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VCF problems on M.Koster x0xi0
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Please be positive and constructive with your questions and comments.

Re: VCF problems on M.Koster x0xi0

by antto on Sun Jan 22, 2017 2:49 pm

soundcloud says those tracks don't exist or have been removed

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Re: VCF problems on M.Koster x0xi0

by Neyah on Sun Jan 22, 2017 3:20 pm


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Re: VCF problems on M.Koster x0xi0

by antto on Mon Jan 23, 2017 1:23 pm

nope, there's no actual resonance going on here
there's something wrong in the filter... it seems to work kinda, it lowpass filters but with no resonance
check the components for proper values and orientation

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Re: VCF problems on M.Koster x0xi0

by Neyah on Mon Jan 23, 2017 3:33 pm

the component have the propers values and orientation, isn't a problem of building because the xox works fine since few years, but it would be rather a defective component or a short no ? If there is no resonance, the problem come from the feedback path more than other spot, am i right ?

Ok so I could test all the resistors, all transistors and some capacitors which are through hole, but it will be complicated with some capacitors which are shitty smd...
What the best way to test the circuit, test all components one by one to find if there is one burned, or test voltage in circuit ?

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Re: VCF problems on M.Koster x0xi0

by antto on Mon Jan 23, 2017 3:56 pm

did this x0xb0x used to work fine before?
because then this can't be a matter of wrong components or wrong orientation

yes, the issue is in the filter and most probably somewhere along the feedback path

a shot in the dark: socket the dual transistor 2291 or what was it.. search the forum for a thread where pulsar suggested substitute with two BC337 transistors, you could try that (i have that in both of my x0xb0xes btw)

replace the transistor with a 5-pin SIL socket (it might be a bit hard to plug it into the holes since they're not spaced out to fit a socket but it can be done)
then you could use another 5-pin socket and solder the BC337 transistors (with their legs bent around to match the needed pinout) onto that socket, and then you can plug that thing into the socket on the x0xb0x - like a little plugin
if it doesn't change anything you can plug the original transistor back in (but make sure you desolder it carefully so its legs survive)

now, do this only if you can't find another problem, because this is just a guess and it might not actually fix the situation (and desoldering the transistor is not fun)

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Re: VCF problems on M.Koster x0xi0

by rv0 on Tue Jan 24, 2017 9:39 am

I will repeat myself: Look for shorts!
In my case, there was a short on the cents pot, middle leg was touching one of the sides
also one cap (C33) wasn't properly soldered.

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Re: VCF problems on M.Koster x0xi0

by Neyah on Tue Jan 24, 2017 11:31 am

@Rv0 I have double check and I don't find bad soldering... but in r110 and r111 (which have bad soldering) I test the voltage and i compare with the voltage map found on this forum and I don't find the good result :
r110 (and so pin 4 of q21) : 1.74V
r111 (pin 2 of q21) : 11.71V

It may 5.92V at both

So I have no resistance on R111, but why the continuity test fall when I test the two side of r111 and the opposite side of r111 and q21 ?
and for r110, a bad soldering can increase resistivity for 3 times ?

Sorry for all this noob questions but that permit me to learn more about electronic.

And again great thanks to you two to be patient with my case.

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Re: VCF problems on M.Koster x0xi0

by antto on Tue Jan 24, 2017 1:14 pm

R110 and R111 don't connect directly to 12V, there's a series resistor in the way (R95) which is 100ohms
on the other side of R110 and R111 (on the pins which go to the dual transistor) - i have 5.94V on both

when the x0x is off, measure the resistance between 12V (for example at J4) and those two resistors
on one of their legs, you should get 100 ohms (that would be because of R95), while on the other leg you should get 22.1K (that would be the actual resistor (22K) plus R95)
then also measure between 12V and the actual pins on the dual transistor, and also check if there's a short (solder whiskers or something) between the legs of the dual transistor

other than that, the audio signal from the ladder passes thru Q19 (which works as a buffer, i think) before it goes to C14 as we already know
the base of Q19 happens to be connected at R110 so maybe that has something to do with the problem too (there could be a short between Q19's pins), or the resistor (R96) could be with wrong value

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Re: VCF problems on M.Koster x0xi0

by Neyah on Tue Jan 24, 2017 4:42 pm

R110 & R111 from r95 side : 100 ohms both
r110 & r111 from q21 side : 22K both (I havn't a top multimeter, only a small velleman so it isn't hyper precise)
Pin 2 & 4 of q21 : 22k

No short at sight or multimeter on q21 & q19.

R96 good value (10k) and connect to ground normally.

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Re: VCF problems on M.Koster x0xi0

by antto on Tue Jan 24, 2017 6:00 pm

okay
and the voltage on R110 (on the side towards the dual transistor) is still lower than normal?
i'm out of ideas.. if i were you - i'd replace the dual transistor with a socket and try another one in
if that doesn't change anything - replace the other transistor too (Q19)

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Re: VCF problems on M.Koster x0xi0

by Neyah on Tue Jan 24, 2017 7:03 pm

I found something ! r116 : 7.22k, r112 : 7.22k, r109 : 7.23k instead of 10k each. I don't know if it's what we are looking for, but it's anormal.

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Please be positive and constructive with your questions and comments.