[SOLVED] Crazy resonance

[mangros]

I'm not sure if this is anything like the 'warble' problem - it's less 'warble', more 'insane modulation'! All the old sample links referring to warble on this forum seem to be dead. Here's what mine sounds like:

http://soundcloud.com/radiantsuicide/x0 ... em/s-nO4IR

When playing in keyboard mode at various knob settings, it's clear that the point when the resonance would normally become nice and squeaky is instead going into self-oscillation whistle. At certain settings with low envelope the whistle is steady, but it mostly wobbles like the sample when the envelope is up.

Otherwise it behaves properly. It's my third x0xb0x. VCO, non-resonant filtering, envelope, accent and headphones seem fine. Aside from the 7805, none of the components get hot. All digital section seems fine. I'm using 945s and 733Ps with hfe around 300-305, 5% resistors, and yellow box polyester caps instead of green film. So far I've tried the following with no difference:

- Reheated all solder joints in the filter
- I have tried two different AC power packs at 9V, one 500mA, one 1A - no difference.
- I have tried different combinations of 6562s (I have four in total) - no difference.
- Different parts in IC12, Q8, Q9, Q10, Q11.

I've taken some readings to compare with the stickied voltage map - everything is relatively close except for the emitters of Q22, where one side (the side with Q23) is about 0.3V larger, and this difference goes down that side of the ladder across all the caps down to Q12. Q21 has a smaller imbalance (0.1V) on the collectors. Having said all that I have no idea if a slightly unbalanced ladder would cause this extreme effect, and those readings were taken with resonance at 0.

Before I descend into desoldering and socketing hell, any ideas would be great...

[antto]

i don't know exactly, in my x0x i can get it to do this same thing when i connect between some points around the VCF (using croco-clips)
thus maybe check for any tiny shorts around the board.. i don't know

[MMM]

I've taken some readings to compare with the stickied voltage map - everything is relatively close except for the emitters of Q22, where one side (the side with Q23) is about 0.3V larger, and this difference goes down that side of the ladder across all the caps down to Q12. Q21 has a smaller imbalance (0.1V) on the collectors. Having said all that I have no idea if a slightly unbalanced ladder would cause this extreme effect, and those readings were taken with resonance at 0.

I've seen the problem of "overshooting" resonance with one xoxbox; this was caused indeed by imbalance of the ladder. So my guess is that Q22 has a problem.

[mangros]

antto: That's interesting... I've checked the joints a few times with a magnifier but can't see anything wrong. Can't hurt to do it again I suppose... Can you remember which bits you connected to get the effect, or was it random poking around?

MMM: that's enough evidence for me to desolder and socket Q22 and try a 2291 from one of my working x0xes. Cheers for the info.

[antto]

random poking, but it happens in quite a number of places.. the behaviour is exactly as in your case - this "instability" is strong when resonance / envmod / cutoff are all high (or in other words when the filter cutoff frequency is highest..)

[mangros]

I removed Q22, put a socket in, populated the socket with a 2291 from a working x0xb0x - FIXED

Thanks for the advice! Now off to x0xsh0p I go...