Guitar Effect Test Box

I’m in var­i­ous stages of com­ple­tion for sev­er­al gui­tar effects at the moment and I’ll cer­tain­ly try to write a post for each of those in turn. How­ev­er, I first fig­ured I should post about my gui­tar effect PCB test box I put togeth­er. I by no means first came up with the idea. Paul of DIY Gui­tar Ped­als in Aus­tralia is who I first saw use & rec­om­mend one. In search­ing around for fur­ther ideas, I came across some notes on DIY Stomp Box­es about adding the probe, which can be used in diag­nos­ing PCBs that aren’t working.

A MXR MicroAmp cir­cuit hooked up to the test box

As you can see, I went with a fair­ly large enclo­sure for this project. As it’s real­ly just the off-board wiring stan­dard to most any ped­al project, with no cir­cuit board, this is some­what a waste of space. How­ev­er, I want­ed to leave a bit of space for poten­tial­ly adding some more fea­tures at some point in the future1. This is a pow­der-coat­ed, alu­minum enclo­sure which is not at all nec­es­sary for this, as the wiring is out­side so the met­al box isn’t shield­ing any­thing. So the enclo­sure was a bit of a splurge. But as Mam­moth cur­rent­ly sells these 1590BB enclo­sures pow­der coat­ed for under $10, it’s not exact­ly a bank-buster. The entire test box is less than $25, and many of the parts I already had in my parts bin.

I cut up some cheap alli­ga­tor clips I bought off of to use for the con­nec­tors. They have lit­tle cov­ers over the clips, so they work quite well even when con­nect­ing into close­ly spaced wiring leads. I did knot these just inside the box to pro­vide some strain relief (though it’s not as though this thing is get­ting roughed up much). I used a Mam­moth Elec­tron­ics bypass wiring board just to sim­pli­fy things a bit. I tend to use a stan­dard wiring col­ors for all my projects: red for 9v, black for ground, green for sig­nal to board, and yel­low for sig­nal back from board.

The spa­cious guts of my test box

The one trick my box has is that I added a tog­gle switch to use a test­ing probe. This switch basi­cal­ly hi-jacks the sig­nal return (yel­low) and con­nects the probe (white) direct­ly to the box out­put jack. So if sig­nal isn’t com­ing back from the cir­cuit, I can flip this switch and then use the probe (which is noth­ing more than a 1μf capac­i­tor) to touch along the cir­cuit to trace where the fault is. It’s very sim­ple but incred­i­bly helpful.

So to quote Paul of DY Gui­tar Effects, if you’re going to even build just more than a cou­ple of gui­tar effects your­self, you’re going to want to build some­thing like this. It’s so invalu­able to be able to test your PCB as soon as you get the com­po­nents installed but before you try to com­plete all the off board wiring & stuff­ing it into an enclo­sure. It’s also extreme­ly fun to hook up to a bread­board and test that way!

A Bazz Fuzz bread­board cir­cuit on the test box
My enclo­sure drill pattern
Wiring dia­gram for my test box

  1. For exam­ple, I also saw this post where some­one has added in the abil­i­ty to change the volt­age and add a volt­age sag (to sim­u­late a dying bat­tery), which is real­ly cool. []

By Jason Coleman

Structural engineer and technical content manager Bentley Systems by day. Geeky father and husband all the rest of time.


  1. I was won­der­ing if on the mam­moth elec­tron­ics bypass wiring board if you actu­al­ly sol­dered those 9 “prongs” that the foot switch actu­al­ly push­es through. I just fin­ished their devi­ater octave fuzz and it’s not turn­ing on lol. Sig­nal pass­es through when it is off but noth­ing when it is on. It’s my first kit build, and I’m hop­ing I did some­thing sil­ly instead of fry­ing the board. Thanks

  2. Hi Matt! Yes, I used a PCB-style 3PDT footswitch. If you have a sol­der-lug style, it prob­a­bly won’t fit into the open­ings (though they are quite large, as I recall). I sol­dered each and it does take a fair bit of sol­der to fill up the holes. 

