Melody Mill 1V/Octave output problem

Hello everybody, I’m new here! :slight_smile: 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.

Any ideas what could be the problem in this case?

Thanks for any Help!
Regards,
Jerome

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.

Thanks for any help!
Cheers,
Jerome

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?

Thanks a lot!
Jerome

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:

https://www.digikey.com/product-detail/en/texas-instruments/LM358DRG4/296-31668-1-ND/3505872