Audio probe distorted on input tip

[twebb6778]

Hi all, long time follower of the page and first time poster. I've build a few pedals but still consider myself a relative novice.

I'm building an MXR Blue Box / Distortion + combo pedal (builds linked below) and I've run into a problem while debugging.

The Distortion + side of the pedal works flawlessly, but I can't for the life of me get the Blue Box to work. With the effect engaged it has very low output with a high pitched squeal. with it disengaged, signal bypasses normally. All my components are in the correct position and orientation, I've run a razor along the track to remove solder bridges and all my IC and transistor look fine.

I've used an audio probe and with the effect engaged the signal is distorting from the input tip, which I'm finding really confusing. With the pedal engaged, but no power supplied, I'm getting a clean signal through the tip.

Is there any reason why my signal would be distorting at the input tip with effect engaged, but sound crystal clear with it bypassed?

Any help would be appreciated - this one has me stumped!

Blue Box: https://tagboardeffects.blogspot.com/2013/04/mxr-blue-box.html?showComment=1550662842355#c906734356862387828
Distortion +: http://tagboardeffects.blogspot.com/2013/01/mxr-distortion-w-wampler-mods.html
Wired using dual effect wiring here: http://tagboardeffects.blogspot.com/2012/02/offboard-wiring.html

[patrick398]

Welcome to the forum!

Do you have a DMM? Time to take some voltages i think.

You say it's a blue box/distortion+ combo, in what order are they? and are they wired independently with two footswitches?

Did you test both circuits separately for function without footswitch wiring? It's a very good habit to get into as when you know the circuit board works but it doesn't work when it's boxed it's most likely the switch wiring

[twebb6778]

Thanks for the quick reply!

Yep, both effects wired independently to separate footswitches. The BB is first.

Didn't test them before wiring them to footswitches though, I guess I won't forget that next time.

Voltages are looking good as far as I can tell - measured o  a wall wart that came in at 9.5v

Ic1
1 - 4.8
2 - 4.8
3 - 2.4
4 - 0.0
5 - 4.5
6 - 4.4
7 - 5.4
8 - 9.5

Ic2
1 - 4.8
2 - 4.8
3 - 4.7
4 - 0.0
5 - 4.7
6 - 0.0
7 - 0.0
8 - 0.0
9 - 4.7
10 - 0.0
11 - 4.3
12 - 4.7
13 - 4.7
14 - 9.5

Tran 1
0,0.2,0.9

Tran 2
0,0.5,4.3

Tran 3
0,0.2,2

[DIY Bass]

What are you using for a power supply?  I had this once and it turned out that the power supply I was using had a noisy ripple in it at audio frequencies, and that was getting straight into the pedal.  If you are not using a battery or a known well regulated power supply maybe try with a battery just in case. I was definitely getting horrible noises right at the input.

EDITED to say that your post appeared while I was typing.  A standard wall wart would definitely have potential to do this, unless you know how well it is regulated (I have a fancy regulated one now just for pedal testing)

[twebb6778]

I'll give the battery a shot, cheers!

EDIT. Power supply I'm using is a 9v One Spot wall wart, not regulated as far as I know.

[DIY Bass]

From what I know of One Spot I would have thought it was regulated, but then if it's putting out 9.5V then that is not what you would expect from a regulated supply - would expect 9V on the dot

[patrick398]

I always though the one spot was regulated, can't find any mention of it on their website though, just says it's triple filtered. Mine is putting out 9.37 fyi.

Do you measure any DC on the input socket? Maybe i'm barking up the wrong tree here

[duck_arse]

can we see photos of your build, please, and welcome. might be your f/switch wiring ......

[GGBB]

From what I know of One Spot I would have thought it was regulated, but then if it's putting out 9.5V then that is not what you would expect from a regulated supply - would expect 9V on the dot

I can't say for certain about the one spot specifically, but it is not unusual for regulated "9V" pedal supplies to provide more than "9V on the dot". The BOSS puts out 9.5 I think, as do the Voodoo Lab units. I think most are aiming at the voltage level typical from a fresh 9V battery which is always more than an actual 9V. It's best to read the specs to be sure.

[twebb6778]

Just about to head off to work, but I'll check for DC on input when I get back.

I'll also grab some photos, but it's wired exactly like the diagram below with the Blue Box as the first effect. I used this because it's my first dual effect build. I've tested for continuity between all points of the footswitch, inputs, outputs, grounds and power, but nothing seems out of the ordinary.

