I recently purchased an official Shruthivox from @Adam, I was aware that the cutoff didn’t shut 100%, but I was sure it would be an issue with the analogue board. However, after probing it, replacing some chips, and checking the resistor values 3 times, I decided to finally try a firmware update by powering it on and holding button 6 as per usual. Despite it being firmware version 1.02 it refuses to go to the update screen. I tried changing the control mode too, but they all didn’t allow me to boot into firmware update mode.
I began to suspect a faulty firmware installation when I assigned a longer decay time (103) to no sustain. It would gradually fade out then suddenly drop the volume to zero as if I had removed my finger from the key. This would also explain the odd behavior that the resonance Q parameter has where the resonance will just magically click on around 57. There just seems to be too many curve errors for this to be an analogue board issue at this point.
Any recommendations as to what I should do or check next? I could try attaching another Shruthi Control board to my polivox board to see if it will work, so I may try that next.
Ok, I used my 4PM Control Board, and the same audio issues persisted. I did set the filter from 4PM to PVK before testing.
The big difference is that I could initiate firmware update mode with this control board.
Clearly, I will have to reconsider what coudl be wrong with the filter board.
Exactly what parts of the Polivox circuit have to do with controlling cutoff?
What’s your problem with the cutoff exactly?
When the firmware-update does not work (no reaction to pressing the button combo) that usually means no bootloader is installed.
Oh flipping ek, I hope you get it sorted Matthew. It drove me to distraction but then I don’t really know what I am doing when it comes to electronics…the guy who fixed it for me originally was a recomendation by TubeOhm, when it came back all that was wrong was the filter issue…I really wish I knew how to fix the damn thing
Hope you get it sorted soon mate
@pichenettes: The cutoff range is very limited. When it is set to 127 the cutoff point is at about 4kHz. When it is set to 0 the cutoff point is at about 1kHz. And that is it. That is as far at the filter will open or close.
I am beginning to think that the issue(s) are with the analogue curve converters as the cutoff, resonance, and VCA signals from the digital board are correct, but they do not interact with the circuit as they should.
Here is an audio demo showing the issues with the cutoff range, resonance, and envelope.
@TheSlowGrowth: I figured as much. However, I can try to compile the code and reflash it or just get a properly programmed MCU as it is socketed.
@Adam: There is a bit more wrong with it than just the cutoff range. The good news is that the digital board is putting out the right signals. Do you remember who attempted to repair it before me?
@audiohoarder, re the person who fixed it before, yes, I sent it to Germany and it came back fine, only the cut off didn’t actually cut off! I can give you the guys details in a private message if you like?
here is a video I made a while ago showing the filter issue I had HERE
Yep, that is the same range of cutoff I am getting.
There are three digital to analogue CVs that are generated by the Shruthi: F for cutoff frequency, Q for resonance amount, and VCA for the volume of the VCA.
It seems that these three signals all have issues. IE: The resonance turning on when no key is held while self oscillating, and the VCA envelope just suddenly dropping to 0V. Those are both easy to miss if you aren’t looking for them.
Does this help Matthew>>>
Now I know what the previous repair was; that is always good.
You won’t have to compile it yourself. The firmware is available in a compiled format already.
@TheSlowGrowth: Good point! I totally forgot about that. For now I still have to decide what is wrong with the filter board. The schematics above are much easier to read than the PCB itself, so I now have a much better idea of what is going on in each part of the circuit.
Ok, sanity check time:
Is it true that with both the TL084P and the LM13700N removed that the digital board should not be able to affect the cutoff?
When I remove just the TL084P the resonance will no longer be able to be affected by the resonance signal.
IC2 is required for VCF frequency, VCF resonance and VCA control.
That is what I figured from the schematic. So I have a short somewhere. How fun!
The cutoff does sweep though the same frequency spectrum without the LM13700N in. It does sound a lot quieter, but it is the same spectrum.
Would the cutoff signal be able to leak though IC9 to affect the cutoff of the filter?