Trouble with Big Muff Pi

[killerkev]

drop the breadboard and etch a circuit board....

[dxm1]

You're using your audio probe, which is good.

I checked the sound for the first transistor and it gets sound at the base, but none comes out at the emitter or collector.

This is bad. The transistor is not working correctly.

It's also getting (from battery to each point) around 8.4 volts to base and 7.8 to emitter and collector. This doesn't make a whole lot of sense to me. It's supposed to be much smaller at the base and emitter, right?

Yes, this is bad, and probably why the transistor is acting cranky. The voltage on the base should be less than that measured at the collector. This is the first problem to fix. You will have to check (again) all connections to the base. There are four, and only four.

The base connects to one end of a 100n (0.1 uF) cap. This appears to be good, or you wouldn't be hearing signal here.

The base connects to one end of a 100K resistor -  the other end of that resistor connects to ground.

The base connects to one end of a 470K resistor, the other end connects to the collector.

The base connects to one end of a 500pF cap, the other end connects to the collector.

Double and triple check these connections with your DMM. Also, verify that the 100 Ohm resistor connects to the emitter and ground only. The emitter should have less voltage than the base.

You may not SEE a solder bridge, but your base voltage is too high. You either have a solder bridge or a wiring error.

And that seems to be the case for almost all of the transistors (which were ordered from this site, by the way).

Where you got'em from doesn't matter. Let's debug this in a systematic manner...

I've replaced the first and last ones and no change has come about, so I doubt that they're broken.

Some of the most productive techs I've worked with have been disciples of the 'Shotgun Approach'.  Repair experience tells you which components are most likely to fail, so spend 15 minutes replacing the most likely suspects. This works in a remarkabe number of cases. However, you never know what the original problem was, and never will. Since you aren't getting paid for fixing this board, rather than blindly swapping parts, why not find the root cause, and fix it? Everyone will learn something...

I also cannot see any solder bridges where there shouldn't be any. WHat could be the problem here?

None of us can see your board, so you're going to have to help us. Use your DMM to verify every connection like Pete said. Start with the first transistor stage - if this isn't working, nothing downstream will be either. Let's get Q1 working first.

[Radamus]

Thanks. I'll get the voltages up here as soon as I can. I've been a bit busy with school. Would pictures help?

[Radamus]

I tested that first transistor. The way it works is like this: It has an 8 volt bias on the base. There is around 7 volts on the other two leads. The resistance from the collector group to the base wire when I disconnect the 470k resistor is around 209k ohms. The 470 is around 460. I did the math and it's a total of 140k ohms from hot to base with that setup and that's what I measured. Is the resistance from collector to base wrong? The resistance to the collector is only 13k ohms and it gets less voltage. Does this mean that I'm somehow getting voltage to the base that is not coming through the collector or the 470k resistor? That's what it seems to be telling me, though resistance to hot is 140k ohms. It still gets greater voltage than the collector, which only has a 13 ohm resistance.

This is a dumb question, but which end of the battery am I supposed to measure voltage from and how much of a difference does this make?

The reason I'm asking is because I seem to be getting the same voltages on a pedal that is currently working (or at least similar voltages).

And what voltages should I get?

Thanks

[Radamus]

I've worked on this pedal quite a bit today. I cleaned up a lot of my soldering and I double checked all of the connections and parts. I have a digital effects processor (Zoom piece of crap) and I ran its drum machine function through the pedal. I got sound all the way through earlier, but it quit, so I'm still working on cleaning up the soldering. The problem that I'm facing now is that the sound coming out of each stage is quieter than from the stage before. The voltages on the transistors are correct but sound at the collector is stifled. As I said, the voltages are correct (C= 8.xx, B=1.xx, E=B-~.6), so I know it's forward biased. But why is it not amplifying?

I'm going to go back to fixing the connections, but that first stage should be fine and it's not. The second stage should also be fine, but sound become too quiet to accurately discern at that point.

What's wrong with my transistors?

[Barcode80]

unless i'm mistaken on my transistor-as-distortion logic, you need that supply on the collector to be roughly half of the supply. check your resistor values going to the collector from the +9v. also, though not likely, you could have fried the transistors if you left them hooked up with supply voltage for that long. they should be rated for at least that voltage, but i've done it before.

[killerkev]

You should repost your voltages again. 8 + volts on the collector is still way too high. They should be around 3 to 4 volts.

[Radamus]

This is not the kind of stuff the average electronics guide tells me. Thank you. I'll double check all of that.

[Radamus]

Okay, I'm ready to get back into this. I'll start with a quick admission: I did not realize that the stereo in was to break up the ground so current only flows when there's something plugged in. So all the stuff about voltage being about the same on all parts of my board was because of that. That's what I get for copying and not knowing why.

