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Sawbench issue

I (finally) completed my assembly of the sawbench synth and this is what I am experiencing:

The light chase sequence at the beginning happens as described but it takes almost a minute for the vco to calibrate and the LFO light to go on as opposed to a second. as described in the manual. Inputting midi from either a keyboard or sq-1, all I hear is a "thump" sound that is very drum like but there is no fluctuation of pitch. In general, none of the knobs are responsive to alter the sound other than VCA attack; if it is not in the far left position, there is no audio.

Besides re-checking my soldering, I am at a loss with where to start diagnosing this.


  • Sounds kind of like a problem with the Atmega. Check the crystal, reflash the atmega.
  • indeed.. could be a badly flashed atmega, but we check that. the atmega needs the vco reset signal from the analog board. measure the vco opamp pins. BTW good news. we're open sourcing the sawbench hardware so you'll have access to the schematics.
  • I will start with reflashing the atmega on the weekend; can you please provide more info with regard to measuring vco opamp pins- which pins and expected voltage?
  • look at the digital board. there's a 6 pin header called VCO&VCA_CV1. on one of the pins there should be a periodic spike from the VCO saw discharging.
    Pin 1: VCO CV
    Pin 2: VCA CV
    Pin 3: DISCHARGE
    So, that should be pin nr 3.. Please inspect it with your scope.
  • Reflashed the atmega and that made no change. I don't have a scope but I can see a constantly fluctuating voltage on the discharge pin between roughly 3.2 and 4.2 volts when I am inputting midi. with no midi, the output of the pin is a constant 4.29 volts.

  • Without an oscilloscope, you're not going to learn much. The VCO is being calibrated which means it's doing a frequency sweep. You should see the pulses increasing in frequency. Please buy or borrow an oscilloscope.
  • Assume that I manage to save up some money and buy an oscilloscope in a few weeks time. If I do not see a sweep from pin 3 on the start up calibration, what are next steps?
  • You can get a second hand for 100 EUR or so if you look well ;)

    If you don't see that check pin 1 (VCO CV) on that same header. That should be increasing in voltage as well.. If not something is wrong with the Atmel or the RC filters inbetween.

    Just this morning we've open sourced the schematics.. have a look:
  • Is this the schematics here?

    If so, how should they best be viewed?
  • Yep. Use EAGLE. I think a hobby license is still free.. Otherwise we may put PDF's up someday..
  • OK thanks. I'll save a copy just in case.
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