Tri-Vibe Debug Help?

[ncusack]

Hello everyone,

I recently built a Trivibe pedal using the Andre Schaap layout. So far its working but I'm getting a fairly decent thump on the last 20% of the depth control. I took a few scope measurements and things look pretty close to the same as the graphs found on the ROG Trivibe page. The only measurement that seemed a bit off was the LFO signal. On the ROG page it has the wave ranging from 2vdc up to just below 7vdc. On my unit at max depth the measurement goes from 2vdc and up to 7.7vdc. Would the extra 0.7 volts account for the thumping I'm getting or would it be something else? Also if this is the culprit what can I do to bring things back in line?

Cheers,

Neill

[Pojo]

First, don't forget the links which helps everyone get on the same page! 
ROG article: http://www.runoffgroove.com/tri-vibe.html

Anyhow, I have a good working tri-vibe build. Let me know if there's anything you'd like me to measure on mine and compare with what you have (will have to wait a little though, at work now and band rehearsal right after work) Your reasoning about the extra .7V you're measuring sounds rational to me. 0.7v is the same as a silicon diode drop, so I would triple check over the 4148's surrounding U2b and go from there.

[ncusack]

Yeah I suspected the diodes too. All of them are reading normal drops of about .625V and they are all oriented correctly. I'm using three in series for the red LED in the schematic and thats giving me 1.8V drop across those. Would increasing or decreasing the voltage drop in that string bring things in line?

[Pojo]

That's a good question, unfortunately much of the circuit is over my head...including the LFO. Do you have an actual red LED you can try? I know the article mentions the 3x 4148's as an acceptable sub but might be worth trying the LED just to rule it out if nothing else (mine use the LED). I remember while I had it on breadboard that I experimented with reversing the LED. IIRC, when I had the LED backwards it seemed to change the shape of the LFO waveform to my ears (no scope available yet). It also didn't light up at all like that, but with it in the correct orientation the LED pulses opposite to the rate indicator LED.

Hopefully someone from ROG can pop on and be of more assistance.