Troubleshooting build mistake and audio input problem

[keyth72]

Hi, I am working on my first build of Pi-Stomp Core, and I made a mistake on the board and I'm trying to figure out the best way to proceed. I went through the entire setup (including power on and software/wifi setup) with the C6 capacitor in the C5 position (nothing in the C6 position). When I plugged in my guitar and enabled the pedalboard, there was no sound out, except for a quiet hiss. I hear the clean guitar signal when I bypass, and I hear sound out when I use generated sounds such as the drum beat generator. After unplugging and finding the capacitor mistake, I unsoldered/re-soldered the capacitor in the correct C6 position and tried again, but I am getting the exact same results. I also went through the steps in the troubleshooting guide (adjusting input level, restart sound engine, checked my guitar cables/connections and amp/guitar volume). I also rechecked the audio input jumper cables and solder joints.  On the pedalboard in the browser, when I use the gain pedal mentioned in the level setup step it's showing "-inf", so I'm guessing the input signal is not being processed.

My question is, did my capacitor mistake damage something and I need to get new parts, or is this unrelated to the capacitor and I should keep troubleshooting what I have?

Thank you,

-Keith

P.S. I don't think this is a problem but wanted to mention it, when I used balenaEtcher to write the image, it always completed writing but failed at the end of the verification step that balenaEtcher does.  Now that I think about it I may try getting the latest Etcher, the one I have installed is about a year old. Everything seemed to work in setting up the pi-stomp software, and the lcd screen operation and communication with a remote browser works fine.

P.P.S. This issue sounds like what I'm experiencing:  https://treefallsound.com/forum/index.php?topic=53.0
Although I should note that I'm using v2 core and this issue is on v1. I'll can try the steps mentioned on that thread if it's pretty clear that the capacitor in the wrong place didn't damage anything (I wish I knew more about electronics!)

[keyth72]

It’s the input buffer, I did a direct guitar input to the audio card and it worked fine! I do want to used the buffered input, so I will go through the input circuit and figure out what part I broke. That’s what I get for trying to build it too fast and too late at night!

-Keith

[Randall (Admin)]

Sorry you've hit a problem.  I would seriously doubt the capacitor created any problem.  It's fine for it to be in either or both locations, but it has a more profound effect as C6 (load side of regulator) than as C5 (source side), where with some power supplies, it seemed to create noise.  I should probably remove C6 from the next board iteration.

Anyway...

Since bypass works for you and generated output works for you, that narrows it down quite a bit.  Here are the things that could be the cause:
1) Audio Card Input (could either be not configured correctly or not working physically)
2) Cable from pi-Stomp Board to Audio Card (common cause could be that the wires were swapped: L goes to L, G to G, R to R)
3) Buffer (U3 chip, R1-R4, C1-C2)
4) Buffer supply (Regulator, C3, C4, C6)

#1 seems kindof unlikely if you're getting generated sounds.  I might start with #2.  If L and R got swapped, then you won't hear audio because it's misrouted.  But you can load up that "Default" pedalboard and instead of routing "Capture1" (Left) to "Playback 1", route Capture 1 to Playback 2 and Capture 2 to Playback 1.

Would be good to test to see if both inputs are dead so if you can use a TRS jack to apply a signal to either input, not just Left. 

If none of that results in passed signal, then it's likely the buffer.  Make sure the buffer op-amp U3 is oriented correctly into the socket.  It it wasn't, you might have killed it.  If it was, then you might need to do some signal tracing.  A crude tracer is to unplug the amp cable, ground the sleeve to the pi-stomp (alligator clip jumper?) and use another jumper to the tip and use that to "probe" points along the signal path.  If you wanna do that let me know and I'll diagram which points to probe. 

[Randall (Admin)]

My last reply was before your last reply.

So good to know that the audio card is working.

You could also test the buffer supply with a voltmeter between ground and the power pin on the IC (pin 8 - next to the notch on the "R3" side).  Make sure it has around 3 volts.

[keyth72]

Thanks for the detailed response! The op amp is oriented the correct way, so I'll do some probing and see what's going on. In the meantime I wired the input directly to the sound card, sounds good to me using guitar, but I suspect some brightness is being removed without the buffer. This thing is so much fun! I could spend days just exploring all the plugins.

I made a time-lapse video of the build, will share on the forum tomorrow.

[Randall (Admin)]

I look forward to seeing your video!

Yes, your guitar will sound noticeably more clear/bright going thru the buffer.  The input impedance of the audio card is only about 28KOhms, where the buffer is 1MegOhm.

I added a signal tracing section to the Troubleshooting Guide:
https://www.treefallsound.com/wiki/doku.php?id=troubleshooting_guide#no_processed_audio

Hopefully that, or your own debugging reveals something.  If not, let me know.