Tonepad tubescreamer - no output

[notid]

Hello, I'm building this: http://tonepad.com/getFile.asp?id=81

I was hoping I could pick your expert brains for advice.

I am getting no output when the effect is on. Working backwards from output with an audio probe, I have found that the signal stops at the left side of the first 1 uF capacitor (The point where the 7-pin from the IC touches). I have confirmed that it doesn't short to ground at this point, but I'm not really sure what to do beyond that. This seems like a very strange place in the circuit to stop getting a signal.

Any ideas as to what the issue could be?

Thanks,
Bryce

[PRR]

Is it shorted to Vb?

Is the IC in the right way around?

[notid]

Unfortunately no to both. I checked if it is shorted to ground, to vb, and the +9 in. The IC chip is definitely the right way round.

[Govmnt_Lacky]

I have found that the signal stops at the left side of the first 1 uF capacitor (The point where the 7-pin from the IC touches).

Sounds to me like you have narrowed your problem down pretty well.  The next step is going to be checking traces, component values, and component placement for everything in the signal path AFTER that 1uF NP capacitor. It could be a short or bad trace/bad component anywhere after that cap causing your problem.

Good Luck 

[MikeH]

What are the voltages on your IC and transistors?

Also, the first thing I'd do is try a new cap.

[notid]

"After" meaning working backwards, i.e., to the left of the capacitor on the schematic?

Thanks again,
Bryce

[Govmnt_Lacky]

"After" meaning working backwards, i.e., to the left of the capacitor on the schematic?

Thanks again,
Bryce

If the circuit works correctly (according to your findings) from the Input up to the "left side of the 1uF cap," then AFTER would mean from the left side of the 1uF cap to the circuit Output.

Stands to reason that if it works up to a certain point, then the problem must be AFTER that point. 

[starekase502]

this is a long shot but i made this screamer from tonepad and put 100k ra instead of 100ohm so maybe check all of your color codes against the layout. and if that dosent work check them again.  95% of the time when something dosent work for me its one resistor that i got the multiplier wrong.

[notid]

Okay, so that problem is solved -- mostly. It turned out that one of the copper lines was very faintly torn or something. I had to put a little solder over part of a copper trace to get it to correctly connect. Not sure if that makes any sense.

So now, I can plug a guitar in, run it through the pedal, and get a signal! The signal sounds pretty vanilla though- The tone pot seems to work, the level pot seems to work, but the drive only *slightly* changes the sound -- maybe like a level pot that effects the signal by 10%. We're talking barely at all. It's not screaming any tubes yet, but it's exciting to have something.

Any ideas as to what this problem could be? My initial thought was backwards diodes (they look correct), but I'm not sure. I checked resistor values.

[MikeH]

Double check the wiring on your gain pot.  Make sure it's the right value too.

[notid]

Not seeing anything off hand -- values look correct. If it helps, the sound is still very, very clean. Almost like there's no drive at all, and the drive pot is just acting as a tiny level pot. This is the most complicated part of the circuit so I'm sort of stumped.

[notid]

*bump*

Still no more progress on this. I keep checking for shorts to ground, voltage source, or adjacent traces. I'm still getting a very mild level effect from the drive pot, no 'dirt' whatsoever. i did a diode check on my multimiter on the diodes and it seems correct. I'm totally out of ideas. I also double checked that the IC has power and is grounded appropriately.

Thanks for the help! Even general ideas like, 'it might be in this section' would be helpful...

Thanks,
Bryce

[twabelljr]

What are the voltages on your IC and transistors?

Could you post the voltages please? You say you dont have any shorts to ground or power so measure them and post em up. That is usually the biggest help. Some people can spot a problem right away. Really check the clipping section of the circuit. Make sure you have Vb at pin 3 and at the 4.7k resistor. You can remove the drive pot and substitute a 1meg resistor across the pads in the layout that connect to the 51k resistor and the diodes. That should provide tons of gain. I am no expert but would like to be able to help and like I said, with voltages posted in the format of the "What to do when it doesn't work" thread, some people on this forum can spot a problem right away. They are that good! Good luck.

