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Saw Bench output problems

What should be the output voltage be when there is no MIDI input? I'm trying to debug my build of a Saw Bench. I've used the other suggested locations for voltage measurements and get the expected voltages.

Comments

  • Think that should be 2.5V .. but not certain. It's been a while since we've ran diagnostics on the good old bench.
  • Best way to check if the signal path is alright: use an oscilloscope and probe the LT1013 opamp pins. The VCO always has a saw running, MIDI input or not.
  • I found that I have a square wave followed by 1/2 sines and the 1/2 sines have decreasing amplitudes. This is true for both outputs on all 4 op-amps. Changing the settings on the box front made no difference. Also the output voltage I measure at the jack is 5V.
  • There should be a big sawtooth on one side of the lowest opamp. Probing the diodes should yield a filtered sawtooth. On which IC exactly do you measure the square wave?
  • I measured at all 4 of the LT1013CP. I measured both OUT1 and OUT2.
  • If there's no saw, there's something fundamentally wrong. it could be stuff like bad resistor values? Is this measured with digital board on top, or not?
  • Yes, the digital board is in place. I always see the lights do the blinking during startup. By bad resistor are suggesting a misplaced resistor or a failed resistor? Other ideas for over the weekend for me to look would be great. Thanks for the help so far.
  • Failed resistor is very rare. Misplaced is more likely. Color codes are easy to confuse.

    Could also be the IC's are destroyed, but.. Best place to start looking is power mostly.. is there a solid 5V on the opamp supply pins? No ripples?
  • I measured on the order of 3V for all the Vin of the opamps. The measurement is very sensitive. What is the correct place to connect for the ground? That may solve some of the sensitivity issue. At the moment I think ground is acting like an antenna.
  • GND? clip to the 7805 regulator lump of metal
  • After almost a year of not being able to work on this I have some time. I now have more information including some oscilloscope plots taken at the 4 opamps. Are you still able to help?
  • Pufff.. it depends if I can remember everything, hahaha! :)
  • Thank you. If this becomes a time eater please stop. I'll send under email a pdf with the oscilloscope plots. Voltages read correct at Vin for opamps. The box continues to properly go through the startup routine with the blinking blue lights. The two boards are electrically connected to each other. The diodes have been checked and all work. Resistors by color code are correctly located. As best as I can tell from the pictures diodes are correctly oriented.
  • Does the LFO LED start pulsing _directly_ after the blinkenlights? Or is there a delay (10+ seconds)? If there's a delay, it cannot measure the saw output.

    Also note, all sawbench sources are now public domain and available for download. So you can use the schematic to easily check which signals go where. https://github.com/pietervandermeer/sawbench_firmware

    Awaiting your email.
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