Tube Screamer not distorting

Ponchus · November 30, 2005, 01:05:45 PM

Hi everyone...

Just "completed" my first build, a TS clone (I went with the 808 for now)! So the good news is that the LED lights up, the volume knob does indeed adjust volume, the tone knob does indeed adjust tone, and I saved a bunch of money by switching my car insurance to Geico (sorry, I couldn't resist).

The bad news is that I'm not getting any distortion when I adjust the drive knob. Doesn't seem to do anything at all, in fact. I'm a total, and I mean total, newbie at this, so I don't even know where to begin. I'm guessing that the diodes are possibly the culprits? They are definitely pointing in the right direction, and I touched the joints with my soldering iron just in case there was a cold joint. No luck. I used two diodes for symetrical clipping (BTW, I used the layout from Tonepad for this project, etched my own board, etc).

I found on General Guitar Gadgets.com, a list of what the correct voltages should be at the IC chip, as well as the transistors. All seems to match up in that department. Could it be that I have a bad diode? If so, how do I find out for sure? Also, from reading through the mountains of good info on this site, I constantly see people who troubleshoot for days, only to find out down the line that the problem was a bad cap. Why would a cap go bad, how do I find out, what are the red flags I should be looking for, etc? I know this is a pretty broad question, sorry for that...

After I get this working, I'll post another thread regarding my PT80. I actually built the TS and the PT80 at the same time, but had to put the TS down while i waited for some parts. So I "completed" the PT80 instead, which doesn't work yet either. From researching that issue, EVERYONE's opinion seems to be "Don't build the PT80 as your first build"...so I put that down, got my parts for the TS, and am now in limbo on both projects (and damn my house is a mess for it)...

Anyway, thanks for any help.

Ponchus · November 30, 2005, 03:20:35 PM

Anyone? I didn't stump you guys did I

?

plucker · November 30, 2005, 03:25:21 PM

Hi M8 what value have you put C4 (51pf On layout) I had the same problem as you I substituted C4 for a larger value as I coudn't find 51pf in the UK and got no o/d but cured it by using a bog standard 47pf ceramic and it worked a treat did a load of other add on mods to it and it has great tone and a tone switch to boot.

Plucker

Ponchus · November 30, 2005, 03:32:25 PM

I followed the layout exactly as is...the only thing I changed was that with Tonepad's projects, the offboard components are listed as a separate project. Due to this, I overlooked needing a 2k2 resistor for the LED. I put in what I had on hand, which I believe was a 4K7. Other than that minor change (I think that's minor anyway), everything (including that 51pF cap) is as stated on the layout.

I could give that a try as you suggest, but I'm still concerned...I've read build reports where people followed the layout exactly as is, and had no issues. I'd like to get it running before modding it...

Roobin · November 30, 2005, 03:49:36 PM

QuoteI touched the joints with my soldering iron just in case there was a cold joint

Did you heat sink the trannies and diodes? Ive forgotten loads of times to do this...

AL · November 30, 2005, 03:53:44 PM

Read this.

http://www.geofex.com/Article_Folders/TStech/tsxtech.htm

Also, build the Audio Probe at GEO.

Also, build the Audio Probe at GEO.

Also, build the Audio Probe at GEO.

http://www.diystompboxes.com/pedals/debug.html

This little guy has saved me uncountable hours of frustration. YOU NEED THIS. Thank you RG Keen.

AL

johngreene · November 30, 2005, 04:02:32 PM

Sounds like either the drive pot is wired wrong or the wrong value or the diodes are somehow shorted.

--john

ocelot · November 30, 2005, 04:11:16 PM

The drive pot should only have 2 wires on it u havent used the other leg to ground ?

Ponchus · November 30, 2005, 04:15:46 PM

I did read that Technology of the Tube Screamer article. Mostly, my eyes glossed over and I was completely lost. It's really not written for a newbie like myself. For example, it just jumps into the Input Buffer (which is a "plain vanilla emitter follower") but I have no idea what an input buffer is, or an emitter follower for that matter.

As far as building an audio probe, I've been meaning to. However, maybe you can advise me as to what I'll be doing with it once I make it. Because the problem for me is that the signal isn't distorting. But the signal IS making it through to the output. From the way I understood the concept of the audio probe, it would be good for cases where the signal doesn't "make it through" to the output. you probe around and try to figure out how far the signal gets. In my case, it's getting all the way through, it's just not distorting...I hope I'm making sense.

Regarding the last 2 posts, I have double checked the connections of the pots, and their values, and they're correct. Not sure about the diodes being shorted though. What does that mean exactly, and how do I test it?

OCELOT: The drive pot has 3 wires on it, not two. This, however, is exactly how it's listed on Tonepad. Check out the PDF at: http://www.tonepad.com/getFileInfo.asp?id=81

Thanks for all the great info, please keep 'em coming!!!

