I'm having some problems with a Madbean Runt build.
The issue is that the output volume of the engaged effect is very low. With the gain and volume all the way up, it's still not nearly as loud as the bypassed signal. The effect does work though, all of the pots operate as they should and it sounds correct, just at very low volume.
It worked briefly at the correct volume when I first tested it, however I had wired up the LED backwards. After I rewired the LED the correct way, it now is at very low volume. I've quadruple checked all of the joints that i changed for the LED, and I see no problem with them. I've even completely removed the LED from the circuit to check if that was causing an issue, and there's no change. I'm guessing that the timing was just a coincidence.
I have double checked my parts values and orientation, as well as reflowed any joints that I thought might have been suspect. I've triple checked the wiring and it is wired correctly. My guess is that a part isn't working correctly, but I'm not sure how to narrow it down to the problem. For some reason, my gut is telling me that it might be the Q1 transistor however this is just a guess and I'm not sure how to check that with my meter.
I have used all stock parts except I have dropped my CLR for the LED down to 1k.
Here are my voltage measurements:
Positive lead at jack/battery. = 8.96
9V in at board = 8.96
Ground in at board = 14 mV
IC pins 1 through 8 = 8.27, 4.61, 4.5, 8.91, 4.04, 4.5, 0, 32 mV
Each pin of transistor = D 8.91, S 5.48, G 4.04
If needed, I can post some pictures tomorrow. Thanks for any help you might be able to give, I'm starting to pull my hair out over this thing!
I think you're numbering your IC pins wrong. Looks like you started in the right place, but alternated left-right from the top down. If that's what you did, your IC voltages are good, but you should be starting to the left of the dimple and moving counter-clockwise around the chip.
If that's not how you counted them, this is a stunning coincidence and your IC pin voltages are @#$%ed.
Your Q1 voltages are reading high, though. You should be getting no more than half your VC there. You're getting an appropriate VB voltage on (the real) pin 3 of your IC, which makes me agree that your JFET might be wonky. Did you use a 2n5457?
Pictures always help, if you can. Good luck!
:-[ you're right, I alternated the pins on the IC. So if I got the pin #s right the correct order would be:
1-8: 8.27, 4.5, 4.04, 0, 32 mV, 4.5, 8.91, 4.61
I am using a 2n5457 ordered from smallbear I believe. Unfortunately if it is the transistor, I'm going to have to wait until I order another one to swap it out. I only have a spare J201 and an old NTE85.
I'll try to get some pictures posted tomorrow afternoon and hopefully that will help. Thanks!
It sounds like you may have signal leaking to ground. Did you double check with a magnifying glass for any solder bridges?
An audio probe would be a good tool to use to help trouble shooting as well. You can work your way through the circuit and see where your loosing signal.
Josh
So I'm trying to debug again tonight, and while I was rechecking everything again I noticed that I do in fact have a part substitution. For C9, the 1 uF capacitors I ordered are electrolytic caps, and are polarized. The schematic shows a non-polarized cap in there for C9. After looking at the schematic I oriented the cap negative "up", or facing towards the volume pot lug #3.
Which brings me to the question, could this be causing the problem? Is it important that the cap there is non-polarized? Or perhaps I have wired it in backwards?
Anyhow, that's the only thing I have found so far, even if it isn't the problem. I'm off to make an audio probe now and take some pictures!
Thanks all!
If I were going to put an electrolytic there, I would orient it the way you've done it, with the + side facing the DC current on the JFET and the - side facing the circuit output. I doubt that's your problem.
Ok, I've since found a couple of resistors that were reading ~55k when they should be 100ks, and I've replaced them. However no change yet. I've taken pictures, I hope they help:
(http://img.photobucket.com/albums/v418/cpuhacker71/Runt%20Wiring/P5211263.jpg)
(http://img.photobucket.com/albums/v418/cpuhacker71/Runt%20Wiring/P5211262.jpg)
(http://img.photobucket.com/albums/v418/cpuhacker71/Runt%20Wiring/P5211268.jpg)
Your soldering looks good and parts seem okay from what I can see. I think an audio probe at pin6, lug3 of the Cut pot and the gate and source of Q1 will tell the tale here.
Quote from: NightOwl on May 21, 2012, 09:56:33 PM
Ok, I've since found a couple of resistors that were reading ~55k when they should be 100ks, and I've replaced them. However no change yet. I've taken pictures, I hope they help:
If you were measuring R10 and R11 while they were in-circuit they were probably fine. You can never get a good reading without isolating them. (You can also really monkey things up by taking resistance readings in-circuit if the board is powered up. Be careful!)
Your soldering looks great. There does appear to be a little crud on the input leg of C1. Probably just flux, but I'd see if I could gently pick that off. You're going in the right direction with the audio probe, though. Just make sure you have your amp set pretty low before you probe pin 6!
OK, I've built the audio probe and have been probing like crazy. I know just enough about this stuff to get into trouble, so I'll put my results up and perhaps someone more experienced than myself can help me diagnose the problem.
Lug 3 of the Cut pot: Same signal as the output, low volume effected. Adjusting the pot makes no change in the sound
Pin 6 of the IC: Louder than the output with more low end, not as clear or bright as the output.