[PRR]

> 9.5V then that is not what you would expect from a regulated supply - would expect 9V on the dot

No, you expect "9V battery".

Which is six "1.5V" cells.

A 1.5V cell, fresh, used to be 1.54V. (You remember this from calibrating VTVMs and some Dynacos.) Apparently they have been fiddling the formulas, or the raw materials....

"zero-load voltage of a non discharged alkaline battery, however, varies from 1.50 to 1.65 V, depending on the purity of the manganese dioxide used and the contents of zinc oxide in the electrolyte."

So 9.0V to 9.9V.

9.3V-9.6V would be safe and typical values. This also leaves about 5% slop-margin for part tolerance without going outside 9.0V-9.9V.

[twebb6778]

The power supply works flawlessly for my regular board and it's powering the Distortion + side of the pedal without any issues, so I think I can rule that out.

[Rob Strand]

So 9.0V to 9.9V.
9.3V-9.6V would be safe and typical values. This also leaves about 5% slop-margin for part tolerance without going outside 9.0V-9.9V.

I guess it's hard to "expect" any exact figure.   Anything between 9.0V and a "new battery" would be fine.

It's true new batteries are always more than 9V.  In the last 20 years don't think I've see Alkalines over 10V (1.67V/cell).    I have seen Non-Alkalines at around 10.5V.    Most data sheets for Alkalines show about 9.7V (1.62V/cell) when new.     I have an unused 9V Alkaline (Duracell) which is just over 2 years old  and it measures 9.5V (1.58V/cell).   Certainly in the ball-park of your values.

One interesting point is a battery only stays above 9V for a very short portion of its life!

[twebb6778]

Photos of the both footswitches (blue box is the one next to the dc jack, distortion is the one next to the output jack) and board wiring. I realise I should have tested this before putting in the enclosure, rookie mistake I won't be making with the next build.

[twebb6778]

Bumpity bump.

Any other ideas on why the signal would be distorting at the input tip? I really don't want to abandon this build.

[DIY Bass]

I think I would be disconnecting the board that is giving you trouble from the jacks/switches etc and make sure it is working independently and then go from that point.

[Rob Strand]

Bumpity bump.

Any other ideas on why the signal would be distorting at the input tip? I really don't want to abandon this build.

I suspect the audio probe is distorting *because* you have an oscillation issue.   When something oscillates it can produce a very high output at frequencies above the audio range.     You might not hear it but the electronics is coping a beating with the massive signal level.  So you need to solve the oscillation issue.

Putting two pedals with gain in the one box is asking for trouble. You need to take great care with the layout,    possibly even using shielded wires.  In some cases the oscillation is caused by the switches in this case you might need to change the way the switching is done or add extra capacitors.   Visualizing the problem is quite difficult when you are starting out.

So I suggest building things up gradually and finding when it breaks.   Here's a way to approach it:
- hard wire the input of the distortion+ to the input socket and the distortion+ to the output socket.   Crank the gain and the level up, also try intermediate settings.   Does it work correctly?  Is the oscillation gone?
- Repeat the same test with the blue box.  Does it work correctly?  Is the oscillation gone?
- Next, hardwire the output of one effect directly into the input of the other.  Use the same ordering that you have in your pedal.  Hard wire in the input and outputs.  Does it work correctly?  Is the oscillation gone?

One point here is you need to get each board working individually as well - it makes life easier.

So depending on how you go with that, the next step is to start putting the switching back in.  Do this one effect at a time.  Then combine the two.

At any point the oscillation appears you need to debug that setup.  The idea is you have a simpler set-up to debug.   If it *only* oscillates with final set-up then that's the problem to be solved.  Without knowing each component is working individually you will find it difficult to isolate the problem(s).

[Edit: cleaned-up some stuff.]

Tips: keep wires short and don't let the input wires and output wires of the same effect get close to each other.  That includes any wiring to level/volume/gain/drive pots.   Near the foot-switch keep the input/output wires as far apart as possible.  If you are using a metal enclosure sometimes running the wires over the metal surfaces can help.

[twebb6778]

Thanks for the tips Rob!

I think my next step is definitely removing the pcb's form the enclosure and testing them separately, the debugging from there.

I'll report back with any progress, thanks again!