"Official" tuning of VCF

[handyandy]

Hi all,

Sorry if this has been discussed previously. I am currently building my x0xb0x kit from Willzyx ( great service ) and I have just finished building the VCF and I'm now testing it with my scope. All seem's ok thankfully, but I'm a little confused over the section on the "official VCF tuning" section. I'm building each section and testing as I go along so I want to get this correct.

I have set the controls as mentioned in the fab manual and applied 3.0VDC via R90, and I'm probing VR4 pin next to C13. I can see the waveform on my scope changing with turning controls etc, but turning TM3 gets me no-where near the stated resonant frequency of 500 Hz.
When I do turn it the waveform changes, but the frequency does not ( I have even tried this on a calibrated frequency counter ). Am I missing something here?
I understand that it also mentions that this tuning can be carried out when the whole build is complete, maybe that is best? Is it then just a case of measuring the mix/headphone out frequency and trimming TM3?

Any help/pointers would be greatly appreciated.

Thanks

[handyandy]

***UPDATE***

I have actually just noticed that the manual states that this tuning is performed by playing C1 ( 65.4 Hz ). But earlier on in the manual it states in the VCO section that C1 is generated by 2.0V. Maybe this has some bearing on my questions.

[antto]

this is probably taken from the TB-303 manual about calibrating the VCF
so here's what it tells:
set the knobs as shown in the picture (ehm look at the manual) iirc it was cutoff and envmod centered, and resonance at max (maybe this explains why they left so much resonance on a bass guitar emulator, for calibrating the filter!!!)
play some C note the oscilations should have a period of 2ms which in other words means a frequency of 500Hz
adjust TM3 accordingly
what the manual DOESN'T say is <when> to take the measurement of the resonant frequency:
1) during the beginning of the note (when the filter envelope peaks)
2) during the end of the note (when the envelope has relaxed down)
3) somewhere in the middle of the filter envelope?!

so without this important detail, the whole "calibration" is a bit WTF..

[handyandy]

Thanks for the reply annto

I have got it sorted!

I took a look at the TB-303 service notes and as soon as I seen the little diagram there all became apparent.
As the x0xb0x fabrication manual says- play C1, cut-off = 50%, saw waveform, resonance = 100%. BTW probe point is the pin next to C13 on VR4.
But, if you are tuning the VCF after building the complete unit also put envelope, decay and accent at 0%.

Well now I have got the whole build complete and boxed up, tuned etc, and all I can say is GREAT
Love it!
Many thanks to willzyx for supplying the kit, very happy