Hello everyone, glad to join the forum!
I’ve just finish to build my cv-12 and ONE thing doesn’t work properly for me, it’s the pitch bend port C1 (c0 in the diagram ) it’s calibrated 0v > 5.40v < 10.52v ( and it looks that the pitch is inverted ) instead of 0v > 2.5v < 5v. Don’t know where to find the trimmer/pot of the pitch bend everyone is talking about, is it the R2 5K trimmer, but if touch it will it not affect my note cv calibration? I surely miss something here.
Hey, shouldn’t a pitch bend be going in both direction ?
-5V < 0V > 5V
I guess i would use somehting like an attenuverter to aachieve this.
From your description it reads like you have an opamp dividing your voltage by 2 somewhere…
You could try to track where in your circuit you drop from 5V to 2.5V.
Hey Bpbby, indeed I meant 0v < 5.40v > 10.52v and in the manual it is 0v < 2.5v > 5v with no negative value , so I did try to find on the schematic wich component doesn’t work but with no luck yet…
In fact it’s the only cv-out that doesn’t work properly on this port, kinda strange…
I’ll try to come back with a solution soon
One important thing I forgot to say , it is inverted , by inverting the signal with an external module I have something that start to looks like a Pitch bend …
It’s just voltage followers on the output of the pic chip, only thing I can suggest is that some bad joint around that chip or a faulty LM324. The PIC chip itself can only output a MAX of 5V, but the LM324 are powered from +12V so its possible, measure pin 12 of U5, that’s the input to the sample and hold for the pitchbend. It should be 0-5v there before it is buffered by the LM324. Pins 13 & 14 of U5 should be joined together to form the voltage follower, also swap two of the LM324 chips around and see if the fault moves or stays.
It’s not possible as it is a unipolar supply to the LM324 and the PIC chip. Most cheap MIDI to CV converters have a minimum voltage of 0V and cannot go negative.
Ok, it acutally makes more sense to have 2.5 V bend, 5V would be quite extreme. It is weird then, you have more voltage than needed.
Maybe one resistor (from a voltage divider) is not soldered to ground properly… good luck with this.
For me, 0-2v with 1v center is better for pitch bend, then you can swing an octave up or down at full range in a 1v/octave system. I designed my midi to CV converter to be 0-2v for bend and 0-2v for modulation as that is useful for controlling a VCA to let the LFO through.
Hi craigyb, thanks for your help, when I said inverted I didn’t mean negative, just that when I use the Pitch Bend Wheel on the midi controller the pitch goes down when I scroll up and the pitch goes high when I go down. Sorry for my bad english I just hope you understand
I never thought for one moment it could go negative, but again that is weird behaviour for the bend range, have you tried just swapping a couple of lm324 chips around to see if the problem persists with the pitch bend and did you measure pin 12 of U5, and pins 13 and 14 should show the same voltage, if they are different then something is wrong around that 324.
Looks like it! I dont think that the LM324N is the the problem because all the other PCB ports (c2-c3-c4-c5-c6) gave me good results, maybe another element on the board circuitry , I think I’m going to find out…
I did check the R9 100k as it was next in the chain…
So you are assuming that the chip is ok because other outputs work, I would test with a quick swap unless the chips are soldered in and not socketed, I have had 324 and 074 where one quarter of the chip is blown, I have a simple tester just for that which lights up an LED for each working quarter or half etc depending on the opamp I’m testing. Like I said, measure pin 12, 13 and 14 of the pitchbend output to be sure its behaving itself. I don’t even own a CV-12 but that’s where I would start based on the diagrams.
I complety agree with you. At this point my voltage problem on this pin is more the fact that it’s acting upside down than the fact that i have a 1v signal. I just have to find why
No no do not change the PSU.
Everthying else is working, as @craigyb said, it most likely is a very located problem, chip, resistor, cold joint somewhere…
Looking at the circuit, there is a 10.6V (close to your 10.52V). It looks like the pitch bend is connected to this rather than the 5V.
I’ve just taken another look at the schematics, my bad, the MC14504 are level shifters accepting 0-5v in (probably PWM) and upscaling that to 10V (10.6v supply). So maybe the fault lies on the output of the 4504 before the opamp which are just acting as voltage followers and smoothing the PWM by the looks of it.
Swap the 4504’s over to check them as well as the 324 chips.
good luck with this.