Q1 Gate: Same as output
Q1 Source: Identical to the gate
Q1 Drain: Louder than the input signal sounds like Pin 6 of the IC
Other observations: When the positive of the probe is not connected, I can hear the effected signal. When I touch the probe to the casing of the gain pot the signal is much louder and clearer than the output signal. Lug one of the gain pot is incredibly loud and a bit muddy. (this was done with the gain on full) I also notice that touching lug 1 of the gain pot, makes the effect seem to function correctly when the positive of the probe is disconnected!
My inexpert guess: Due to the casing of the gain pot, could a bad pot putting signal to ground be causing this?
I hope this helps, thank you all for your assistance so far. I feel like I must be getting close to the root of the problem!
Is your signal at IC1 pin 6 as loud as you would expect the pedal to be at close to full volume? Or just louder than your input signal. It should be loud as hell at that point.
I don't think you should notice the Cut effect if you're probing pin 3 of the Cut pot -- you're short circuiting the signal before the sweep of the pot. Does adjusting the Cut pot affect the output sound?
If you're getting a significantly lower signal at lug 3 of the Cut pot than you are at IC1 pin 6, I recommend probing either side of R6 and C7.
OK, so R7 is definitely the right value, it's a Vishay 1k5 that I've double checked the number on the part as well as tested the rest of them I ordered.
The negative side of C7 is the same as the output signal, and the positive side of it is very loud and clear.
I appreciate the help!
Sorry! I edited my original comment while you were writing. Bad form on my part.
C7 should not be killing much signal, so my guess is that either C7 or one of your diodes is wonky.
The sweep of the Cut pot doesn't affect lug 3 to my ears.
Pin 6 is blow your eardrums out loud. R6 tames it down a bit, but still quite loud. C7 kills it to nothingness. I'm going to replace C7 and see if that fixes it. I will report shortly!
OK, replacing C7 changed the tonal character a little bit at the positive lug, but no overall change. Same problem, and the volume drops considerably across C7. I measured it with my MM before I put it in, and it was reading ~5.1 uF, so a bit higher than it should be.
So that makes you suspect one of the diodes is a problem? How would I go about confirming that and pinpointing which one? FYI the effect sounds great when I attach the positive probe to lug 3 of the gain pot, I just can't get that to the output jack it seems :-\
Well, the diodes are the only other thing in your signal path between the negative lead of C7 and lug 3 of your Cut pot. Process of elimination! I'm not saying I'm 100% sure that's where your problem is, but it's really the last place we can effectively test components.
Diodes only pass current in one direction, but even when they pass it in that one direction a little bit of voltage is lost. This voltage that doesn't pass through is called the forward voltage. The maximum forward voltage rating of a 1n914 or 1n4148 is 1V. Your normal guitar signal is probably ~10-50mV, so to achieve clipping your op amp amplifies the signal to greater than one volt and then your clipping diodes let all the signal greater than 1V (or thereabouts -- the forward voltage is current-dependent) gets shunted to ground. This leaves you with a signal smaller than 1V with the tops "clipped" off the sine wave of your signal: distortion. Then you use the Volume pot as a voltage divider to lower the signal back down to something reasonable. Google "diode clipping" -- the Internet can do a better, more accurate job of describing the whole process than I can.
If your clipping diodes weren't conducting at all, you would get no diode clipping at all. You would essentially get that signal that you're probing on your Gain pot. Any distortion there is op-amp clipping, which is supposed to be tasty in the LM308. If your clipping diodes had too high of a forward voltage the signal would be louder than normal and it might be harder to get any diode clipping at all. That's obviously not your problem. If your diodes had too low of a forward voltage, your signal would be quieter, but since more of the wave form would be chopped off you would get a synthy, buzzy "square wave" type of distortion.
You see where this leaves us? None of these scenarios really sounds like your problem -- very quiet, but normal sounding distortion. That's why I'm not 100% sure it's the diodes. However, you can easily test this by removing them from the board. If you remove the clipping diodes (D1, D2) and you get essentially the same signal you were getting at the Gain pot, then we know that the diodes had something to do with it. Good luck!
I pulled one side of D1, and the output volume jumped like crazy. It sounds essentially the same as before, only with adequate volume. So can I infer from this that specifically D1 is bad, or just that one of the diodes isn't working correctly?
I think that as long as you have one leg off the board, you should be able to do a valid continuity test on the diode. Try it in both directions, and if it conducts both ways you've got a bad diode.
The 1n914s will clamp the volume for sure. One thing you could do to increase volume output is put two 1n914's in series on each side, or use a couple of LEDs instead.
It's also possible you had one of your diodes oriented the wrong way.
Jacob
It is finished and working beautifully now! D1 was a faulty part, it conducts both ways and I assume it was shunting a lot of the signal to ground. I've replaced it with a spare and the circuit is working great, and I've just finished boxing it.
Thank you all for your help, especially mgwhit: you sir are a gentleman and a scholar. I sure have learned a lot through this troubleshooting, hopefully someday I will be experienced enough to help out newbies myself.
I'm putting up my first build report now, whoohoo!
Congratulations! You did a fantastic job of debugging it: voltages up front, good photos and great work with the audio probe. I know it can feel like like it's taking years off your life while you're working on it, but nothing teaches you more about these circuits than successfully debugging with a multimeter and an audio probe. Pat yourself on the back, crank that mother up and rock.