SOLVED Arachnid with loud hiss/noise (yes, another one!)

[p_wats]

I remember a time when I used to build pedals that worked. In fact, this WAS one of those pedals! I built this double Arachnid last year and we used it heavily both live and while recording, but then the rotary control on the right in the image below failed (as confirmed way back in this thread).

I was able to get the defective switch out without issue and finally got a replacement. I popped it in last night and things seemed fine. I also added a toggle to access the internal programs (connecting ground at R5--the angle may make it look like it could be touching R2, but I assure you it's not) and again things seemed fine. However, after getting excited and trying it my usual rig I noticed a very loud hiss/noise that was not there before the repair.

What I've learned so far:

• The noise changes depending on the power supply (can get worse), but is still present with battery power
• The noise is NOT present in the circuit on the (inside) left---that circuit is as quiet as the FV-1 can be
• Using my audio probe I can verify the noise is not present as far as pin 2 of the TL074, but seems to come in at pin 1 and is affected by the volume pot
• I have changed the TL074, to no avail
• Thankfully, I've got a perfectly working Arachnid right next to it, so I have confirmed all voltages seem normal (4.5v on pins 1 & 2)

After coming up empty trying to troubleshoot my last FV-1 build, I'd love it if folks would chime in with any ideas here!

 

[zgrav]

Does the noise change when the Mix pot is changed? Or is it constant?

Does the noise change if you toggle the internal programs on or off?

Make sure the 220pf cap is connecting through the PCB to lugs 1 and 3 of the volume pot. Also maybe try temporarily attaching another cap to those lugs to see if it makes any difference in you noise.

 

[p_wats]

Does the noise change when the Mix pot is changed? Or is it constant?

Does the noise change if you toggle the internal programs on or off?

Make sure the 220pf cap is connecting through the PCB to lugs 1 and 3 of the volume pot. Also maybe try temporarily attaching another cap to those lugs to see if it makes any difference in you noise.

Thanks @zgrav.

• The noise is constant (does NOT change with the mix knob, only with the volume knob).
• Toggling internal effects has no change.
• There is continuity between both sides of the 220pf cab and lugs 1 & 3 of the volume pot. Temporarily adding an another 220pf cap did not affect the noise.

Curiously, and maybe unrelated, the mix pot seems to remove the wet signal as you dial back, but some wet signal reappears at the fully dry position (ie, all the way counter-clockwise).

 

[zgrav]

do you think the noise may be constant relative to the sound level? so it only sounds noiser because the overall volume is increasing?

not sure where your noise originates and how it would have anything to do with replacing your rotary switch and putting in the toggle.

 

[p_wats]

do you think the noise may be constant relative to the sound level? so it only sounds noiser because the overall volume is increasing?

not sure where your noise originates and how it would have anything to do with replacing your rotary switch and putting in the toggle.

It's possible the noise is relative to the sound level, but hard to say. Audio probing doesn't seem to indicate any unusual noise as the signal enters the TL074 at pin 2.

Basically, if I set the controls of both circuits to the same positions, the one is very quiet and the other is very loud. It also seems to get a lot worse depending on power (completely unusable with my TC Electronic Fuel Tank Jr.--just a loud hum, which may be down to how much load the power supply is under, but why would it work fine with one half and not the other?), so I was thinking there maybe be something to run down there.

My assumption is maybe something bridged or got loosened when I had to move the board to install the new switch. All voltages and continuity seem to check out though.

 

[zgrav]

you could try replacing the filter cap for the board and see if it makes a difference.

 

[p_wats]

you could try replacing the filter cap for the board and see if it makes a difference.

I temporarily clipped in another 100uf cap after the 1n5817 to ground and it didn't make a difference.

However, your question around noise relative to sound level had me playing with the volume knob a bit and I notice it has the same behaviour as the mix pot: when full-counter clockwise the signal doesn't turn off, it decreases as you dial it back, but then has a small uptick at the end.

The other circuit goes fully dry with the mix pot and fully off with the volume knob dialed back, so I'm curious if this is a symptom that could be related to the other problems.

 

[p_wats]

@zgrav, I Found it!

Looks like moving the board in/out to remove/reinstall the switch pulled on the mix pot and upset the trace at lug 2 (VREF) which meant VREF also wasn't getting to C17 (and not being filtered to ground). I only saw it when I had my magnifying visor on and was carefully following the circuit for continuity.

That seems to explain both my pot inconsistencies and the noise.

I've run an extra jumper from lug 2 of the mix pot to R6 and it seems to be fine now.

Thanks again for all your help! I couldn't let a 3rd FV-1 in a row defeat me. Ha. (now maybe I'll have the guts to revisit the other noisy one).

 

[Gordo]

Well done!

 

[zgrav]

Glad you got it working!

 

[p_wats]

Glad you got it working!

Thanks again for the support. I owe you a beer/coffee/whatever for chiming in on my various threads.

I've got one more very old FV-1 build to re-conquer now (high-pitch whine) and will be sure to take a very close look at the traces around the board-mounted pots.

But I may just actually make music for a while. Ha.