My first post, my first stompbox and a short circuit

[lautmaschine]

First of all, I'd like to say hello to everyone, as this is my first post.  I really wanted to report that I had made my first stompbox and everything was working beautifully....  but not is all well...

I made a Blue Magic and after some fiddling it worked great for a few days.  Just this evening though, as I was plugging in the DC I  noticed an intermittent short.  I fiddled with it a bit and quickly discovered that my intermittent short had become permanent.  I realized quickly that I had probably done a poor job of insulating my metal DC jack from the enclosure (I'll not likely be buying one of those metal jacks again, but they sure look cool). 

Now I can't get the pedal to work at all.  I took things apart and it seems that the short is in the board.  Ie, when I connect power and ground to my board from DC, the LED on my power supply goes dead.  I'm completely new to this, so I'm not really sure where to start.  Is it likely that I've blown something in the board?  If so, how can I isolate it?

Thanks for your help, and as soon as I get my pedal up and running again (and decorated), I'll be sure to post a pic. 

Paul

[petemoore]

  Sorry I don't have the silver bullet, that type of problem is a hunt 'n search task.
  The power supply is touching where it shouldn't, and that can be anywhere...the only isolation...you're already doing that by taking things apart.
   A close in light, eyeing especially the PS traces, and maybe a magnifying glass.
  Sometimes the problem pops right out, other times it takes awhile to find it.

[lautmaschine]

Thanks for getting back to me.  that's the thing though, I've checked the board with a bright light and magnifying glass, and I honestly can't see a problem with it.  Remember, it worked for three days perfectly until the power jack started to short.  The power jack is no longer shorting, the short is now in the board, so i'm wondering if I've blown one of the parts -- like a transistor or something, and if so, how do I actually test for a blown part. 

thanks again....

[Alex C]

Hello, and welcome. 

It is possible that you fried something by shorting to the enclosure.  Do you have a DMM (Digital Multimeter) or voltmeter?  A DMM is an invaluable tool for testing and debugging.  After you get this pedal all straightened out, you will become addicted and need to build more, so you'll need a DMM in the future.

This thread (click) has a lot of information about how to go about discovering the source of the problem(s) you're having, as well as how to relate it to other forum members. 

Are you getting any sound at all?
If you have a DMM, check the voltages on Q1 and Q2, and post them here. If you don't have a DMM, do you have another BS170 or J201 that you could sub in to check?

Good luck, and I hope you get it working soon.

-Alex

[Alex C]

I would like to add that I highly recommend that you build and use an audio probe immediately.  It should help you pinpoint the section of the circuit in which you have your problem.

http://geofex.com/FX_images/audioprb.gif
http://www.diyguitarist.com/PDF_Files/DIY-AudioTester.pdf

[lautmaschine]

Thanks for the advice, Alex C.  I realize that I probably wasn't clear enough.  I'm not getting any sound through the board at all, because I have a short in the board, which is preventing me from powering it.  I did get my hands on a DMM though and immediately discovered that the first diode (D1) has been fried.  It's a 1N914; although I don't have any spares, I do have a couple of 1N4148's lying around, and I'm hoping I can substitute on of those. 

If this doesn't solve the problem, I'll move on to more analysis, and will likely build an audio probe as you suggest. 

Paul

[petemoore]

  Pull the fried diode and test again for the short?

[Alex C]

Going by the GGG schematic in the PDF, if D1 is fried in the sense that it is no longer functioning as a diode, but as a jumper, then you have a direct short between 9V and ground, which is a bad thing.  If you have applied power to the circuit for lengths of time, it is possible that you "fried" other components by shorting.

If D1 was the only problem, and everything else is intact, you should be able to remove D1, power it up, and see what happens, like Pete suggested.  D1 is for polarity protection, and is not critical to the circuit.
A 1N4148 should be fine.

Hope you get it going!
-Alex

[lautmaschine]

Guys, thanks a lot.    I put a 1N4148 in D1, and it fired up just fine.  I didn't get a chance to play it for long, but it seemed to sound okay.  Is i t possible I might have damaged the two transistors without completely frying them? 

thanks again....

Paul

[Peter Snowberg]

Welcome to the forum  , and congratulations on your build+debugging.

When you damage transistors a little, I think the most noticeable effect is the addition of noise.

The diode in that circuit is purely for reverse polarity protection. I use 1N5818s for that purpose because they're rated for more current and because they have a lower forward voltage drop (~0.3 volts vs. ~0.6V).

[lautmaschine]

Is damage something I can measure in transistors, or is it something I just have to listen for?

Paul