DuoPhase blew a capacitor -- what did I do wrong? Plus weird PCB layout error?

[bengarland]

I'm a total newbie at building pedals. I built the CaptainBit, SuperHeterodyne, and Arcana successfully but I just finished the DuoPhase and when I plugged it in it blew the 220uf cap at C20B. I had my guitar plugged into the switched input and the output next to it went to my amp (green arrows on photo). Nothing plugged into the other jacks.

----> Image here

----> Here's the build doc for reference.

I noticed one anomaly with the specified layout, since apparently both sides of the board are supposed to be the same -- I'm wondering if Mr. PedalPCB can comment... the resistor pairs of R38A+R46A are flipped compared to R38B+R46B. But on the schematic it looks like they are supposed to be the same on both sides. Is this intentional? I populated it according to the values printed on the PCB and the PCB layout on the build guide. This seems to be the only pair of parts that aren't the same on the left vs right side of the PCB.

Could that be the problem? (I would think not since I'm sure other people would have reported it by now...)

I plan to take it apart and check everything again, but I'm wondering:

1) Is there anything that's "most likely" (such as... a short somewhere?) to cause a capacitor to pop? Let's assume that I have populated the parts correctly on the board.

2) What's the chance that this has totally screwed up my entire board? Like let's say I find a tiny solder bridge somewhere that's the problem, fix it, and replace the cap... (and nothing else is wrong)... what are the chances that it will then work fine?

 

[zgrav]

what was the voltage rating on your 220uf cap? it probably needed to be at least 25v or higher. odds are good that nothing else on your board was damaged, but the most likely thing to have been damaged would be the charge pump (IC7B). If you remove the charge pump IC and bad capacitor and power it up, does the other half of the board work?

 

[Robert]

A short doesn't usually cause any excessive load on a filter capacitor, typically unless the capacitor was just defective (not extremely likely, but possible) the main reasons for an electrolytic to pop are overvoltage, or reverse polarity.

The charge pump on the DuoPhase creates a -9V supply, but it's not routed in a way that comes in close proximity to that cap.

I'd make absolutely sure it wasn't installed backwards.

 

[bengarland]

Well geez you're right... I mistakenly installed the cap backwards. D'ohhhhh. That should have been the first thing I looked at.

Still learning!

 

[Robert]

In that case, replacing the cap (and cleaning off any of the leakage) should be all you need to get it going.

 

[p_wats]

Don't worry, we've all done it at some point!

 

[the_grumpy_gnome]

Well geez you're right... I mistakenly installed the cap backwards. D'ohhhhh. That should have been the first thing I looked at.

Still learning!

Yep. Glad you figured it out.

 

[Robert]

I noticed one anomaly with the specified layout, since apparently both sides of the board are supposed to be the same -- I'm wondering if Mr. PedalPCB can comment... the resistor pairs of R38A+R46A are flipped compared to R38B+R46B. But on the schematic it looks like they are supposed to be the same on both sides. Is this intentional?

The circuit is the same on both sides, but the component layout on the PCB might not be 100% symmetrical.

The reason is because of the traces entering each half of the board from opposite sides coming from the toggle switches in the middle.

 

[bengarland]

Replaced the cap in the correct orientation and all is well! This is probably my favorite build so far -- sounds awesome. Thanks for the help everyone.