SOLVED LGSM - what did i do?

[Untro]

Hey gang, I think I need some help with this run of LGSM I just finished. After a long haitus of the hobby, I finished three I had started months ago. Two worked, one with a lot of fanaglling, but the third...

The third had one pot that had lugs, and i forgot to heatshrink those lugs. When I tested this pedal (outside of the enclosure), it created a screaming self oscillation, the LEDs in the circuit glowed bright green, and then it stopped making any noise at all except a soft rhythmic 'chck chck chck' kinda sound. I wonder if the lugs on the wired pot shorted against the board in a critical way. It still passes dry signal.



I really want to learn how to fish when it comes to trouble shooting. It had been awhile since I had made a pedal and I forgot some of the finer points of offboard wiring, but I've gone over the parts again and again and everything looks correct, the points on the pcb look fine. I know I'll need another tlo72 and diodes in there, but want to make sure I dont just short another.

I don't know how to trace or measure signal or how to compare what's what, I've tried to learn but it just eludes me. Any outside perspective/advise on this one, or leads to resources that worked for you when it comes to learning troubleshooting, I'd very much appreciate it.

 

[Brett]

You can verify off-board wiring with a simple volt meter that has continuity testing. Audio probes serve a purpose, but it may be faster to just probe a few off-board junctions first to verify the problem isn't there. Set your volt meter to continuity mode and place one probe at ground on the circuit, and probe each location where a solder joint exists for ground. You should have continuity for all of these points.

Next, in continuity mode with the pedal bypassed, probe the tip lug of your input jack and the tip of the output jack. You should have continuity. Now, put the pedal in its active state. Probe the input jack tip lug and the IN pad on the circuit board. Probe the output jack tip lug and the OUT pad on the circuit board. You should have continuity in these locations. Go ahead and make sure that neither your input jack tip lug or output jack tip lug share continuity with ground. If they do, you'll want to verify the pads on the board aren't grounded unintentionally. Put the pedal back into its bypassed state. Check if you have continuity between the OUT pad and ground. If yes, desolder the wires from the PCB that go to the 3PDT breakout board. Afterwards, check it again. If it's still grounded, your problem is on the board. If not, you need to replace the 3PDT switch. If your OUT pad does not share continuity with ground at any point, you'll move on.

Put your volt meter in DC voltage mode and verify that the pedal is receiving 9V at the + pad.

If you haven't found an issue up to this point, you can check a couple other points on the circuit to see if voltages look correct. Start by getting voltages for each pin of the IC.



Pin 8 should be very close to VCC (the voltage you're putting into the pedal), slight drop due to protection diode losses. Pin 4 should share continuity with ground. The remainder of the pins should have some voltage, but you'll want to measure and list them here for further help.

I don't know how to trace or measure signal

This part isn't as complicated as you might think.

Build yourself an audio probe following the instructions here: https://diy-fever.com/misc/audio-probe/

If you have a looper pedal, record a couple lines and pass the output from the looper into the input jack of the pedal in question. Don't plug anything into the output jack of the pedal in question. If you don't have a looper, you can use anything that outputs sound/music that can be connected to the input of the pedal in question, using a 1/4" to 1/8" plug adapter is perfectly fine here. This is your SOURCE.

After building the audio probe in the aforementioned link, you'll plug the 1/4" jack of the audio probe into an amplification device (headphone amp, low-wattage practice amp). Connect the audio probe ground to the circuit ground of the pedal in question. From there, you'll just need to apply power and start sending sound/music to the pedal in question. You'll use the probe side of the audio probe to touch junctions inside the pedal in question to determine if audio is present at that point.

Generally, you'd want to check the off-board junctions first, as outlined above.

Using a combination of the schematic and your eyes, you can generally trace the path that the audio should flow. When you get to a point in the circuit that the source audio stops and you're sure you've followed the signal path, your problem is typically adjacent to the location you last had audio from the source.

In some cases, it may be easier to start from the end of the circuit and work your way backwards. Although it's less methodical, you can also just probe around the board (just be mindful to avoid power junctions) until you find a point where you DO have audio, then proceed from there.

Good luck!

 

[Untro]

Holy smokes, thank you so much for that! It makes a ton of sense, thanks for the clear and concise reply. I'll tackle this first thing tomorrow morning with fresh eyes. I think building an audio probe too will help me understand some more circuit concepts. I have another pedal, a portly provoker from Effects Layout, that doesnt pass signal or turn on, and I'll try looking at both and seeing if I got any more info on the issue(s). Thanks again, I'm feelin' a little less frustrated about this goof up!

 

[Mcknib]

It looks to me like your in jack tip is wired to ground but it's difficult to make out

You'd still get bypass signal even with the jacks wired incorrectly albeit you would get a little bit of static when at idle ie not playing but you wouldn't get effect signal

Post clear images of your in and out jacks and DC jack so we can clearly see and trace your wiring

 

[Barry]

I think your output jack is wired backwards, hard to tell from these photos.
Edit: Actually I think both are backwards

 

[Untro]

heh, so you guys were totally right, it was absolutely backwards. Fixed that, popped new IC and LEDs in it and its working fine. Dont solder at 2am I guess

But Bretts help there was still super valuable to me and I hope others, I got (many) pedals that ive bungled through the years that Ive always said I'd go through one day... guess Ill start digging in!