Help sought debugging Eurorack TH-555VCO

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Help sought debugging Eurorack TH-555VCO

Post by alphonsom »

Apologies if this is not the place for this kind of request. I have built a Fonitronik TH-555VCO, and am trying to get it to do what it is supposed to. Output of any waveform only occurs above the halfway point of the coarse frequency pot: below, all is flat. I have been unable to find recommended values of R_LED or the speed indicator LED. Also F1 and F2 do not appear on the the only schematic I can find (at the link in the build doc). Has anyone any advice?
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Delta T
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Re: Help sought debugging Eurorack TH-555VCO

Post by Delta T »

F1 and F2 are 10 ohm resistors mounted on each side of the power connector (on the commercial pcb. One in the -12v line and one in the +12v line). The schematic at electro-music is the original Thomas Henry schematic/design. The Fonitronik version has been modified to work as a vco or lfo. The speed led is a dual pin bi-color (two color) type. It changes color depending on the polarity of the voltage across it. The resistor for it needs to be selected to set the led brightness where you want it. It’s not critical. Start around 2k to 5k and see where you’re at. Too bright - increase the resistor value. Too dim - decrease.

As for the other problem with wave not coming out…double check all of your wiring, solder connections, component orientation. Are you getting both + and - voltages to all chips, etc? When you turn the frequency up and the waves appear - are they correct for each output - Triandle, Ramp, Sine and pulse? Also - are you building this from a kit, diy pcb, etc?
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