Timmy signal problem!! Sounds awful after 15% volume

[nate77]

I've been sitting on a populated vero timmy clone for a while and I've been dying to get this boxed up. I boxed it yesterday but have a problem that baffles me! It sounds as it should (I think...) at clean boost levels but as soon as there is any breakup it turns to complete crap. When I hit the strings with any force it pops and distorts aggressively. I went through it with an audio probe and the signal is intact until pin 6 of the opamp. At pin 7 it is slightly worse. The only components it travels through are a couple of 3k3 resistors (and a 10n cap to vref) but I've checked the board a dozen times, reflowed solder and cleaned any suspect areas but it is unchanged. I'm still dealing with a faulty photobucket account. Do any of the vets out there have any ideas what could cause this? It seems to be wildly obvious that the problem is somewhere in the "right" side of the opamp, the jumper and the 3k3 resistors but I'm at a loss. FWIW, the layout I'm working with is from the guitar fx compact layout. I'm stumped and dying to hear this contemporary legend so any help is much appreciated. Thanks!

[dwmorrin]

Voltages on all the opamp pins might be useful.

[nate77]

I tried to take some voltage readings on the pins but couldn't seem to get any readings at all. I have a pretty cheap but well rounded DMM but still seemed to get nothing. To be honest, I'm only about 20 builds deep and haven't had any issues that needed voltage reading from the opamp. Input the negative to ground and positive to each pin but got no reading. Is there a better way to do this?

[dwmorrin]

Double check that you're on volts dc, and not volts ac.
Sometimes it's two different positions on a selector switch, sometimes you switch to just "volts" and then there's a toggle push button between ac and dc.

You should be able to verify the +9V power at least.  Black lead to ground, red lead to pin 8 (assuming the common DIP dual opamp pinout).  You may have to dig in a little, and hold still if your meter is really slow.
Maybe start with a 9V battery by itself just to make sure the meter is functioning properly.

Most likely the voltages are OK since it at least gives you clean signal at low gain.  But it is always a good starting point when debugging to check voltages.

[nocentelli]

The 10n cap off pin 5 should go to ground, not vref.

[dwmorrin]

...the signal is intact until pin 6 of the opamp. At pin 7 it is slightly worse. The only components it travels through are a couple of 3k3 resistors (and a 10n cap to vref)

The 10n cap off pin 5 should go to ground, not vref.

And the cap should not be connected to the 3k3 resistors... if you've connected the cap in parallel with the resistor to Vref, then your second opamp stage is getting a huge gain boost there.  Disconnect that cap and see what happens.

[nate77]

Sorry guys. I meant 10n cap to ground. Late night and typing from memory (poorly). So too more accurately state where I've narrowed the problem to; signal leaves pin 7 > goes through signal splitting 3k3 resistor > into pin 6 &
Through another 3k3 resistor (resistor to vref) then as it leaves pin 7 it travels through a 1uf cap > volume pot > output. What could happen in this small ring? More accurately, the issue arises between pin 5 and 6 of the op amp. Where are the only points of possible signal degradation here? How to I accurately check for component failure?

[GibsonGM]

Learning to read schematics is the #1 way to get 'em right, Nate! Please brush up on that!   It gets really, really hard to troubleshoot when you're trying to work off a layout.   It's all fun when assembling, and easy, but then if it's got issues - you have no way to communicate what's going on!

"pin 5" is non-inverting input #2 (2IN+), "pin 6" is inverting input #2, or "2IN-".      "pin 7" is opamp section #2 OUTPUT. 

It's all in the language, my friend....if you have bad distortion on pin 6, I'd check that the VRef voltage is ok there (WORK that meter!), and that the other 3.3K resistor is attached to pin 7 properly....a bad IC could have one section not working, too.  Not common, but hey - possible.  Any blobs of solder could cause shorts, too...are those REALLY 3.3K resistors, or could they be something other, like 33K??  You never know - gotta look CLOSELY and CRITICALLY.   

Why don't you refer to this, see if it is what you've been working off of:  http://3.bp.blogspot.com/_y4AYtND8Hz8/SDNMRfHYKEI/AAAAAAAAALI/LjTrmxycb4c/s1600-h/tream2ed5.png

If you look at the opamp on the right - is this what you're trying to describe?  You have signal right up to the "+" input on the 2nd section?

Don't get too stressed, the problem is 99% most likely right in front of your face; you'll eventually find it. We ALL have this stuff happen!  )

 

[nate77]

I'm on the road mike, but when I get back ill dig into the dark and scary world of reading schematics. At this point I can read schematics in the same capacity I can speak spanish; I can ask for the bathroom, find a library and pay for beer. I've been putting this off for longer than I should have. I'll get into this right away. The inability to use common language is killing me. I know the problem is RIGHT in front of me. Thanks for the kick mike. It'll get this thing handled!!

[nate77]

Also, yes. The signal is intact up to the "+" part of the opamp (pin 5)
Then it goes screwy on me. I'm going to double check the resistor value tonight and see if I may have had them mislabeled. Thanks again mike

[nate77]

Aaaaaaand, yeah, that's the schematic I'm working from. 

[GibsonGM]

OK then, Nate, you should be focused on pin 7. That would be 'full volume' so to speak.   The inverting input, pin 6, will have a lesser signal after going thru the resistor.  Not all of it is being sent back to the input in the feedback loop (the 3.3K resistors).     A cruddy solder joint there, off-value resistors, or incorrect VRef can do a lot to mess things up there. 

