Faultfinding DIY Pedal Builds

Hi Everyone,

While there seems to be quite a few threads about building DIY pedals in this forum section, I noticed that there isn't a thread about faultfinding them, so, I thought that I would take the initiative and start a thread about faultfinding DIY pedal builds....so here it is.


Okay, you bought a DIY pedal kit online, assembled it, and for some reason it doesn't work as it's supposed to, so what do you do?, whatever you do, don't give up, because believe it or not, it is quite possible to get a non-functional DIY pedal build working as it's supposed to, all you need to do is go through the fault-finding steps one by one until you discover what's causing the malfunction.

Most of the time when I build a DIY pedal, I've been pretty lucky and my DIY pedal builds have usually worked first time, but sometimes I do come across a pedal build that doesn't appear to work as it should, here's a procedure that I usually follow to get the pedal working:


1, First thing I do is give the circuitry of the pedal a good visual inspection, I do this by having a close-up look at all the solder-joints on the circuit board with a X10 Magnifier, sometimes the cause of a malfunction in a DIY pedal build can simply be a cold solder-joint, this is where the solder hasn't properly adhered to either the component lead, or the copper track on the circuit board, or it can be both, it can even be a joint that hasn't been soldered, I'm sure that I've done that at least once ever since I first got interested in Electronics.

The fix for a cold solder-joint is pretty simple, first I de-solder all the existing solder, then I remove the component, then, using a stanley-knife blade, I scrape the component lead so that it is all silvery and clean all over, this will help the solder to adhere better, next I remove any residual solder from the copper track and gently scrape it so that it is shiny and clean, you can use solder wick to soak up the residual solder, I recommend buying a roll of 3mm Goot Solderwick from your local Jaycar Electronics shop, I find that Goot Solderwick tends to work really well, better still, buy two rolls.

Once both the component lead and copper track are clean and shiny, put the component back in it's position on the circuit board, making sure that you get polarized components oriented the correct way around, and then re-do the solder-joint, if the solder-joint looks bright and shiny it should be good.

Another cause of a malfunction in DIY pedal builds could be a broken wire, in that case you simply re-solder the wire connection.

Or, some part of the circuitry might have inadvertently shorted-out to circuit ground.

2, Once I'm satisfied that everything looks good after doing my visual inspection, I get my digital multimeter out and use it to measure the DC voltages present in the circuit while it's powered up, if I get any unusual DC voltage readings in any part of the circuitry then that immediately tells me where the fault is most likely to be and I start investigating the cause of the unusual DC voltage readings, sometimes these unusual DC voltage readings can be caused by a faulty component, or, a copper circuit board track that might have a hairline crack/break in it that's very hard to see, in the case of a faulty component most of the time replacing it with a known good one is enough to fix it, in the case of a hairline crack/break in a copper track, bridging it with a short piece if component-lead cuttoff that's soldered in place is enough to fix it.

I've also come across a faulty 9V battery connector a few times too, replacing it completely with a known good one fixed the non-functional pedal I was working on.

3, A Component getting abnormally hot is another cause of DIY pedal malfunction, it could be as a result of the component failing, or it could be something related to the component that's failed, so it's always a good idea to check for any components getting abnormally hot (making sure that there are no dangerous voltages present, which is unlikely in a DIY pedal designed to run on 9V DC), sometimes a component will get abnormally hot due to it oscillating at a very high frequency, this can happen in small amplifier circuits.

4, If I haven't fixed the malfunctioning pedal at this stage I usually take a break from it and come back to it later on with a clear head, this is not only a good idea, it's also important to allow your most powerful faultfinding tool, Deductive Reasoning, to work at it's maximum effectiveness, also, trying to faultfind a malfunctioning DIY pedal build when you're in a bad mood is an exercise in futility, because you're unable to think logically when you're in a bad mood, or upset, so, always try to keep your cool, I'm sure that I've been guilty of loosing my cool when faultfinding a non-functional DIY build on a few occasions, and when I've come back to it with a clear head, the cause of the non-function turned out to be so obvious it was staring me in the face, making me feel like a right goose.

Faultfinding a malfunctioning piece of electronics is a lot like solving a mystery, make sure you make a note of the symptoms that a malfunctioning DIY pedal is displaying, because the symptoms can be valuable clues to what could be causing the malfunction.


More info to come so stay tuned.


Note: All comments/contributions are most welcome.

