Tube Screamer not distorting

[guitar_199]

(I hope this works!!!!!)
Ponchus,

Here is what I am getting at and I figured a picture is worth a thousand words....so here goes.

If you pick the pot up and hold it by the shell, with the shaft facing your eyes, with the leads down, you are aligned with this picture.

If you touch DMM probes to 1 and 3 you should read the whole value of the pot (500K) and it should not change.

If you touch DMM probes to 1 and 2 you will read the amount of resistance between terminal 1 and terminal two and it should go higher as you turn
clockwise, lower as you turn counter-clockwise.

As far as the pot goes, you are using it as a variable resistor.  The schematic on the Tech of the Tube Screamer usees only pins 1 and 2.
The schematic on GGG uses all three..... the PC board ties 2 and 3 together on one side and pin 1 remains "au solo" on the other.  I don't know
if there is any real difference which way you go.

The way an op-amp works the gain is roughly equivalent to the amount of resistance in the feedback loop (the 51K plus the setting of the 500K pot)
divided by the resistor on the input of the chip, I believe a 4.7K.  Thus, at a minimum pot setting, the gain would be roughly 51K/4.7K or just a shade over 10).
As you turn the pot up you can see the effect. 551K/4.7K is more like 117.

If I were fighting this for myself,  I'd be right on that gain pot with a magnifying glass.  You are getting sound through.....clean....but sound... so I would tend to think that the basic build is okay....or at least not the problem.

I would make sure that:

1) The pot works as I describe above (I'd unhook it and measure it to be sure!)

2) I would make sure that I didn't tie the two ends together on the PC board and put the wiper alone.... or some other permutation of the pins.....

3) I'd follow the PC traces fromn the points where the pot solders on the board touch the pins on the chip.......and make sure each and every component that touches those traces is the correct part, the correct value, and that it goes to the correct place.

Man, I'm really pulling for you...........  I hope this turns out good.

[johngreene]

2. I tested with my DMM. What am i supposed to see here? If I probe at the signal INPUT, should I see voltage readings, or would I only see voltage on the +9V line? I'm getting a weird read. At the input, I get 0. The signal then travels into the .027cap. At both leads of this cap, the voltage still reads 0. Then the signal goes into the 1K resistor, where I get a read of 8.4V on both leads. So the voltage magically starts appearing at the input to the resistor. Is my cap bad?

I think this is where a lot of your problem is. The cap and the resistor should be connected together so you should read the same voltage on both at that node. Chances are you weren't making good contact when you probed the resistor side of the cap or they are not connected. Second, the voltage at the point in the circuit should be 1/2 of the supply. You are reading the supply voltage, or close to it. So you should check the Vb divider and make sure that is wired correctly and that  you are getting 1/2 of Vbatt at the junction of the 2 10K resistors.

If you could measure the DC voltage at each of the transistor pins and each of the IC pins and post those, we may be able to narrow down the problem much quicker.

--john

[Ponchus]

Hey guys, thanks for the responses, I really appreciate all the help I can get here. I've been super busy the last two days (rehearsal with one of my bands one night, wake for a friend's father tonight) so I haven't had a chance to work this any further (or to check these responses until now).

After I posted, I took a break for a while, and later when I figured I'd give this one more chance for the night, I started getting different results than what I had before. So I was getting results ABCD, then took a break to cook and eat dinner and watch TV, then I started over and was getting results CDEF. Even though I hadn't changed anything...<sigh> Something is definitely shorted somewhere, or a component is bad. For example, at the input, when I'd probe with the audio probe, I'd get a nice strong signal. Just as loud as at the input jack. Then out of nowhere   (such as after my break), the signal at the input would be very low. Things just aren't consistent, and that's what's killing me. At the first transistor, for example, I was having huge issues, where the signal would enter the base, but would not come out of the emitter. I didn't do anything significant, maybe touch up solder here and there, and all of a sudden I've got a perfect strong signal coming out of the emiiter. Take a dinner break, come back, no signal out of the emitter again. Who said that building pedals is fun?! 

Quick question before I go to bed. The first Cap is .027uF. I forgot to order this when I bought all the components. Radio Shack doesn't carry that value. The closest is .022uF. To order this 15 cent piece from Mouser will carry a $5 shipping charge. I got lucky and found this guy in Brooklyn (which is out of the way for me) who sells components, so I went and picked up this cap. Turned out to be his last one. The cap looks a little fishy, like it's been sitting there for a while. If it's faulty, what sort of "signs" would I see? And more importantly, can I use a different value for now, until I place my next Mouser order? If so, what values? Will the .022 work?

Thanks again everyone. I WILL get this freaking pedal to work even if it leads to people attending MY wake! 

[johngreene]

First of all, you are a lot closer than you think think you are! Second, a .027uF substituted for a .022uF will not make a difference in the circuit working. It may make a difference in how the effect sounds, but that's about it. So, I really think your problem has to do with the biasing of the circuit and therefore the importance of the voltage readings.
--john

[Ponchus]

Success!!! I removed the pot like you said...tested the pot and it tested fine. I looked really close for any sorta short (wire wisps and whatnot). Nothing. However, when I resoldered the pots to the board, the pedal now works!!!

Now I have to reinstall the switch and LED and power jack and I'm good to go (I hope).

Thanks for all your help, I really appreciate I!! Now to see about this PT80 I'm building...

[Paul Marossy]

Yeah, sometimes reflowing some solder joints will do magic. Glad it's working for you now. 

[guitar_199]

Good deal, Ponchus.   Glad that worked out for you.

