Red Llama clone - will bad LED kill output?

[tcr77]

Built the Red Llama replica kit from General Guitar Gadgets

http://tinyurl.com/RedLlama

I assembled everything except the battery clip within the enclosure and plugged it in. Lo and behold, it worked (this is my first attempt at building anything electronic). The only issue was that the LED did not turn on when I engaged the pedal.

So, I went back to my workbench and resoldered the LED connections and put in the battery clip.

Now, the pedal doesn't work at all when engaged (it passes the signal fine when bypassed).

I checked the voltages on the IC pins, and everything was fine. The odd part is that I get Zero volts at the point where the wire from the footswitch connects to the board (labelled "I" on the board). But I get plenty of voltage readings on the other components (and IC chip, obv.) If it's not getting any power at "I", then how is it getting volts on other parts of the board?

I get 9.2v on the ground side of the LED, and nothing on the other (going to the switch). But I assume that's not what is causing the failure, since it didn't light up anyway when the pedal was working.

Any ideas?

Do any Red Llama owners have a voltage list for all the solder points on the board? That would be helpful for determining which component (if any) has gone bad.

I get zero volts at "I", R1 (both sides), the south end of C1, north end of R3. Does that mean anything to anyone? Like I said, this is my first attempt at anything like this, and I'm a bit confused and rather frustrated.

Many thanks!

[MikeH]

To answer the initial question- Yes, it's possible.  But that's not likely to be the problem here.  If an LED somehow became a short circuit it would cause your power supply to short out and your circuit wouldn't work.  I've never heard of this happening, but anything is possible.

The pad labeled "I" is the input.  There should be no voltage there, unless you're feeding it an input signal.  And in that case it really should only be AC.

I get 9.2v on the ground side of the LED, and nothing on the other (going to the switch). But I assume that's not what is causing the failure, since it didn't light up anyway when the pedal was working.

This is a problem.  You should be reading 0 volts here and somewhere around 2-3 volts (I think) on the positive side.

Couple of 'silly' questions:

1- Do you have a Multi Meter?  If so, take your voltages and post them along with other helpful info in the "Debugging- what to do when it doesn't work" thread.
2- Have you built an audio probe?  This is the tool I find 99% of my problems with.  Easy to build- just use the search function and you'll find instructions.
3- Did you use a current limiting resistor on your LED (1-5K range)?
4- Are you sure you've wired your battery snap right?  I'd try removing it and see if you can get it to work again.  Whenever I had something
working, and then I did something that made it not work, I go back and undo what I just did to see if that was the problem.

[MikeH]

Another thing:  Your circuit should work independently of the LED. In other words you can remove it (delsolder it) from the equation all together and the pedal will still work if everything else is wired up correctly.

[Kearns892]

As Mike said the LED should be independent of the circuit, so my guess would be something went wrong when you attached the battery clip. I would check there first, and if you cant find the problem, go back to your previous set up to make sure the circuit still works and there isn't an intermittent problem caused by something like a poor soldering joint.

[tcr77]

Mike,

1 - Yes. The voltages on the IC all match what GGG says they should.
2 - No, but that will probably be my next step
3 - No. Should I? Where does it go?
4 - I took the battery snap out already and still it doesn't work

As for checking for cold solder joints, is there some trick to getting the PCB out to get to the solder points on the bottom of the board? I'd hate to have to desolder all the wiring just to look, because that just invites more problems when I resolder all the wiring.

The pad labeled "I" is the input.  There should be no voltage there, unless you're feeding it an input signal.  And in that case it really should only be AC.

Ok, good to know about lack of voltage on the input. What about the lack of voltage on R1 (the next component in the circuit, if I'm reading it right)?

I'm thinking the LED issue could be related to the switch? How do I use the multimeter to verify the switch is working correctly? I find no volts on any of the lugs.

Thanks so much for your help!

[MikeH]

You need to put a resisitor in series with the LED, otherwise it will burn out.  Most people use something from 1K to 5K; lower values yield higher brightness, and vice versa.

The switch only routes signal (with the exception of the pole that handles the LED) so you wont find voltage there.  Generally you're only concerned with voltages on active components like ICs and transistors, and to a lesser extend diodes and electrolytic capacitors, because they have polarity.  You won't figure out much debugging-wise from voltages on passive components (not at this point anyway).

As for checking for cold solder joints, is there some trick to getting the PCB out to get to the solder points on the bottom of the board? I'd hate to have to desolder all the wiring just to look, because that just invites more problems when I resolder all the wiring.

When I build an effect, first I build the board, with any potentiometers hooked up as well.  I don't box it up or wire up any jacks, switches or leds until I verify that the main guts of the circuit are working first.  So, I build the board (pots attached) and just have the leads soldered on for the input, output, ground and v+.  Then I use my breadboard to rig it up to a couple of jacks that I have mounted on my breadboard to see if it works.  If it does I just box it up.

For you, you can just dismount all of the pots and jack and the switch and flop the guts out to look at the solder side.  But, since you said it was working at one point, it is unlikely that you problem lies on the solder side of you board (unless you removed it an fiddled with it).  I'd guess it's in your offboard wiring somewhere.  But thats just a guess.

To verify that your switch is still good, you need a continuity tester on your multi meter.  It's the setting that looks like a little diode symbol: ->|

At least on mine it does.  If yours is like mine, it will produce a tone when there is continuity between 2 points, otherwise it might just read '0' or something.  Anyway, just check for continuity between the center lug and the corresponding outer lugs for each pole.  You should find that they are all connected one way, and not connected in the other way, then after you push the switch, you should find that all the connections are flip flopped.

If your DMM doesnt have a continuity setting, you could use the resistance setting and make sure the connections are a (or VERY close to) zero ohms.

[tcr77]

Well, I feel like an idiot now. The no sound problem was that the volume knob was not screwed down tightly to the post on the pot, so turning it up didn't turn the pot. I guess I just never thought I would get all the soldering right on the first try.

The "bad LED" turned out to be wired in backwards. Thanks for the advice on adding a resistor to extend the LED's life, though.

Meanwhile, on to the next project!