Also note that these faultfinding procedures can be used to faultfind malfunctioning electronics in guitars too.

Excellent idea for a thread. I need this info for sure!

I need this more than anyone, I just started building a pedal earlier today and I've already gooched it, luckily the Chinese seller is giving me a free replacement with all components.

One thing I check pretty early on is that I've put the components in the right way - electrolytic capacitors, diodes, LEDS, and transistors.

Quite often I've had a pedal not work, and the simple solution was that I'd put a transistor in backwards. :rolleyes:

Building yourself an audio circuit probe is helpful..
http://3.bp.blogspot.com/-_ba-mH6img...udio+probe.jpg

Thanks for posting doc

No worries Andy40.


An Audio probe is definitely a handy piece of test gear to have alongside your soldering iron and multimeter, I need to build myself one or maybe two of them, so when I do build one I'll do a mini-tutorial while I'm at it, a Signal Generator and Oscilloscope are very handy to have as well, you might be able to pic up a decent Scope on eBay for a reasonable price, but an Audio Probe works out cheaper.

That audio probe is a great idea! Will have to make one sometime soon

Quote:

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Originally Posted by Joe3334

I need this more than anyone, I just started building a pedal earlier today and I've already gooched it, luckily the Chinese seller is giving me a free replacement with all components.

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Joe, I meant to ask, what did you do to gooch it?

Sometimes, when faultfinding a non-functional effects pedal, there'll be a need for being able to see what a signal looks like when tracing out the signal path in order to locate the fault that's causing the malfunction, of course, you can get away with just using an audio probe, or, you can use one of these handy pieces of test equipment:

Attachment 24938

This handy piece of test equipment is a device called an Oscilloscope.

Why is it handy?, because it displays on it's screen what the signal looks like, and, from looking at the display you can learn a lot about how a circuit is actually behaving, it is also very handy to have because some types of effects pedals need to have adjustments done on them in order for them to operate properly, as a demonstration, I will be posting some screenshots of the Oscilloscope you see in the pic I posted, I will be using it to look at the output signal of some of the effects pedals I have in my collection, some factory made ones, and some that I made myself, I thought that it would be interesting to to post the screenshots so you can see what the pedals actually do to a signal coming from an electric guitar.

Stay tuned.....

Rightio, what I'm going to be doing today is I'm going to post some screenshots of my DSE Q-1804 20Mhz Oscilloscope being used to look at the output signal of some of the effects pedals that I have in my collection, I'm going to focus mainly on pedals that produce a distortion effect since you can clearly see on the Oscilloscope's display screen what the pedal is doing to the signal from the guitar, in each case I'm going to be using my Fender Strat set to it's neck pickup, and with a capo placed just behind the 12th fret, as my signal source, the capo is so that I can free up one of my hands for holding and operating my digital camera.

The pedals I will be posting screenshots of are as follows:

Red Jim Dunlop JD F2 Germanium Transistor Fuzz Face.

DIY Tonebender MkI Germanium Transistor Fuzz Pedal.

DIY Tonebender MkII Germanium Transistor Fuzz Pedal.

DIY Rangemaster Germanium Transistor Vintage Treble Booster Pedal.

Ibanez TS-9 Tube Screamer.

Boss BD-2 Blues Driver.

DIY Baja Real Tube Overdrive Pedal.

DIY Doctor Overdrive Pedal.

DIY Big Muff Distortion Pedal.

Boss MT-2 Metalzone Distortion Pedal.

First up, it's my Red Jim Dunlop JD-F2 Germanium Transistor Fuzz Face.

Attachment 24945

Here's what the un-effected guitar signal looks like on the Oscilloscope's display screen:

Attachment 24944

Here's what the output signal of the JD-F2 Fuzz Face looks like with the pedal engaged and the Fuzz control set to minimum:

Attachment 24946

And here's what the signal looks like with the Fuzz control set to maximum:

Attachment 24947


At minimum Fuzz settings we notice that the bottom of the signal is gently rounded while the top is pointy, when the Fuzz control is turned up to maximum we notice two things, firstly the top and bottom are sharply squared-off, but they are also un-equal, this is called "Asymmetrical-Clipping" and is largely responsible for the reedy, nasally sound that the Fuzz Face produces.

Next up it's my DIY Tonebender Mk I Germanium Transistor Fuzz Pedal.

