Mojito PCB traces question

[G.G.]

Hi guys,
I just put together my Mojito build and I'm not getting any sound at all from the output. Oddly, when I use an audio probe on Lug 2 of the Output pot it sounds like the circuit is working fine, all the controls work and it sounds great. When I test with my DMM I'm getting continuity from Output Lug 2 to the tip of my test rig output jack. But I also get continuity to the jack ground and also to the PCB pad right below the LED anode in this pic which I believe is also ground. I noticed the PCB trace from Output Lug 2 to the Output pad has a small spot on that looks like maybe the ground pour is touching the trace, is that possibly the problem? I can just run a wire from Output Lug 2 to my switch easy enough, but it's such a weird problem I thought I would run it by you. Any thoughts?

 

[zgrav]

remove the output wired from the test rig and test your output wire to see if it is shorted to ground. if that is good, use your audio probe to see if your sound is OK at the output wire where it was connecting to your test rig. if that sounds ok you should see if the output and ground are shorted somehow on your test rig.

 

[G.G.]

I'm still getting continuity from end of the output wire to the ground pad after removing it from the test rig. I don't see anything that looks like it could be a solder bridge anywhere else. Maybe I'll try making a small cut on either side of the Output trace in that one spot

 

[zgrav]

Are getting continuity between lugs 1 and 3 for the output pot? does the resistance measured between lugs 1 and 2 vary with pot rotation? how about between lugs 2 and 3?

 

[Robert]

That is very unusual, and most likely your problem. Let me check the other boards in the bin today and see if more are affected.

 

[G.G.]

zgrav, I didn't get continuity between lugs 1 and 3. The resistance didn't change between lugs 1 and 2, but it did between 2 and 3 (not 100% sure my measurements were accurate). I removed lug 2 connection from the PCB and ran a wire from it to my test rig jack and it all works and sounds excellent. Cutting through the solder mask & ground pour accurately looked like it was going to be harder than I thought so I just left it alone. Thanks for the debug help, I'm not always very systematic and thorough so that was very helpful. Cheers guys, looking forward to boxing this up!

 

[zgrav]

Glad you got it working and your problem solving steps were all spot on!

 

[G.G.]

forgot to add, after removing the lug 2 connection from the PCB I got voltages changes on both 1&2 and 2&3. This is a great sounding circuit, super-psyched to get it up and running!

 

[Chuck D. Bones]

My Mojito board is black and has a mfgr's code printed on the lower right corner. This one definitely looks like a bad etch to me.

 

[Robert]

I checked through the remaining PCBs here, none of them are affected so this must have been a one-off error.

I'll get you a coupon code for another PCB of your choice since you had to patch this one up.