Spirit Box low frequency oscillation issue with depth knob

[azzblaster666]

Everything seems to function properly except for on the effected signal there is a low frequency wobbly oscillation noise that increases as the depth knob increases. The off signal passes through fine.

I've seen a couple forum and reddit posts with similar issues, but the suggestions there have not been able to fix my issue.

All component values appear correct. I've changed R4 to 18K to pull up the volume closer to unity. Changing back to 10K and the issue still persists.

When troubleshooting, I've used a line out from a computer > pedal > Fostex powered monitor. The problem is not noticeable in this situation, but I'm unsure if it's because of the line level, or the low frequency is too low for the Fostex to produce.

Video w/ sound

Extra note: It's one of my earlier builds and is the only pedal I've been unable to get working properly. I had originally put the chips on backwards, and had to unsolder to redo since I didn't socket them. In doing that I exposed a few traces that you can see in the picture. Then I had a small speck of solder shorting one of the other Belton brick pins to the pin5 output send trace to the depth knob. Cleaning that was what got the effected signal to pass through, but now I have the low frequency oscillation.

Any suggestions on where to go from here?

Thanks

 

[giovanni]

Given the issues you had with this one, it’s hard to tell where the noise may be coming from. The board in the picture looks relatively dirty: not sure it’ll help, but try cleaning solder up with IPA and a toothbrush? As Chuck would say, next thing would be visual inspection of all joints and component values…

 

[giovanni]

I listened to the clip again and realized that the tone you are hearing is exactly around 60 Hz (a slightly flat B) so my guess is that it’s coming from the power supply. Either your supply is extremely noisy or the filter circuit has some problem. I would start the inspection from there.

 

[fig]

You may want to consider beefing up your electrolytic stash. 10V-rated caps in a 9V circuit is cutting it awfully close. A bit of bleed could introduce 60Hz ?

 

[azzblaster666]

I listened to the clip again and realized that the tone you are hearing is exactly around 60 Hz (a slightly flat B) so my guess is that it’s coming from the power supply. Either your supply is extremely noisy or the filter circuit has some problem. I would start the inspection from there.

Thanks for the replies.

The 60hz is a great observation. The power supply I've been using when I get the issue is different than the one I use when troubleshooting, so I will try a known good PSU or battery.

That would be great if that's all it is.

 

[azzblaster666]

You may want to consider beefing up your electrolytic stash. 10V-rated caps in a 9V circuit is cutting it awfully close. A bit of bleed could introduce 60Hz ?

Thanks, fig,

Everything else I have is 16, 25, or 50...not sure why/how I ended up with those 10s.

That makes sense. If swapping power supplies doesn't solve it, swapping out those caps with higher voltages will be my next plan of attack.

 

[azzblaster666]

giovanni, great call. It was dirty power causing the problem.

giovanni and fig, there is an increase of noise as the depth increases, though it is a far more tolerable amount and is now actually usable. Where would you recommend to go to get less noise in the wet signal? Would swapping out those 10v 100u caps to 25v's be the first thing to try? Anything else you could think of?

Thanks again for your time and help. You all are amazing.

 

[giovanni]

Hard to say. You could audio probe the power path to see where the noise is leaking through. It could be a short somewhere or a bad capacitor?

 

[fig]

I concur with what @giovani has said. C17 would be a likely candidate being that it is between the PT2399 and the L7805. Those chips are notoriously fickle and faked. Try another [2399] and see if the noise level changes?