[notid]

Sorry for the delay. In response to having Vb at pin 3, I'm seeing it connected in series with a 10k resistor, then Vb. Is that correct?

Here are the voltages (fresh battery).

IC1
1 - 4.7 V
2 - 4.7 V
3 - 4.7 V
4 - 0V (seems correct, since it's ground)
5 - 4.7 V
6 - 4.7 V
7 - 4.7 V
8 - 9.4 V

Q1
1 - 9V
2 - 2.4V
3 - 1.8V

Q2
1- 9.4V
2 - 3.4V
3 - 2.9 V

Hope this helps. Ic1 seems interesting to me that so many voltages are the same, but the pins that should be grounded and have voltage source seem correct....

Thanks for all the help.

Bryce

[notid]

*another bump*

[notid]

Hi,

I've been doing some research... Still no success!

But I have tracked down another person who have posted voltages...

http://music-electronics-forum.com/t8877/

This person has IC voltages that seem roughly equivalent to mine, and he had drive working okay, but bad levels.

Found this: http://www.generalguitargadgets.com/pdf/ggg_its8_instruct.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

Seems like my IC voltages are correct, but the Q1 and Q2 voltages are incorrect. Are these pedals similar enough that I can assume it's something with the transistors?

Thanks for the help,
Bryce

[PRR]

> interesting to me that so many voltages are the same

The IC is, for DC, two unity-gain followers biased with half supply voltage. In theory, at "9V", all terminals should be 4.5V. Since your 9V is a hair high, all those nodes are where they should be.

Your transistor pins do look right, though Q1 seems to be a bit slack. For any non-reject 2N3904, that emitter voltage really should be 2.5V to 3.5V.

Check transistor ORIENTATION. The flat-sides facing the way the plan shows. Note that to simplify the copper, they face opposite directions.

I'm thinking: 1.8V is 7.6V below 9.4V battery. That's like the expected 0.6V junction drop plus a 7V drop. There IS a 7V breakdown in a transistor, if you put the emitter where the collector should be. With E-B-C pinout, if you put it backward C-B-E, the over-volted emitter breaks down. No permanent damage, but the input signal is clamped solid, can't swing, which matches "no output" and "signal stops.. first.. cap".

[notid]

> interesting to me that so many voltages are the same

The IC is, for DC, two unity-gain followers biased with half supply voltage. In theory, at "9V", all terminals should be 4.5V. Since your 9V is a hair high, all those nodes are where they should be.

Your transistor pins do look right, though Q1 seems to be a bit slack. For any non-reject 2N3904, that emitter voltage really should be 2.5V to 3.5V.

Check transistor ORIENTATION. The flat-sides facing the way the plan shows. Note that to simplify the copper, they face opposite directions.

I'm thinking: 1.8V is 7.6V below 9.4V battery. That's like the expected 0.6V junction drop plus a 7V drop. There IS a 7V breakdown in a transistor, if you put the emitter where the collector should be. With E-B-C pinout, if you put it backward C-B-E, the over-volted emitter breaks down. No permanent damage, but the input signal is clamped solid, can't swing, which matches "no output" and "signal stops.. first.. cap".

I much appreciate the help. The transistors do have the correct orientation (at least, according to the pdf (left side:flat down, right side:flat up)), however, I'm tempted to swap them around just to see.

I believe I said this up above, but I did ultimately get a signal. The "first cap" issue no longer applies. Right now, the tone knob works, the level knob works, but I'm getting absolutely no clipping. The drive knob does work, but it is acting like a level knob that affects the signal about 10%.

So for the past few days I've been going over and over the clipping section. I haven't found anything yet, but I'm probably missing something stupid. I am pretty sure the diodes are in the right way, but I tried swapping them just to see, and no luck.

Could it be that the signal isn't loud enough when it gets to the diodes, causing the clipping to not occur?

Thanks again.

Bryce

[Schappy]

Are you getting normal volume output from the pedal?

[notid]

Are you getting normal volume output from the pedal?

Yes. It's almost as if when I turn it on, the level increases just a tad.