I'd audio probe right after that 1u output cap, forget the volume control, and see what you get.    But first, as I've had a chance to think more, do this stuff below:
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- Do you get clean signal all the way to the output if you play very lightly? Like, "normal" sounding to you?  I re-read your original post, where you said: "...as soon as there is any breakup it turns to complete crap. When I hit the strings with any force it pops and distorts aggressively".    This could mean something is up in the FIRST feedback loop, with the diodes.  They turn on when you play above a certain threshold.  So, you could be barking up the wrong tree and might need to go back to THAT area.  First check that the diodes are 'pointed' in the correct directions (use their black bands as an indicator...).

Did you put the switch in and all that as in the schematic?

If the first 1/2 is wired up ok, you should be able to audio probe at pin 1 and get clean-to-overdrive guitar sound as you play.  Won't be awesome, but it should BE there without poo sound, minus the nice treble cut etc.  and the 1M drive should work normally.

Getting a reading on your supply voltage, and especially your VRef voltage, would be awesome!  Try to get it right from the top of the 47u cap...

This should at least give you some stuff to do, which will get you some answers, even on what's NOT wrong. Good to eliminate things like this!

[nate77]

Thanks Mike, I very much appreciate the in depth problem solving. So; the diodes are all correctly placed, the signal is coming through the whole curciut without crappy breakup and distortion when volume and signal are low, the signal sounds good at pin 1, 2 and 3 of the op amp, the resistors are correct value, I did not opt for the diode clipping switch, and when trying to measure the op amp pins for voltage I get no reading (oddly enough!). I will measure the signal after the 1uf output cap tomorrow and see what I get. I will also take a peak at the first feedback loop as well. I've been focused on the second feedback loop and the second inverting input assuming that's the source of the problem. Thanks you very much man, seriously invaluable insight.

[GibsonGM]

It takes time to build up, it's an experience thing, Nate!     Out of curiosity, I put together a Timmy last night (using that schematic).  Everyone's been talking about them, so what the hey.  

Wow, I'm seriously impressed, very transparent but this thing SINGS!   It's evened out my "pretty-good" blues tone - now I'm going from a NOT boomy low end to scorching highs without being glassy.   I like this pedal!  You can run from AC/DC back to clapton and get good results.  Dunno about with a more modern rig, but I bet it would shine there, too.    Definitely will replace the old TS-9 I sometimes use for this purpose!

Honestly, Nate - in this case, the audio probe will be your best tool.   You need to hear output at pin 1.     Then check pin 7.    If no output, something is probably shorted or not connected right in between.

For voltages, you need the opamp power supply (~9v, at pin 8, measured from ground) and the VRef voltage, measured right at the 10k/8.2k/47u junction....then again at the two 3.3K resistors by the "-" input at opamp section 2.    My VRef is about 4.8V with a battery voltage of 8.73V.     If it's close, it is ok.   No voltages at the opamp = a short or problem with your power supply.   I'd lean more towards SHORT - check that voltage divider/47u cap area for that.

Doing these first on breadboard is, for me, the ONLY way to go.  If something's wrong, you can move stuff. Then you KNOW it works, and when you do the PCB (I use perf), you just go A-B-C with the parts.   Maybe something to start working into, man...

[nate77]

I'm doing the family thing until tomorrow afternoon, but then I'm gonna go back and rebuild this thing. I've got goof signal at pin 1 and can't seem to track down the error otherwise. I've heard nothing but great reviews like yours about this curciut. I'm psyched you dig it, I'm hoping that this will be my new go to as apposed to replace my tube screamer army of derivatives I've got. I've got a breadboard on the way, that should ease the pain of post build debugging! Thanks for all the advice and prodding to move me in the right direction, much appreciated. Thanks for the time to help me work my way through this. Have a great holiday season man, and I'll post back once I resolve my issue with this build. Merry Christmas (or whatever your flavor is)!

[nate77]

I'm also having problems getting any voltage readings from the op amp pins, silly right?! I'll figure that out too. Thanks!!

[GibsonGM]

Sounds like the best way to go, Nate - just keep working it.  If your meter is REALLY on the blink, you might have blown a fuse...try just measuring batteries to see if it works.
It will get easier as you go along, just do NOT give up!  Let us know how it's going. 

Have a very Merry Christmas, man!

[nate77]

Hey Mike, after the festivities yesterday I found myself with a bit of time so I laid out a zendrive and a SHO on vero, stuck them in a 1590BB. Great combo and very therapudic. I needed a win after batting that timmy! I decided to keep my good luck flowing that I'd whip up a new timmy and teace the disfunctional one. I desoldered the old one, rewired the new one and BAM! It fired right up. Now that I'm out of the depressive funk of getting beat up by the first timmy I'm recharged to see what the hell I did to the first one. Also, this process has forced me to learn much more about the language, thought process and theory behind all this! Thanks Mike, I hope
You had a great Christmas!

[GibsonGM]

Awesome, Nate!  Glad to hear it!  Usually you get a little beat up, stick with it, and then get several wins that way!  Shortly, there will be few boxes that can beat you - you'll go into the problem A, B, C - BAM and done!  I was glad to help, and now I have a Timmy, too, ha ha...

Merry Christmas to you, too...keep working schematics, finding the patterns, and reading up on here about new stuff!  ) Happy New Year.