Paul Marossy · November 30, 2005, 05:48:04 PM

Did you try a different opamp? Maybe something is wrong with it...

johngreene · November 30, 2005, 06:07:10 PM

Quote from: Ponchus on November 30, 2005, 04:15:46 PM

Regarding the last 2 posts, I have double checked the connections of the pots, and their values, and they're correct. Not sure about the diodes being shorted though. What does that mean exactly, and how do I test it?

Try removing the diodes and see if you now get a lot of gain when adjusting the drive pot. Also, check the 4.7K - .047uF. If either of those isn't making contact, you'll have a unity buffer.Also check the 47uF to ground (Vb).

--john

Ponchus · December 01, 2005, 10:37:36 PM

I took out the two diodes and there is still no gain. Was I supposed to jumber those pads though? I did not.

I built an audio probe, as was suggested. MAYBE I'm on to something, but I'm not sure, maybe someone here knows. I'm able to follow the signal that I fed into the TS with the audio probe until it hits the first transistor. I'm not quite sure what should happen after that. Here's what I'm thinking, according to the layout at Tonepad, avaiable here:

http://www.tonepad.com/getFile.asp?id=81

When the input signal reaches the first transistor in the circuit, it enters into the base. That's the last that I hear of the signal. Is it supposed to come out of the emitor (toward the left according to the tonepad layout)?

johngreene · December 01, 2005, 10:48:09 PM

Yes, an equal amount of signal should be on the emitter as is on the base. Something must be wrong with the input transistor. Orientation?

--john

Ponchus · December 01, 2005, 10:55:54 PM

unfortunately they're in the correct orientation...and my signal is not coming out of the emitter at all

johngreene · December 01, 2005, 11:20:44 PM

Take the transistor out, remove the 10K to ground, short the base pad to the emitter pad and then see if it works.

--john

Ponchus · December 03, 2005, 05:26:59 PM

OK, I fixed my transistor problem, turned out to be a cold joint.

The TS still isn't distorting though, but I'm sure that with my newly build audio probe (that thing is freaking awesome), I will find the problem. Question though...when I follow the circuit, should I be hearing audio on ground? If you look at the layout for the TS from Tonepad, there are three colors being used. I assumed that blue was the audio, red was +9V, and green is ground. Where should I be hearing audio? On all three, just the blue lines, etc?

Thanks for the help

guitar_199 · December 03, 2005, 05:59:53 PM

Do you have a meter? If you pull the diodes out and still have not gain/drive function.... I question the wiring on that pot. If you have a meter... the way that pot is wired, you should be able to probe each of the end terminals and see the resistance change as you sweep the pot. If it is shorted you would effectively have the 51K resistor as feedback but the pot would not have any effect. If you dont have a meter, just examine the soldering both ON the pot tabs and on the PC pads where they solder onto the board. Make double sure that a solder blob or a wire "whisker" isn't bridging across the pads connecting all three together.

Ponchus · December 03, 2005, 07:18:12 PM

The wiring on the pots is definitely ok, I retouched both the pots and where they connect on the PCB. When testing the pot terminals with a DMM, the two outer lugs show a change as I turn the pot. The middle lug does not change. Any ideas? Also, should I be hearing sound (when using the audio probe) on traces that connect to ground?

guitar_199 · December 03, 2005, 09:53:30 PM

"When testing the pot terminals with a DMM, the two outer lugs show a change as I turn the pot. The middle lug does not change."

THAT is way weird. What kind of pot is it?

Most of them..... the outer two lugs represent the whole total of the pots resistance and dont change. The middle one is called the wiper and slides long the resistive element changing resistance as it goes.

And what else I think is weird is that, if this pot was hooked up when you tested it...., the wiper and one end were tied together on the PC board. That means.......that if you touched the one end that was alone........ and EITHER of the other end, or the wiper you should see the same thing.

If I was you.,...I would consider unsoldering that pot and giving it a good once over with that DMM to be certain of what you saw. Again...the outer two should measure the whole value of the pot.......always! (Now that is with all of the leads separate. the middle one to either end should change value as you move it.

Another thing you might consider..... the schematic that is drawn on the Tec of the Tube Screamer.....shows using only the wiper and one end of the pot. ONce you measure this one and identify the wiper.... I would hold the pot so that the shaft is facing me..... then....take the left lead....and hook it where the two holes are together........ and take the wiper... and hook it to the other one that is by itself... and try it.

Good luck...you are almost there I think.....

Ponchus · December 04, 2005, 01:25:26 PM

OK, so by now I'm super frustrated!!!

Here's where I am with this thing. I've removed the switch and LED, so i just have the input and output and battery going into the board. I'm getting no sound making it to the output at this point.

1. I've been testing with an audio probe. The signal comes into the input, makes it through the .027cap and then the 1K resistor, then the base of Q1. At this point, it comes out of the collector, which is connected to the +9V line. It also makes it out the emitter, then the 1UF cap, then goes into pin 3 of the 4558 (which is socketed by the way). I'm assuming at this point that I should hear the signal come out of pins 1 and/or 2?

Pin 4 is connected to ground. I am NOT hearing anything out of pins 1 of 2...

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?

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Tube Screamer not distorting