SMR4mk2 output issue

Hey guys,
I’m assembling my first Shruthi and am having some problems. The output sounds nasty and FM-ed, even though I have double checked the voltages at the points near the ICs and have a 400milliamp, 9v center positive power supply. Also, I’ve tried probing the circuit with an audio jack and there is no signal even at point 1.
I need some help!
Thanks!

Could you post a recording?

It would be worth trying with another PSU too.

Is the control board attached to the filter board when you are checking the audio path?

Sure, I will post a recording in the morning. The two boards are connected when I actually attempt to play it, but when I’m probing with the jack they are not.

You need to use jumper wires to connect the two boards when troubleshooting the filter circuit.

Keep in mind that the filter board does not generate any sound by itself: it processes a signal generated by the digital control board.

Ok, thanks! I’ll get back on it as soon as I can.

Ok, tried it again today on some different power outlets, and the signal does not sound quite as nasty as before. It still has some hum though. Here it is. Also, checked the audio path on the filter board and it’s all good! Unfortunately I don’t have access to another compatible PSU; maybe I could piece something together with a 9v battery?

100% sure it’s power-brick related. I can tell you’re in a 60 Hz country :slight_smile:

Hahaha! Great! I’ll find another then. In the meantime, can a battery be used somehow?

Yes!

Ok, soldered a battery to a connector and it works great! Thanks!

Any ideas why my filter resonance barely comes through when set to self-oscillate? When I mute both oscillators and play the synth, the oscillation is very quiet.

Are you still using the battery or a wall-wart?

Wall wart. When I was finishing construction of the synth, I thought I noticed this function flickering between this behavior and the normal behavior. Also, I realized last night that I had a few resistors switched that were causing MIDI I/O not to work. R19 and R14 are swapped, and at R18 is also a 10k like R14. So maybe when I find the one that should go at R18, and swap them, the filter issue will be fixed too? Or maybe it’s just a bad solder point somewhere?

Ok, I have a real problem I think. I have no MIDI I/O. I looked and saw that I had swapped R19 and R14, so I swapped them back and then went over every solder joint between there and the ATmega. I also have tried it with the 6n137 in both orientations.

Fixed! Found the solution on another thread.

Man, I am just running into one thing after another. There is a break in the audio chain somewhere between points 1 and 2 as defined by the troubleshooting section of the instructions. I replaced the 10k and 100k resistors between those points, because I had some extra, but to no avail. I had just been soldering some wires to the pole selectors for the mod. Could that have done this?

Maybe resolder all pins of IC2?

Ok, I’ll give that a shot!

Nope, that didn’t do it. Maybe I need a new TL07? Or any other suggestions?

Check the soldering of all parts around the TL072.