Timmy - Drive does nothing...

[bsas]

Hi all,

I am stuck here. Here is the design I followed (and that is the 8th one, I already did 7 successful ones! :D)



Basically the pedal "works". The volume boosts a lot! Treble and bass kind of works. The treble makes a very subtle difference, but I can hear it and also I can see it using a function generator in a oscilloscope.

But, the drive knob does nothing! I already measured the resistance across the lugs and looks correct: it goes from 0->1M between drive [1-2] and between 0->3k3 between [2-3].

Basically, when I am fully CW it kills the signal (actually, not entirely, but drops volume a lot). Also, this "volume drop" is just at the end of the knob, let's say, on the last 10% of the pot. I know that not just by hearing but also by checking on the oscilloscope as well.

So, I don't know what to look for. I can measure the voltages, but I think that is pretty much it. Any ideas?

Thanks!!!

[Frank_NH]

Upload some photos of your build so we can have a look.

Based on your description, I would check for a short or low resistance between pins 1 and 2 if the IC.  Also, check your clipping switch (or remove it temporarily).  My theory is that there is a short between the output of the op amp and the inverting input such that the gain pot is bypassed and hence no gain is obtained from the clipping stage.

As for the treble control, it is just a simple LPF treble cut (rolling off treble above 309 Hz at the low end and ~10 kHz at the high end).  I don't like this arrangement as it drops the signal level as you roll off the treble (through the 50K pot).  But it should work OK here given the gain from the preceding op amp stage.

Good luck!  

[bsas]

Thanks! I will test them, and I will post pictures of the build ASAP.
Also, do you know what are the expected voltages on the IC pins?

[bsas]

Ow! Two new infos I just forgot:

1) I removed the clipping switch to test. I was doing all the tests without it basically to isolate that variable;

2) When I remove the "treble 2/3" from the "drive 1" I get no sound whatsoever. I don't know if this is expected.

Again, thanks for you help! :D

[Frank_NH]

"Also, do you know what are the expected voltages on the IC pins?"

Pins 1 - 3, 5 - 7 should be close to the reference voltage (4.5V).  Pin 4 should be ground (0 V). Pin 8 should be supply voltage (9 V).

"When I remove the "treble 2/3" from the "drive 1" I get no sound whatsoever. I don't know if this is expected."

Yes. You're disconnecting the output of the op amp since it runs straight through the treble pot.  See schematic below.

 

[bsas]

OK, here is my debug:

1) The resistance between pin 1 and 2 are as follows:
- Fully CW it is 3K3 fixed;
- Fully CCW it slowly "builds up" (like a capacitor slowly charging) to around 670K.

2) DC voltages (supplied a 8.5V battery):
- Pin 1: CW: 4.3V, CCW: 4.1V
- Pin 2: CW: 4.3V, CCW: 4.4V
- Pin 3: CW: 3.7V, CCW: 3.8V (that is weirdly low I think)
- Pin 5: CW: 4.3V, CCW: 4.2V
- Pin 8: 7.5V (so, around 1V lower then the battery)

Here are my pictures (I just resolder the Drive 1 one line up to be sure was not a bad soldering, so, you will see a "out of the design" solder, jumper and cut on line 2):





Thanks!

[Frank_NH]

Check for a solder bridge on the right hand side of the flipped board, rows 3 and 4 from the top (near pins 2, 3 of the IC).  That's the only thing I can see that might be the problem.  You can also try reflowing the solder joints just in case there is a cold solder joint somewhere.

[bsas]

Fixed!

It was the freaking 100p capacitor :(

Thanks a lot dude! :)

[rocket88]

looking at the picture of the component side it looks to me like you have an additional link that shouldn't be there. look at row 2 right above the 100pF ceramic cap, it looks like you have a diagonal link from there to row 3, right next to the 100pF ceramic cap right under the d4 diode.

[bsas]

I know, I did this extra link because I was checking for bad solder and I got my rail destroyed. But I did made an extra cut on that line.

Anyway, it is working now. It was a bad 100p cap!

Thanks for taking a look :)

[Frank_NH]

Cool.  So the 100pF cap must have been shorting.  You never know...glad it's fixed.

[rocket88]

agreed. i've had to do that before. it was something that caught my eye. i love the timmy, and i hope you do too man. it's an awesome OD.

[bsas]

It is an awesome pedal man, I am having a blast! :)

My only question is (since I never tried a real Timmy) after like 12, the drive gets quite distorted, almost like a fuzz. This is expected right?

Thanks all! :)

[Frank_NH]

Depends what you mean by fuzz.  Folks like Zach (Rocket88) take their fuzzes seriously around here!  

The 1M gain pot should give you quite a bit of gain.  That combined with the different clipping diode arrangements means you should be able to get heavy distortion from the Timmy.  If it's too much, go ahead and substitute a 500K gain pot for the 1M.  One of the cool things about this hobby is the ability to mod pedals to suit your tastes.  Have fun!

[rocket88]

Haha. I am a man who loves fuzz, lol.

But, I don't think it should get I to the tell of fuzz, but as frank said it should get some nice thick and heavy distortion. Check out some of the demo videos on the tubes and compare it to how yours sounds. I've played a real Timmy, built a clone, and have the danelectro cto-1 (controversial pedal), and they all give a heavier distortion, but not really fuzz territory IMHO.