    As for the ped­al not work­ing, don’t get too dis­cour­aged. If you haven’t already fig­ure it out, I’d start with the off-board wiring between the switch/or switch PCD and the main effect PCB. My very first build, I man­aged to swap the input and out­put jacks. If your LED comes (assum­ing you have one), then you’re at least get­ting pow­er. You can try using a capac­i­tor like I’ve described above on the cable to your amp and probe the cir­cuit (just turn the amp down because it can get loud!).

    Last­ly, you can post some images on ped­al builder forums. Folks that know a lot more than I do are great help­ing to trou­bleshoot issues.

  3. Hi! Hop­ping on this qui­et thread, hop­ing to revive it for a lit­tle help. I’m a begin­ner and very much stum­bling through all this.

    I’ve built a test box and am cur­rent­ly try­ing to test this board (‑2350–2355/i‑2354/i‑2354_Interval_Fuzz_0920.pdf).

    What I’m con­fused about is that you, Paul, and every­one else who I’ve seen build and use a test box have effect boards with 4 off­board con­nec­tions: pow­er, ground, sig­nal in, sig­nal out. Mine’s got 8 — these four, plus an input jack ground con­nec­tion, an out­put jack ground con­nec­tion, a ground con­nec­tion for the footswitch, and a con­nec­tion for some­thing labeled “SW” on the footswitch board. 

    For the life of me, I can’t fig­ure out what to do with these extra. Do I con­nect them all to the test box? Do I leave them on the board but dis­con­nect­ed from the test box? You seem to have some jumpers on your board, is that what I’m missing?

    As it stands now, I’ve tried test­ing this board 2 ways: with the extra 4 off­board wires not con­nect­ed to the test box; and with the extra wires removed from the board. Each time, the effect worked for a sec­ond or so before fad­ing out and not com­ing back.

  4. Hi Andrew! So, yes, all your ground wires can be con­nect­ed togeth­er. The SW looks like it might be just the LED con­nec­tion in this set­up. Might be good to e‑mail Stew­Mac’s sup­port account to ver­i­fy, though.

  5. Thanks for your quick response, Jason!

    Not­ed, con­nect all grounds. Makes per­fect sense now that I think about it, actu­al­ly — uncon­nect­ed its just a big ol’ break in ground.

    I haven’t heard back from Stew­Mac yet but I googled a bit and it looks like you’re right regard­ing the “SW” pad. But it begs the ques­tion… when test­ing, what do I con­nect that to? Or do I leave it alone?

  6. This is super help­ful, thanks! Also, I know this is prob­a­bly a super basic sim­ple ques­tion, but I’m assum­ing doing this only works with the pots con­nect­ed to the pcb? I’m work­ing on a few builds right now with PCB mount­ed pots so I’m just try­ing to fig­ure out the logis­tics of test­ing those.

  7. @Andrew — Oh, I total­ly missed the ques­tion at the end of your com­ment from last year! You’ve prob­a­bly already fig­ured it out since then, but I did take a look at the Inter­val Fuz­z’s instruc­tions. I’m not 100% sure, but I think this is how you’d wire it based on my tester col­or con­ven­tions (that is, Red for 9v hot, black for ground, green for sig­nal to board, yel­low for return effect­ed sig­nal from board). I’m not 100% sure about the DC jack they use and which is the ground lead (that is, the black and red may swap if I’m wrong).

    Stewmac Interval Fuzz PCB Offboard wiring

    @Jake — Yeah, you typ­i­cal­ly have to have any pots on the board. If you have a bread­board, you could put them on that and them just tem­porar­i­ly sol­der them to the pcb (or some oth­er tem­po­rary means of con­nec­tion). I have loose­ly mount­ed them in the enclo­sure and then sol­dered them to a PCB, but then tak­en the whole thing out for test­ing in the past. That depends on the PCB and enclo­sure, though, I suppose.

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