SOLVED Arachnid - Used tantalum in place of MLCC...

[DougieFresh]

The Arachnid calls for six 1u MLCC caps (I presume because of the tight spacing... but I'd be interested to learn if there are any other reasons why this type are required here). While building I realized that I didn't have any MLCC, but I did have a bunch of 1u tantalums which I thought wouldn't be an issue because they fit in the small space. Live and learn! I didn't realize tantalums are actually electrolytic/polarized... I just oriented them with the labels facing forwards, which I can now see places the positive lead to the right.

There is a fairly loud popping noise when engaging and disengaging the foot-switch, and I measure 1v DC on the output jack (there's no DC voltage on the input). This points to an issue with the 1u output filter cap (C15), which is what made me realize my error in using the tantalums. Otherwise, the pedal mostly works as expected, although I don't have another correctly built one to compare the sound to.

So here's my question... I can certainly remove the tantalums and order/replace with MLCC, but before I do that I wanted to check to see if maybe the tantalums would be okay, and that the issue is simply the polarity (which I don't know how to figure out)?

As always, thanks for the help!

 

[Robert]

Tantalum should work if they're oriented correctly.

I've marked the positive pins here. It looks like C15 might need to be reversed, which coincidentally is the output coupling capacitor and could very likely cause the popping when switching.

 

[jwin615]

Or just use 7.2x5mm box film caps?

 

[DougieFresh]

Tantalum should work if they're oriented correctly.

I've marked the positive pins here. It looks like C15 might need to be reversed, which coincidentally is the output coupling capacitor and could very likely cause the popping when switching.

View attachment 83241

Wow, looks like I got really lucky... Only needed to reverse C15 (since I had already used a box film cap for C6 on the lower-right). My component tester showed no apparent issues so I was even able to re-use it. No popping now and everything seems to be working perfectly. Thanks!

 

[DougieFresh]

Or just use 7.2x5mm box film caps?

Since they are 1u, the box caps (at least the ones I have) are a bit too thick to fit in the available space. I did use one (for C6 at the lower-right) where space wasn't an issue...