What I learned about troubleshooting today

[motterpaul]

Man......   I finished my second Mad Deep Prof Blue delay pedal today, but I wanted to do the one with no volume pot (fwiw, I recommend it).

So, I started building this thing on Saturday, but I just could not getting it working right. I checked continuity (all good), voltage supply throughout (all correct) and then started doing audio tracing.

Now here is one cool thing I found. I was just using a breadboard as a testbed. I had the circuit built with pots attached, but no boxing yet. I made a simple audio probe with just a cable with alligator clips at each end. I hooked one end to the tip of the "amp in" and to the other I just clip a short & stiff wire lead. I started tracing audio, and here was where I realized I had a problem. I had some resistors where the initial guitar audio would not go through. In other places I could hear the delayed guitar, but not the original signal.

Anyway - a couple of things. I have these "audio quality" electrolytic caps.



I found that when tracing audio I could hear if the cap is working just by touching the audio probe to the metal top of the cap. I could hear the sound clear as bell - what a time saver.  That was the first thing I learned. I found one cap not giving me audio. So I decided to replace it. It was at that point - after hours of assuming my circuit was built correctly, that I found an error, that cap was off by one hole.

So - the other thing I learned is that you should never assume your circuit is right, even if you think you have thoroughly checked it, if you cannot find a solder short someplace, chances are you still have an error in the build. It took me literally a day and a half before I found this one - and that is just nuts.

But the last thing I learned is that if you are doing an audio probe and you find "dead" spots, it could be a bad component - OR it could be an error in your circuit. Mine was an error, right where the audio went dead. The audio ended after a resistor, but it was a following component (a cap) that was placed incorrectly. I had replaced two caps and a resistor, but in the long run it was just a build error.

[IvIark]

Great post Paul, I think you've hit on a couple of really important points and I'm sure the info about the dead caps will come in handy to plenty of people when trying to diagnose a problem with a probe.

[Synsound]

Never giving in to the thought that your build is correct is a great point.
The majority of my faults have been traced back to a misplaced component. Sometimes i have had to just put it down and revisit it a week or two later with fresh eyes to find it.

[AC_FX]

Agreed, very good points.  I hate it when I go down the rabbit hole trying to get something to work for hours, only to end up sleeping on it and immediately finding the obvious, basic cause of it the next time I look at it.  It's a damned good argument for simply stepping away from the bench if something's stumping you for more than about 10 minutes!!

For those who haven't built/used an audio probe - do it the next time you have a build that gives you more than a little trouble.  It is so useful to know where the signal stops doing what it's supposed to!  Even if the cause ends up being embarrassingly dumb. . . at least you got yourself looking in the right area of the circuit quickly!

Finally - OP, what do you like more about the DBD with no volume?

[motterpaul]

Thanks for the friendly replies.

When I made the build with the volume control it never seemed like I was getting unity gain. The pot sounded best full CCW, and as I turned it up it just seemed to distort the circuit. A delay should be "clean" and faithfully reproduce whatever you put in. I think the circuit without the volume pot gives you perfect amount (unity) of gain. I used it after my B1 Legend build and it managed the overdrive sound with no issues.

I also just want to explain how easy it is to concoct an audio probe. All I needed was one cable with alligator clips at each end.

You probably have a jack for your "amp in" cable, so just connect one alligator clip to tab for the the tip of that. For the other end clip use a piece of sturdy conductive wire for your probe. Whatever you touch should conduct "clean" guitar (starting at the input) to your amp. Then, just follow the circuit. Look at the data sheets for ICs to see which pins are for input/output. Also check your pots to make sure they are getting sound in & out.

I did this after I checked all rows for clean solder joints and verified the voltages at the regulator & ICs, plus double-checked all resistor and cap values. Also, a fast way to check for good solder joints is to just pull on both ends of each component. If it seems solid, and you can see a tiny bit of wire through the solder, it should be a good solder connection. But what I had missed was the placement of a cap.

As AC_FX said - when the sound ends at least you have found the right neighborhood. Now, I actually substituted a few resistors & caps before I realized that the resistor where I did not hear any output was acting that way because of the capacitor that followed it. I realized that when I noticed I could NOT hear audio from this one caps just by touching the top of it (very cool trick, but in this circuit it only worked on the 1U electrolytics in the audio chain).

I am writing this to remind myself how to troubleshoot, because I can build circuits pretty fast now, tho it is still pretty rare for a circuit to work for me the first time - but, I am getting better.

[Dennis Fox]

Thank you.   I am learning more every day regarding trouble shooting.   I will use this tip.