Doppelganger oscillation

[muddyfox]

Alright... here it goes. This is a c/p from your post and I'll add my values to the right and mark the obvious variations in red.
FWIW:
-all my ICs are TLxxxCN
-all LEDs light up the same intensity
Here's a picture of all switches' positions, just to make sure we're on the same page:
-rates on 1 o'clock
-spans, color, blend maxed
-square, slow to the right
-antiphase, true up
-phase on (I think  )
-one lfo (green LEDs light in unison, I suppose that's one lfo?)



Source: 9.54V  9.33
IC1 (TL074):
1-3: 4.35V  4.29
4: 8.79  8.59
5: 3.71-5.0  3.8-5.2
6: 3.71-4.87  3.7-4.6
7: 3.7-4.8  3.6-4.7
8: 2.6-7  2.7-6.8
9: 3.8-5.3  3.8-5.2
10: 3.4-5.6  3.27-5.5
11: 0  0
12: 2.3-9  3.16-5.63
13: 2.8-8  3.6-5.3
14: 0.4-9  2.1-7

IC2 (TL072)
1: 3.8-5  3.7-4.5
2: 3.8-5.1  3.7-4.9
3: 4-5.3  3.8-5.2
4: 0  0
5: 3.8-5.3  3.8-5.1
6: 3.9-5.3  3.8-5.2
7: 3.9-5.3  3.8-5.2
8: 8.7  8.6

IC3 (TL074)
1-3:4.17  4.1
4: 8.12  8.1
5: 4.15  4.0
6: 4.17  4.1
7: 4.17  4.1
8: 4.29  4.1
9: 4.24  4.1
10: 4.16  4.0
11: 0  0
12: 4.16  4.1
13: 4.7  4.4
14: 5.2  4.1

IC4 (TL072)
1-2: 4.19  4.1
3: 3.4  3.3
4: 0  0
5: 4.17  4.1
6: 4,13  4.1
7: 3.8-4.1  4.1
8: 8.09  8.04

[m-Kresol]

thanks a lot! The voltages look quite ok, I guess. I'll make sure that we have the exact same settings and measure the parts again, where they don't agree.
I guess I have no other option than to disassemble it this weekend and check the back of the pcb, check for solder joints and reflow everything.
Just fyi, swapping the LED that was not quite as bright as the others didn't help. It's the same brightness now, but the noisy breakup is still there.

Try pulling out the dry output run or shielding it. I didn't install it in mine and it looks like Lace didn't have it installed on his prototype. It is running to right before R7 and IC3A which puts it right before the modulation section. 

I don't really see how that could influence the signal tbh. I only put the dry out in for completeness' sake, but isn't it basically a wire to nowhere if nothing's plugged in? There is no circuit than I think. Correct me if I'm wrong. Or do you think something is coupling into the dry output wire which is then fed back to the signal?

[blearyeyes]

I'm no brainiac but it looks like that connection is in the right place to cause an issue. It's in the audio path right before the modulation of LFO1 and from what I can see you're getting oscillation in the audio path that is being modulated by LFO1. So maybe something is feeding back through the wire.. It looks like it runs under the whole board. Seems like a wire to nowhere could be an antenna but I could be completely off base. Tbh I'm just guessing but that is something I would try.

[m-Kresol]

thanks for the explanation. I ran the wire on the top side of the board above the pots, but I'll check it for sure.

[muddyfox]

Do you have a specific knob/switch position that you can set and get immediate oscillation or do you have to coax it out? I'd like to try and replicate it and your vid kinda makes it hard to see (or I'm just being dense today  ).

Also, I shielded my input as well although it wasn't asked for. If you're picking up some garbage on the input, maybe that's your problem that just runs away under certain conditions? 

[muddyfox]

You removed the YT clip? Now I definitely can't replicate it. 

[m-Kresol]

The video is running fine on my computer. It's just some really specific settings as it seems that is kinda influenced by all knobs. I had to fiddle around to find it, but there seems to be more than one setting that produces different kinds of oscillations depending on the switch positions.

[muddyfox]

Huh... it's prolly something at work blocking yt in a funny way...
Anyhoo, I'll give it a shot again but no fiddling I did produced anything unusual the last time I tried it.

[blearyeyes]

I fiddled with mine and never heard a peep. Plus the crunch issue sounds like a cold solder joint.

Look at the components in the audio circuit right after IC4a before the first LFO modulation occurs.
Look at the dry output and how it connects to the circuit on the schematic.

Audio probe would be really helpful as well. Might be able to find it without taking it out of the box.

Good luck hunting!

[m-Kresol]

Hi there, I wasn't really in the mood for debugging on saturday and didn't have time today so I didn't really devote myself to the build. Yet, here's an update:
* I rechecked the voltages that were different from muddyfox' values. Mine were the same as before.
* I poked around with the audio probe of my testing rig. I found that the signal is fine till after the first opamp part (IC4). the output is still crystal clean, but starting after R44 (going to the dry out) or R7 there is some noise.
* I haven't taken it apart yet, but I hope there is only a cold solder joint somewhere at IC3. As the weird oscillation is just at very specific, hard to find positions I could actually live with that, but the scratchy noise is a no-go.

I'll keep you posted on the progress and thanks for all your efforts.

[blearyeyes]

Try removing the dry out

[alanp]

Try replacing IC4 (nearly any dual opamp should work, for a certain value of work.)

[m-Kresol]

UPDATE:

I'm super exited right now and I couldn't wait to tell you. I just removed the dry out and the noise is gone as well as the funky oscillation!! Thanks a ton blearyeyes. Next time remind me to just do what you told me all along first thing.
It sounds way different now as well. No clue how this is possible, but who cares
I'll use shielded now and report back.

[m-Kresol]

Guys, first of: Thank you big time. Especially blearyeyes for guiding me to the real problem and muddyfox for taking time for voltage readings.

Now, here's a funny story:
I replaced the dry out wire with shielded cable and the noise was back. I was confused since I thought the noise issue was the result of some antenna effect of the wire running a long way around the enclosure. I just unscrewed the jack from the enclosure and tried again - without grounding the shield. No noise. Not a tiny bit. Crystal clear sound.
So upon closer inspection of the jack I finally found the culprit... as you can see in the pic in the first post, out and dry out are VERY close together. I feared there was an issue of touching metal parts, so I had put some insulating tape in between. Turns out the tape ripped just a teeny tiny bit at the edge thus making contact to the regular out. I guess some of the output signal was fed back through the dry out or something like that resulting in oscillation. I added some hot glue to insulate the jacks better and now it works like a charm.

This turned out to be an amazing day. I was productive at work (finished my conference poster for Istanbul); time flew by, so over 10 hours of work didn't bother me. I fixed the doppelgänger and tried my first attempt at toner transfer, which came out spectacular and now I'm just really happy with how the build turned out. I just gave it a quick try since it's already rather late, but the sound is amazing so far. Fixing the issue gave it a fuller tone, the colour knob is actually doing something and the build looks killer, if I might say so.

Thanks again! be ready for my build report

[muddyfox]

Glad you got it working, Felix! Good to finally hear some good nooze (as I can't seem to do anything right these days with pretty much everything I touch).