SOLVED Loud pop from Arachnid (it's all gone downhill from there)

p_wats

p_wats

Well-known member
I'm getting a loud pop in an Arachnid build I just boxed up, which is strange, as I've used the relay bypass before and it was absolutely silent---in fact, this exact bypass board/switch was salvaged from a build where it had also been silent.

Here's what I've tried so far:
  • Different power supplies (including battery): same pop
  • Different amps/cables: same pop
  • Jumpered in an extra pull-down resistor on input and output: still pops
  • Disconnected the LED: same pop
Curiously, the pop only seems to occur on power off, not power on.

Also, in this build I'm doing the wet/dry output mod (taking the dry out from pin 7 of the TL074) and the dry out doesn't pop.

Any ideas before I take it all apart and try replacing the relay board (I have a few extras on hand)?
 
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Maybe your output cap is leaking a bit too much? I’ve found that to be a cure for most of my switch pops. If there’s an electrolytic on the output especially I change it to a film. The arachnid calls for an MLCC 1u there so it’s not super likely but it may be worth replacing.
 
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Nostradoomus said:
Maybe your output cap is leaking a bit too much? I’ve found that to be a cure for most of my switch pops. If there’s an electrolytic on the output especially I change it to a film. The arachnid calls for an MLCC 1u there so it’s not super likely but it may be worth replacing.
Click to expand...

Thanks for the suggestion! I tried jumpering an extra 1u MLCC in series with the output and it had no effect. I also connected a new one in parallel with the existing C15 and that made no difference either.

I also tried moving where I'm taking 9v from to power the relay board (pin 4 of the TL074), but that made no difference.

Very strange. Unfortunately it's a very consistent, loud pop.
 
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Nostradoomus said:
Damn looks like we got a gremlin on our hands!
Click to expand...

Robert said:
Ahh okay, that won't be the issue then... this was almost two years ago.

(I see now that you already jumpered in a pulldown, doh!)
Click to expand...

Ha. This enclosure is cursed (it's the same box I had an old Arachnid in but somehow damaged the FV-1).

I guess I'll carefully remove the relay board (it's attached by soldered pin headers, alas) and test the board alone again before trying another relay board.

I've also got the clock module board going, so that adds complexity. That said, the pedal itself works great aside from the pop. The pop just makes it unusable unless left always on, which it won't be.
 
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Bad news: I removed the relay board and the main circuit still pops consistently.

Good news: the relay switching is likely fine and as quiet as I've come to expect and I've localized the pop to the main circuit/clock module.

Some notes on the main board:

  • Aside from the electrolytics, and the 2n2, all caps are MLCCs.
  • I was out of 1/4 1k resistors, so 2 of them are 1/8, but given the voltage/current, I didn't think that was a problem (hopefully that's not wrong, @PedalPCB, I believe some circuits, like the Hydra, use 1/8 size).
 
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I had just the main board jumpered to my test rig with a 3DPT. It was inconclusive though, as another half finished circuit was popping in that set up too.

Unfortunately, before I could do much more troubleshooting, something started going haywire with the circuit itself. I'm not sure if I shorted the clock module or having it disconnected too long can affect things, but I suddenly only started getting the internal programs (again!) regardless of whether pin 13 on the FV-1 was grounded or not or which EEPROM was installed.

I've now tried swapping the 4049 and TL074, but it's only gotten worse. Just loud hissing/static now on the wet side of the mix knob that still demonstrates a change with the rotary switch, and it sounds like (internal) programs start up underneath. The 78L33 is getting really hot too--far hotter than the working double Arachnid build I have beside me for comparison.

I've removed the clock module and slotted in the crystal/cap again (not soldered, just placed in the holes and bent), but I'm not sure what's going on, as everything I touch tonight seems to just get worse.
 
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Figured I'd give this another shot after some sleep.