Bob

[fixr1984]

im having the exact problem as Ponchus did. I also did the tonepad version. I checked all the voltages from the GGG build instructions and Im right where im supposed to be there. My 500k pot took a crap somewhere between opening the package and now so the only thing i had left was a 1m pot. With the pot out i get a good ohm reading on it. with it installed i get 0 ohms everywhere i check on the pot. Ive tried swapping wires around on the pot..took out all 3 diodes(1n4148 from Aron) resoldered the wires to the pot and to the board a few times. Audio probed around the diodes and at the wires going to the drive pot and i get a clean signal thru. I think thats all. Anyone have any ideas?

[petemoore]

  Glad ya got it !!
  You probably can't hear any difference between a .027uf and a .022uf. ... rare to have a problem with small caps like that unless they are really old.

[fixr1984]

Just giving this a bump

im having the exact problem as Ponchus did. I also did the tonepad version. I checked all the voltages from the GGG build instructions and Im right where im supposed to be there. My 500k pot took a crap somewhere between opening the package and now so the only thing i had left was a 1m pot. With the pot out i get a good ohm reading on it. with it installed i get 0 ohms everywhere i check on the pot. Ive tried swapping wires around on the pot..took out all 3 diodes(1n4148 from Aron) resoldered the wires to the pot and to the board a few times. Audio probed around the diodes and at the wires going to the drive pot and i get a clean signal thru. I think thats all. Anyone have any ideas?

[Dave Eason]

Yeah, sometimes reflowing some solder joints will do magic. Glad it's working for you now. 

I have definately found that out.  I made a TS808 clone and the drive was doing nothing, it turned out it was a case of just re working the solder joint where the 0.047 uF low end roll off cap joins the 4.5V bias in the drive section

[fixr1984]

Yeah, sometimes reflowing some solder joints will do magic. Glad it's working for you now. 

I have definately found that out.  I made a TS808 clone and the drive was doing nothing, it turned out it was a case of just re working the solder joint where the 0.047 uF low end roll off cap joins the 4.5V bias in the drive section

I even tried that just now..I touched up all of the joints just in case they were cold and still the drive has no effect. Is my 1m pot to big to be using? I had the same problem even when I used the 500k so I cant see that being the problem..

[johngreene]

im having the exact problem as Ponchus did. I also did the tonepad version. I checked all the voltages from the GGG build instructions and Im right where im supposed to be there. My 500k pot took a crap somewhere between opening the package and now so the only thing i had left was a 1m pot. With the pot out i get a good ohm reading on it. with it installed i get 0 ohms everywhere i check on the pot. Ive tried swapping wires around on the pot..took out all 3 diodes(1n4148 from Aron) resoldered the wires to the pot and to the board a few times. Audio probed around the diodes and at the wires going to the drive pot and i get a clean signal thru. I think thats all. Anyone have any ideas?

Are you saying that the drive pot adjustment doesn't do anything or you are not getting any gain? If you are not getting any gain through the circuit then the opamp is behaving like a follower. There are 2 things in this circuit that could cause that.

1. A short from the output to the inverting input. (across the diodes most likely, if the pot is shorted, there is still a resistor in series to provide some gain).
2. An open between the inverting input and the RC network to ground (or Vb).

You say you are measuring 0hms everywhere you check on the pot when it's installed. Are you measuring from lug-to-lug? and are you making the measurements with the power off?

--john

[christobean]

i just finished my ts-808 pedal, but instead of putting a jumper next to the 2 diodes, i put a third diode, i plugged it in and it works with a little OD, will replacing the third diode with the jumper change the tone?

[fixr1984]

im having the exact problem as Ponchus did. I also did the tonepad version. I checked all the voltages from the GGG build instructions and Im right where im supposed to be there. My 500k pot took a crap somewhere between opening the package and now so the only thing i had left was a 1m pot. With the pot out i get a good ohm reading on it. with it installed i get 0 ohms everywhere i check on the pot. Ive tried swapping wires around on the pot..took out all 3 diodes(1n4148 from Aron) resoldered the wires to the pot and to the board a few times. Audio probed around the diodes and at the wires going to the drive pot and i get a clean signal thru. I think thats all. Anyone have any ideas?

Are you saying that the drive pot adjustment doesn't do anything or you are not getting any gain? If you are not getting any gain through the circuit then the opamp is behaving like a follower. There are 2 things in this circuit that could cause that.

1. A short from the output to the inverting input. (across the diodes most likely, if the pot is shorted, there is still a resistor in series to provide some gain).
2. An open between the inverting input and the RC network to ground (or Vb).

You say you are measuring 0hms everywhere you check on the pot when it's installed. Are you measuring from lug-to-lug? and are you making the measurements with the power off?

--john

I measured with the pot out and it reads fine, with it installed and no power i get 0 ohms..across the wiper to an outer lug as well as the two outer lugs together. As far as the drive, the pedal is acting more like a treble booster than anything else, and the drive pot has no effect to the sound at all.  When you say an open between the inverting input and the RC network, do you mean i have a bad chip?

[fixr1984]

Ive checked the joint around the diodes and it doesnt appear that there is a short to another pad or something, i even tried another ic chip. Still have no distortion. When im audio probing, at what point should i be able to hear the signal start distorting? Any more ideas would be great.

[petemoore]

  If you put up the voltages from ground of the opamp pins, and the transistor, also the battery voltage, I would be glad to try narrowing things down.

[fixr1984]

  If you put up the voltages from ground of the opamp pins, and the transistor, also the battery voltage, I would be glad to try narrowing things down.

Ok heres what i have.
battery=8.89
Q1...c=8.89   b=3.44    e=2.92
Q2...c=8.88   b=3.47    e=2.95
pin1=4.42      pin8=8.88
pin2=4.42      pin7=4.42
pin3=4.41      pin6=4.42
pin4=0.00      pin5=4.42

Is there any more numbers you would need?

[fixr1984]

Bump