OC-7 Octaver : brief crackling noises upon activation, related to the upper octave.

[eh là bas ma]

Hello,

My OC-7 clone worked fine since october 2022.

https://www.delykpcb.com/product/up-down-daddy-pcb/?attachment_id=1089&download_file=k0sve9jvlkveb

Last week it started acting up, with crackling popping noises when the Oct. up control is set somewhere CW before i switch it on. This control also become a bit noisy, with a "Shhhh" sound when i turn it.

No issues with both octaves down.

The noises happen upon activation for 3 or 4 seconds and disappear. It doesn't happen at every activation, but 1 out of 3 or 4 times.
Which makes it quite hard to track down with an audio probe.

However, the effect works perfectly if the oct. up control is set fully CCW before i switch on. Oct. up pot has a quiet rotation. Everything works fine, or so it seems.

I already replaced the Oct. up pot, the output electrolytic cap, Q1, Q2, Q3, to no avail.

Q1 J201

G= 9.46
S= 9.46
D= 4.60

Q2 2N5457

G= 5.36
S= 5.39
D= 0V

Q3 2N5457

G= 5.36
S= 5.39
D= 0V

IC1 TL72

1= 5.15 8= 9.46
2= 5.08 7= 5.45
3=4.68 6= 5.45
4= 0V 5= 4.88

IC2 LM324

1= 4.84 14= 8.27
2= 5.38 13= 5.15
3= 5.33 12= 4.87
4= 9.46 11= 0V
5= 5.34 10= 5.15
6= 5.38 9= 5.38
7= 5.30 8= 0V

IC3 CD4013

1= 5.70 14= 9.46
2= 3.8 13= oscillating from 0V to 9.46
3= 0V 12= oscillating from 4.50 to 7.50
4= 2.1 11= 3.8
5= 0V 10= 0V
6=oscillating from 2.5 to 5.38 9= oscillating from 0V to 9.46
7= 0V 8= 0V


IC4 CD4013

1= oscillating from 4.35 to 9.46 14= 9.45
2= 9.45 13= 9.45
3= 9.45 12= 0V
4= 0V 11= 0V
5= 0.5mV 10= 0.5mV
6= 0.3mV 9= 0.4 mV
7= 0.3mV 8= 0.3mV

IC5 TL74
1= 5.46 14= 5.46
2=5.46 13= 5.46
3= 5.42 12= 4.99
4= 9.45 11= 0.5mV
5= 5.02 10= 5.42
6= 5.45 9= 5.46
7= 5.45 8= 5.46

IC6 TL74

1= 5.36 14= 5.46
2= 5.37 13= 5.37
3= 4.93 12= 4.94
4= 9.45 11= 0V
5=4.92 10= 4.93
6= 4.93 9= 4.93
7= oscillations between 2.3V and 3.5V 8= 7.55

I am not so good at deciphering complex schematics. I wonder where this kind of issue could come from ?

Every observations and suggestions are welcome !

 

[eh là bas ma]

I think i figured it out.

Probably some short between the board and the back of the pots, despite the insulating black stickers from musikding.

I changed all ICs, all transistors, all electrolytic caps, 3PDT, DC power jack, reflowed many pads, performed lots of continuity tests, cleaned the pcb soldering side with some soldering residue cleaner, to no avail.

The effect was fine unboxed, it put me on the right track to understand that the pots weren't perfectly insulated when pressed against the board.
I've already noticed this possibility much earlier in my debugging process, thought i fixed it, but it still wasn't enough :

This project is a very tight build with lots of soldering pads under the pots, unless the soldering is very clean, it's harder than usual to insulate the pots.

More issues appeared because of all my tinkerings : TL72 got hot because i neglected to take it off while reflowing some pads near it, some wires connections became weaker because of the movements, etc.

Now it's finally all fixed, lets hope it stays that way.

 

[jimilee]

I think i figured it out.

Probably some short between the board and the back of the pots, despite the insulating black stickers from musikding.

I changed all ICs, all transistors, all electrolytic caps, 3PDT, DC power jack, reflowed many pads, performed lots of continuity tests, cleaned the pcb soldering side with some soldering residue cleaner, to no avail.

The effect was fine unboxed, it put me on the right track to understand that the pots weren't perfectly insulated when pressed against the board.
I've already noticed this possibility much earlier in my debugging process, thought i fixed it, but it still wasn't enough :

This project is a very tight build with lots of soldering pads under the pots, unless the soldering is very clean, it's harder than usual to insulate the pots.

More issues appeared because of all my tinkerings : TL72 got hot because i neglected to take it off while reflowing some pads near it, some wires connections became weaker because of the movements, etc.

Now it's finally all fixed, lets hope it stays that way.

Woohoo, nice job.

 

[eh là bas ma]

Unfortunately i couldn't fix it for good.

I changed all ICs, all electro caps, the octave up pot, cleaned the pcb from residues, but no improvements.

Here is a sound sample of the symptoms :

https://soundcloud.com/ehlabas-ma-chaussure%2Foc-7-issue

I hope someone can help me narrow it down ?

Once out of 20 times, the up octave effect works, but it's unstable. No issues with the octaves down, but i suspect they don't sound as good as usual.

I am ready to change all the diodes now, but i am not sure it will be helpful...

Do you think this kind of issue can come from a damaged diode ?

After all the time spent debugging this build without much progress, i ordered a new kit, so i can compare both circuits with an audioprobe and narrow it down. But it will take weeks before i receive the order.

Every observations and suggestions are welcome !