SOLVED General Test Box Build Issues

[Mjdcamp]

I am in the process of building a test box with an audio probe, and I have been loosely following the design at this link: http://tagboardeffects.blogspot.com/2014/09/test-box-20.html?m=1

My layout was a little different, so I improvised a bit with the 3PDT and DPDT pins. The box seems to be working and doing its job, but I noticed something odd: when disconnected from an external breadboard, and the effect switch is “engaged”, the audio out plays a very quiet form of the raw input sound. Even when unpowered, the pedal does this. When the effect is “off”, the pedal behaves as expected in bypass, but when it is switched “on”, it does the same thing as before and plays a quiet form of the raw sound.

Without going into too many of the wiring details here, does this indicate a general issue/solution? Does it seem like a problem with the wiring of the switches? A grounding issue? I am new to this, and I appreciate the info! Thanks.

 

[Mjdcamp]

Here are some images of the box. I 3D printed it! Don’t roast me on my poor cable management...It’s probably the main contributor to my issue. All of the red wires are hot, black is ground, and blue is signal. I connected all of the power cabling to stripboard in an effort to organize. I heatshrunk every joint to test out shielding, which also makes it hard to troubleshoot. I hope this helps!

 

[Mjdcamp]

I have cut back most of the heat shrink and have reflowed most of the solder joints. I noticed that when I have the pedal unpowered, and hook up a separate audio jack with the ground to ground on pedal only (the signal lead is disconnected), I can slightly hear the input sound! Does this sound like a grounding issue? I have been checking continuity, voltage, and resistance with a multimeter, and have tried grounding with a power supply. This is also a plastic enclosure; could the enclosure being non-metallic have anything to do with the issue? I appreciate the help.

 

[BuddytheReow]

A plastic enclosure just means that it’s not grounded. Double check switch continuity and see if there’s any stray solder anywhere. You shouldn’t be getting any signal coming through if you don’t have a board hooked up to it. You will hear some ground him since the board you may be testing isn’t enclosure grounded but that will get fixed later

 

[Mjdcamp]

Thanks for the reply, Buddy. I have double checked switch continuity, and it seems to be connecting as expected. I also checked for continuity with adjacent poles, and could not find any, as expected. It seems that there is no stray solder on the switches, or anywhere in the box.

I know they are probably too messy, but how do my switches look?

 

[Mjdcamp]

I have ordered some new switches to try out. Is it possible for there to be an issue internally with the switch, but it does not show as continuity/any resistance between pins that should not be connected? I am just trying to understand how to diagnose in general, and this doesn't quite make sense to me. I have taken everything apart, except the switches, and it seems that these must be broken. Broken switch(es) could make sense, because I used a heat gun for extended periods of time for the heatshrink. Any tips for soldering with 3PDT/DPDT switches? At this point, I have broken 4/4 switches throughout my projects...

 

[BuddytheReow]

Its possible you may have overheated the switch. Personally, I think you put too much solder on.

How are you testing the "on" effect? Hooking it up to a finished board?

 

[Mjdcamp]

I have been testing it with a breadboard Distortion 250, which works when connected separately from this box. I have also been testing the “on” effect with the terminals completely disconnected from any external breadboard, as this should produce no audio signal at all.

 

[BuddytheReow]

Ah, ok. So when you turn it on with no board connected you are still hearing a faint raw signal similar to bypass?

 

[Mjdcamp]

Yes, it’s very faint but audible. It makes diagnosing impossible because some bypass is always audible.

 

[BuddytheReow]

Its hard to tell from the pictures where your issue is. It's either shorting somewhere in the speaker terminal, the IN/OUT jacks, or the 3pdt switch. The jacks appear to be correct based on your color coding mentioned in your first post. Can you send a pic of the speaker terminal? Maybe something is exposed there?

 

[Mjdcamp]

At this point I have actually cut out the speaker terminal, but the connections looked clean. After cutting, the issue still persists, so I am certain the terminal had nothing to do with the issue.

 

[BuddytheReow]

Then I would say try replacing the switch. Don't overheat it; i've heard stories of people melting them! If you've got a solder sucker use it. Otherwise heat each leg until you can pull the wire out of each one. This may just be a case of the smallest piece of solder shorting two lugs together and if true you can reuse the switch.

 

[Mjdcamp]

So I tried new 3PDT and DPDT switches, and the issue still persists! I only have the box partially wired up, so only the bypass is connected. When either switch is flipped, no signal should be audible, but I am still able to hear a faint signal!

 

[Mjdcamp]

Okay this is a really dumb thing but I have to ask... Outside of a pedal, I took two new jacks, connected input cable to one and output cable (amp) to the other. I used alligator clips to connect the ground and signals to act just like a cable. When I disconnect one or both of the clips, I get the same thing I was getting before, where there is a very quiet guitar audio signal through the humming. I think this is the exact same thing I observed before...Is this just due to cable/jack/wire shielding, or lack thereof? Is this just a completely normal thing? Thanks.

 

[BuddytheReow]

Where did you hook the jacks up to?
At the end of the day there are only a handful of parts in this box. 3pdt, dpdt, 2 jacks, a speaker terminal, and the wire itself. You mentioned you replaced the two switches, cut out the speaker terminal, and clipped new jacks on it as a test. By process of elimination that leaves the wiring/connections as the culprit. You have every connection covered in heat shrink so it is hard to see where your problem lies. I'm also not 100% sure how you have your switches wired up. If you can post some different picks that would help.

 

[Mcknib]

Although really difficult to trace it looks to me like you've got the IN jack tip going to the DPDT which would account for the weird behaviour you can actually connect your probe to the IN side and still get guitar signal going through

Perhaps you've not accounted for your enclosure being the wrong way up as we've all done and your IN is on the left instead of the right with the
box the wrong way up

So in DPDT probe mode your IN jack tip is connected to the cap switched the other way your in tip to your out tip

 

[BuddytheReow]

I saw that too but i was assuming that was the out jack.

 

[Mcknib]

I saw that too but i was assuming that was the out jack.

Took a while but in the enclosure image the speaker terminal's at the bottom but I could be wrong it's a 'mare to trace but weird behaviour like that usually means yer probes connected to the IN jack rather than the OUT jack so instead of probing a circuit with audio going through it you're putting audio in it with your probe

 

[Mjdcamp]

Yeah so after taking that image I saw the mistake that I had the IN and OUT jacks swapped..doh. I swapped them back to their correct positions a few days ago.

So today I tried something completely outside of this box, just to do a sanity check. I took two completely new 1/4" audio jacks, unwired to anything, and connected one cable to a guitar and one to my amp. I connected both the GND and signal pins together using alligator clips, to establish that my audio was working. Then I disconnected one alligator clip at a time, then both, and saw the same thing I was having before with this box. The leads being close together or having free wires in close proximity to one another caused a sound to be transmitted, even without any wiring. Did I just discover a basic truth of audio engineering? Does an audio signal always get transmitted when two unconnected wires are in close proximity? Is this just a shielding issue, and is this negligible enough to ignore? Thanks.