Here's where I'm at:

  • Dry signal seems fine.
  • Wet signal varies depending on where the CTRL pots are set, but the only signal passing through is a crackling, explosive mess.
  • The CTRL pots and rotary selector do affect change to the noise, but hard to tell what patches are being used
  • The toggle to ground pin 13 seems to have no effect on the sound
  • The voltage regulator still seems much hotter than the ones in my working builds (I've left a working version plugged in for the same amount of time and it's not uncomfortable to touch, but this one is)
What I've tried:

  • Replace the voltage regulator: no change
  • Replace the 100uf cap: no change
  • Replace the TL074: no change
  • Replace the EEPROM: no change
  • Reflow all the solder joints: no change
  • I've re-added the clock module and the clock control does change the noise (makes it worse at slower settings)
Audio probing seems to indicate everything is fine at pin 1 of the FV-1, but bad coming off pin 28.

The orientation of this new board makes it likely that I could replace the FV-1 without damage to the rest, which is good, but that super hot regulator gives me pause (though it takes 9v in and puts 3.3v out as required).
 
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I subbed in yet another regulator (thankfully I actually socketed it on this board, and it's not getting any hotter than expected, so I'm moving on from that as an issue.

I can hear the internal programs trying to work (the reverb/tremolo setting is very discernible), but it's very distorted and noisy. I can't get the EEPROM to register.

Next up I'll measure and trace all the FV-1 pins and narrow it down further. Worst case I have a Deflector PCB with included FV-1 I could remove and sub in to see if that solves the issue (and a few other damaged FV-1s to confirm suspicions).
 
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More good news/bad news:

The good:

I'm now getting really good at desoldering/resoldering FV-1 chips in populated boards, as I've now removed the FV-1 from this board and tried to replace it with others I had previously also understood to have been damaged while modding. All were confirmed still damaged, but I was able to steal the one that came soldered to my as-yet built Deflector PCB and this board is alive again!

That leaves me with yet another damaged FV-1 (this time making loud noises/crackles and distorted attempts at internal programs).

All my 3 (!) damaged FV-1s seemed to be fine until something went wrong while messing with the additional clock module board (my fault), so be careful!

The bad:

The circuit works fine, but we're back to diagnosing a pop when toggling bypass. I've had it hooked up to two different testing rigs and it pops consistently each time in both (very loudly), while other finished boards do not.

I've checked the pull-down resistor and jumpered others on the in/out, but no change. Anything else worth looking at?
 
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Though I should mention there are a few differences in the other builds vs. this one:

  • This has the clock module
  • This has all MLCC caps aside from the 2n2 and the electrolytics
  • This has 2 1/8 watt 1k resistors instead of 1/4, as I was out (but we already determined that wouldn't be an issue)
 
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I've re-flowed all the solder joints and double checked all resistor/cap values (they are all the right values, though the in-circuit measurements different slightly from my other working build, even though it sounds fine).

I also checked continuity to ground for anything that should ground and it's all fine.

Once again tried jumpering in some extra pull-down resistors and tried putting a new 1u MLCC cap in parallel with all the current ones one at a time to see if there might be one that's bad, but no luck so far.

The sockets for my voltage regular seem a bit loose, but I not sure if that would bother anything.

Basically, I'm not sure what else to check and have already damaged an FV-1 grasping at straws, so I'm open to suggestions!
 
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Nostradoomus said:
Maybe your output cap is leaking a bit too much? I’ve found that to be a cure for most of my switch pops. If there’s an electrolytic on the output especially I change it to a film. The arachnid calls for an MLCC 1u there so it’s not super likely but it may be worth replacing.
Click to expand...
UGH!

Remember that time I said it wasn't this because I tried another 1u MLCC in series with the output and it didn't help? Turns out that was dumb and you were 100% right.

I removed C15 and measured it in my capacitance meter and it was hitting 1.6u. Measured a new one at 1.0u, swapped it in and problem solved.

Boy do I feel like a chump for not just going to that effort wayyyyyy sooner. I measure resistors before using them, but haven't been measuring each cap first. That will change now.

Thanks for all your help everyone! I'll box this back up with all the mods I had planned this week and post in the forum. Too bad I somehow damaged an FV-1 in this process, but could be worse, I guess?
 
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