Here's the important stuff:
Q1
C 7.6
B 1.3
E .85
Q2
C 7.18
B 1.26
E .79
Q3
C 6.81
B 1.16
E .71
Q4
C 3.63
B 1.49
E .91

That's what I have right now. From what you've told me, Q4 is right. I'm testing around Q1 right now. The collector resistance to hot is 14.x Kohms. The resistance to hot at the collector of Q4 is only ten.

With the sound probe, I have noticed that sound loses volume through each transistor, instead of gaining it. Also, the sound at the diodes does not seem to be distorting at all, though there is audible sound before and after each diode.  Sound goes through, but it basically just drops volume. What would cause it to do that?

[Radamus]

I'm going to try and phrase these questions carefully. I think there are some major things I do not understand.

If I measure 7 volts at one part of a circuit, what can I do to lower it?

Why does the 10k resistor in my final stage only pass 4 volts when a 15k resistor allows 7 to the collector in the first part?

What does a transistor need to amplify a signal?

Why would a transistor not amplify a signal? Can a high collector voltage cause the transistor to make a sound quieter?

Why would each stage of my pedal here lose voltage on the collector, compared to the one before it?

I've double-checked everything and I've spoken with an electrician and an electrical engineer (both in my family) and they were unable to help me (mostly due to not having dealt with sound much).

Thanks in advance.

[killerkev]

The only thig I can tell you is that on many muffs that I've looked at have collector voltages of 3 ~ 4 volts, Base except for Q4 are around 0.5 volts and Emitter except for Q4 again is ~0.1 volt. Q4 does look good. What I do when everything goes ary is to disconnect a coupling cap between two of the transistors and check voltages on the first tranny and also probe it with the audio probe. You should really check your resistor values again to make sure you read the color code correctly and placed a 390 ohm resistor in there instead of a 390 K. I've done that before! Also check your traces with your DMM continuity check. It shouldn't sound when crossing a component but will sound when your traces are solid and intact. This is a good way to check for solder bridges.

[Radamus]

I think I figured out what's wrong. I read all of those capacitor values and though, wow, no one would ever use a .1F capacitor in a project like this. And then I used the same logic on the 100 ohm resistors. Now I have to figure out how many 2.2k I have to put in parallel to make a 100 ohm. I knew it had to be some stupid mistake. I'll test that and hopefully, all will be well.

Could I possibly omit the resistor coming off of the emitter? I don't have any 100 ohm. I thought they were 100k and never ordered them. If radio shack has them, I can pick them up there and there's also a fry's electronics if the price different is worth the five bucks in gas to get there.

[Radamus]

Okay, I took all the places where I put a 100k to ground from an emitter and I put them directly to ground. All the voltages are now close to the correct value. For some reason, I have almost no gain coming out of the transistors, though there is plenty of cool fuzz. And my bypass isn't working. That's more than likely independent of the circuit, though. I ran out of time for the night, so I'll try fixing that stuff tomorrow. The sound is pretty damn cool, though. Can't wait to get it louder.

Would the lack of the 100 ohm resistors keep the transistors from amplifying?

[petemoore]

I ran out of time for the night, so I'll try fixing that stuff tomorrow.
  Hard to set down I know, probably a good thing you ran out of time, better than running out of hair to pull out !
The sound is pretty damn cool, though. Can't wait to get it louder.
  I've had cool dam Fuzzes that I then fixed and thought maybe they were cooler before completely debugged, they were 'quiet' too though.

Would the lack of the 100 ohm resistors keep the transistors from amplifying?
  I would get teh 100ohm-ers in, [actually I'd probably put 220 or 470 or 1k there and work my way down by parallel-building resistors on the 'oversized' values long, above board leads..made into solder points above board..that way I'd have a default value, large, to which it's easy to diddle to a smaller value.
  Anyway when you get the 100ohmers in the circuit, you can jumper across them and see what that does.
  Getting a circuit right', involves getting everything right, the voltage measurements just help find what's wrong, from there...measureing everything measurable at & adjactent the problem area, every resistor, connection, non-connection etc. etc.] and fixing/adjusting until it all measures correct, makes it verified as right. Good sound [boost/fuzz] and function [all the knobs work as expected] and voltage measurements at the transistors is generally 'right' enough to call debugged.

[killerkev]

You can put Q1 - Q3 emitters to ground but not Q4. I don't understand what you mean about the capacitor values (o.1 uf). These are the values that I use. The Big muff is a very loud pedal so It's queit, you still have issues. Repost Voltages....

[Radamus]

I fixed a few things and it works now. Sounds great on my bass but I can't figure out why so many guitarists like it. Anyway, thanks for all the help.