities. Congratulations!