Hello everybody, I’m new here! I’ve got a Melody Mill a few days ago and everything seems to work fine, only the 1V/Octave output is always high, no matter what I’m doing/changing/pressing or have it running with the sequencer (I measured 10.47 Volts). the Rectangle output on the right works fine and reacts to all the knob and keyboard changes/information. Also Gate and Trigger output works perfectly.
I have a Doepfer PSU 2 with the Doepfer 5 V Adapter on.
I just observed the circuit board and saw that there is no capacitor on position C13. Can anyone take a look on his/her melody mill and confirm if this is normal or did I loose this one on my pcb? if so, are there any schematics out or does anyone know the value of this capacitor?
I’m not an super expert, but I followed the trace coming out of the bit stm32 chip going to the tip of the rectangle output jack via a smd resistor (R24). Between tip of the jack and the resistor, the signal goes to ground via a smd capacitor (C13) - what I’m missing on my pcb.
C13 should not be a problem… It is a filter cap for the square wave output, and it might never have been installed anyway.
There is small trim pot you can use to adjust the 1V/oct scale, you could try turning it with a small screw driver and see if it changes at all. When you say it is always high do you mean it is never changing? This might indicate another problem / damaged component.
Hey @oweno thanks for your quick response! I tried the 1V/oct adjustment trim pot but still the same output of 10.47V. It’s never changing, no matter what I do, it’s constantly 10.47V.
So the 1V/oct signal is coming out of the LM358, could there be a problem? I’m able to measure or change a damaged component, but I don’t really know where to search / measure.
I did some measurements on the LM358 which is connected to the 1V/Octave ouptut (pin 6+7). The first measurement is with the trim pot in original position, the second with trim 2x ccw, the third with trim 2x cw.
meas.1
pin 1
10,34V
pin 2
6,27V
pin 3
6,27V
pin 4
0V gnd
pin 5
10,32V
pin 6
10,32V
pin 7
10,32V
pin 8
11,65V
meas.2
pin 1
10,28V
pin 2
6,75V
pin 3
6,75V
pin 4
0V
pin 5
10,29V
pin 6
10,29V
pin 7
10,29V
pin 8
11,64V
meas.3
pin 1
10,32V
pin 2
5,92V
pin 3
5,92V
pin 4
0V
pin 5
10,32V
pin 6
10,32V
pin 7
10,32V
pin 8
11,66V
also I programmed a little sequence so the notes are changing while measuring, also I turne the scale and tune knobs, no change. thanks for any help! Jerome
The LM358 scales and buffers the 1V/Octave from the DAC (IC4). You want to check the output of the DAC to make sure it is working, so check pin 8 on IC4 and you should see different voltages for different notes. If that is working, the LM358 might be blown…
Thanks again @oweno for your answer! I measured the output on pin 8 from ic4 and it seems to work. I can measure the voltage difference when notes are playing, also the offset from the tune knob, also the range.
There are only small changes about < 2V but you mentioned the lm358 as a scaler…
So do you think it would be worth a try, changing the lm358? And what substitutes would work?
Yes, the range would be small (0-3.3V total), then gets scaled by the opamp (LM358). So might be the opamp that is the problem. You could try replacing it. This is the part:
So I got the module back to life. I changed the LM358, recalibrated the 1V/oct output and it works almost perfect. There is just one thing, the 1V/oct output jitters a bit, not much but clearly audible. Also the rectangle audio output jitters the same way. Any ideas what this could be?
Thanks,
Jerome
Hmm, if both the audio output and CV output are jittering it is probably not a problem with the output circuitry since those signals are separate. Perhaps it is coming from the Tune knob…I’d be curious to know if it is jittering more or less when the tune knob is up or down.
I will try that and will let you know! Thanks! Yes I know that the rectangle audio output is coming more or less directly from the main chip, the 1V/Oct too (if I’m right) but with its own circuit before the output, so I hope that there is no problem with the main chip
I’ve got a Melody Mill a few days ago and everything seems to work fine, only the 1V/Octave output is always high, no matter what I’m doing/changing/pressing or have it running with the sequencer (I measured 10.47 Volts). the Rectangle output on the right works fine and reacts to all the knob and keyboard changes/information. Also Gate and Trigger output works perfectly.