Attachment 24953

Again, here's what the un-effected guitar signal looks like:

Attachment 24954

Here's what the output signal looks like at the minimum Attack setting:

Attachment 24955

And here's what the output signal looks like at the maximum Attack setting:

Attachment 24956


We can see that at the minimum Attack setting that the output signal shows some sharp asymmetrical clipping, when we turn up the Attack control to maximum, we can see that it just gets more heavily clipped asymmetrically, notice how the's a bit of sloping of the top and bottom of the heavily-clipped signal, that's caused by a fair bit of high-frequencies in the resulting tone so therefore the Tonebender Mk I will sound brighter and buzzier than a Fuzz Face.

Re your source signal photos, or 'un-affected' shots......Are you injecting real guitar or is it from a signal generator?

Real guitar then maybe okay, but if a sig gen then I'd be checking it as it doesn't look right. Even a single note from a guitar should look more sinusoidal...

Quote:

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Originally Posted by Marcel

Re your source signal photos, or 'un-affected' shots......Are you injecting real guitar or is it from a signal generator?

Real guitar then maybe okay, but if a sig gen then I'd be checking it as it doesn't look right. Even a single note from a guitar should look more sinusoidal...

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Yep, it's an actual real guitar signal straight from the output jack of my Fender USA Strat guitar, going straight into the pedal with nothing in between except the instrument lead, I've connected the input of the Oscilloscope directly to the output of the pedal in each case via a short pedal patch cord and a lead I made up using a BNC connector, a piece of Response WB-1530 mic lead and two alligator-clips.

The pickups on the Fender USA Strat are a set of three Kent Armstrong Dual-Blade Humbucking pickups that are Single-Coil sized, and for each pedal I set the pickup selector to the neck pickup, all the controls on the Fender USA Strat were set to full-on.

The strings on the Fender USA Strat are getting a bit old, and I guess I should have put new ones on before doing the screenshots except that I'm out of both new strings and money at the moment.

There's a reason why I decided to use my Fender USA Strat as a signal source, most Signal-Generators usually have a low-impedance output and I've found that some vintage-style Fuzz pedals behave differently if a low-impedance signal source is connected up to their inputs, the Fuzz Face is a classic example, I wanted to show how the pedals behave when a real-world guitar signal is fed into them.


All my DIY pedals use what's called a "True Bypass" footswitching system, that is, the signal doesn't go through any buffers, when bypassed, the signal goes straight from the tip-connection of the input socket through to the tip-connection of the output socket, the effect circuit is taken completely out of the signal path.

Okay next up is my DIY Tonebender Mk II Germanium Transistor Fuzz Pedal.

Attachment 24960

As before, here's the signal from the guitar:

Attachment 24961

Here's the output with the Attack set to minimum:

Attachment 24962

And here's the output with the Attack set to maximum:

Attachment 24963

This time we notice a few interesting things, firstly at the minimum Attack setting the output signal is heavily clipped, it is also a bit more symmetrical (although there is still some asymmetry) compared to the Tonebender Mk I, and notice how the sloping of the top and bottom of the signal is more pronounced?, that means the sound is going to be much brighter and buzzier.

Turning up the Attack control to maximum just clips the signal harder.

So, what we learn from that is that most Fuzz pedals tend to behave in a similar way, taking the guitar signal and converting it into either a square-wave or pulse-wave, the sharp clipping is responsible for the buzzy-ness of the resulting tone, the symmetry affects the overall tonal-colour, or timbre, and the spikiness of the clipping determines how bright the tone sounds.


One thing to note is that what you see in the screenshots I posted is what I saw on the Cathode Ray display screen of my 20Mhz Analog Oscilloscope, if you were to use say a 100Mhz Digital Oscilloscope the clipping would look sharper because of the increased signal-bandwidth of the Scope.

For those of you who are interested in getting an Oscilloscope, you can sometimes find second-hand ones on sale on eBay for reasonable prices, for most signal-tracing work a 5Mhz Single-Trace Oscilloscope will do a good enough job if you're on a tight budget, you can sometimes pick up a good one for about $100.00 or so, make sure it's rated to run on the AC mains voltage of the country where you live though.


More screenshots to come....stay tuned.

What you need to take into account is what you actually hear through a guitar amp compared to those oscilloscope waveforms will be something different again; the speaker acts as a low pass filter, so you won't get many of the upper harmonics produced by the distortion unit, resulting in a much 'softer' sound. A direct signal from the fuzz (or distortion or overdrive) unit straight into your audio interface will sound a lot harsher and not very pleasant. Even that will be filtered by the A/D sampling rate setting. Even at a sampling rate of 192kHz, the audio bandwidth is a bit over 80khz (depending on the filter used), which is a lot less than the 20MHz of the 'scope. And unless your monitors have ribbon tweeters, your playback system (be it a studio monitor or hi-fi speaker) will have around a 20kHz frequency response (maybe double that with ribbon tweeters).

Doc, any way you could show some of that effect? Maybe record the output into your system, then get screenshots of the raw, straight playback and playback through a cab sim waveforms?

Quote:

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Originally Posted by Simon Barden

What you need to take into account is what you actually hear through a guitar amp compared to those oscilloscope waveforms will be something different again; the speaker acts as a low pass filter, so you won't get many of the upper harmonics produced by the distortion unit, resulting in a much 'softer' sound. A direct signal from the fuzz (or distortion or overdrive) unit straight into your audio interface will sound a lot harsher and not very pleasant. Even that will be filtered by the A/D sampling rate setting. Even at a sampling rate of 192kHz, the audio bandwidth is a bit over 80khz (depending on the filter used), which is a lot less than the 20MHz of the 'scope. And unless your monitors have ribbon tweeters, your playback system (be it a studio monitor or hi-fi speaker) will have around a 20kHz frequency response (maybe double that with ribbon tweeters).

Doc, any way you could show some of that effect? Maybe record the output into your system, then get screenshots of the raw, straight playback and playback through a cab sim waveforms?

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That's true and you do raise a good point, I guess what I could do is record the sound of each one of the pedals going into my Marshall amp that's miced up with my Scarlett CM25 condenser mic(it's the only good mic I have), I'll need to sort out my account with Soundcloud first though cause I only have enough play-time on it to upload a 4 minute audio clip, but it is entirely do-able.

Something else to consider is the load impedance that the pedal/guitar is 'seeing'. Most amps are typically 1M ohm input impedance, so is your Cro which is replacing the amp also providing a 1M ohm load to the pedal/guitar? I'm suspecting your Cro's impedance may be a little higher leading to the 'discrepancy' that I think I'm seeing in your bypassed screen shots...

1M per volt is not a 1M ohm load.. and a x10 probe is definitely not 1M ohm i/p Z.

Quote:

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Originally Posted by Marcel

Something else to consider is the load impedance that the pedal/guitar is 'seeing'. Most amps are typically 1M ohm input impedance, so is your Cro which is replacing the amp also providing a 1M ohm load to the pedal/guitar? I'm suspecting your Cro's impedance may be a little higher leading to the 'discrepancy' that I think I'm seeing in your bypassed screen shots...

1M per volt is not a 1M ohm load.. and a x10 probe is definitely not 1M ohm i/p Z.

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Good point, well since I'm connecting the output of the pedal direct to the input of the Scope, I guess it would be seeing the input impedance of the Scope, which according to what the input labeling says is 1M shunted by a 75pF cap, the only time I would need to adjust any high-frequency compensation is when I connect a X10 Scope probe to the input, I do realize that most Scope probes are deliberately designed to be lossy, and that the alligator clip-lead I made doesn't incorporate the deliberate lossy-ness of a proper X1 Scope probe.

From what I understand, a X10 Scope probe is not 10M, but 9M shunted by a small value variable capacitor, this is because it is deliberately designed to form a 10:1 resistive-divider with the 1M input impedance of the Scope in order to divide-down the input signal by a factor of 10, there's at least two reasons why you would want to use a X10 Scope probe that I can think of, firstly, a X10 Scope probe increases the input range of the Scope in terms of signal-amplitude, secondly, it reduces the loading effect of the Scope probe so that the circuit that's being "Probed" continues to operate correctly, some radio-frequency circuits will simply stop operating if they are loaded by a X1 Scope probe, or they will operate incorrectly, high-frequency RF circuits tend to run at high impedances and any loading can cause de-tuning effects.


The passive pickup circuitry in an electric guitar is a high-impedance circuit too, active pickups, like EMGs are low impedance, a circuit that is a high-impedance type doesn't have a lot of current flowing through it, so audio signals are more of an AC voltage signal rather than a current signal.


Hopefully I'm remembering my electronics theory correctly from what I learnt doing a basic electronics certificate at uni in the mid 90's.