Vero Sonic Titan - Output cuts out from 80%-100% on level pot

[cajone5]

Not sure if this is cool to post here since it's not a madbean project.  Just let me know if this is inappropriate for this forum and I can remove it.

Anyway, I built a Sonic Titan over the weekend using this vero layout...
http://tagboardeffects.blogspot.com/2012/01/dam-sonic-titan.html

It's actually the second one I've built and I tweaked it slightly to add low end by changing out the 4u7 caps on the left side out for 10uF.  I also used a linear pot for gain as I liked that better.  My first (4 years ago that is now with a friend) was perfect as is.  This one is almost perfect.  The only issue is that when I turn the level pot up it increases volume and then the last 20% of the sweep (where I should have max output) the volume quickly drops and the signal cuts completely.  Anyone have an idea why this might be happening?  I feel like the answer should be straight forward since it seems to work perfectly otherwise.  I've already tried swapping out pots to no avail.

Here's the pedal...





Any help would be greatly appreciated.

[cajone5]

Not sure if this helps but this appears to be the schematic.  My simple mind can't figure out a scenario that would cause the volume to rapidly cut out in the last 20% of the level pot.

http://s22.photobucket.com/component/Download-File?file=%2Falbums%2Fb324%2Faggressive%2FDamSonicTitan.jpg

Thanks in advance for any assistance.  I will verify all wiring again and turn the board over to make sure I don't have anything wrong.  I also ordered a new IC (LM386-3) to see if that helps.

Thanks!

[m-Kresol]

1) nice build, I like the striped wire

2) it's perfectly fine to post non-madbean builds, you can put them in the build reports section.

3) I think your problem might be a faulty pot. I have never heard of such a behaviour to be IC related, but I might be wrong. try measuring resistance between pin 1 and 2 of the pot as you turn. if the pot is the culprit the resistance will get lower first but then increases as it cuts out.

[cajone5]

Pot tested and it's not the problem.  With meter on pins 1 and 2, the resistance starts around 95kohm and sweeps to near zero and the reverse happens with the meter on pins 2 and 3.

Here's a video of the issue.  Unfortunately I don't own a tripod (going to pick up on amazon now  ) so it's just me strumming the open (and out of tune) strings.

https://www.youtube.com/watch?v=mQwJY-fdWbQ

I'm pulling it apart to check everything.  Any chance it's a transistor issue?

[cajone5]

Photos of the board...

[cajone5]

Swapped transistor (and socketed it this time).  Same issue.

Op amp is the last piece that I can think of at this piont.  Unless there's some component that can get overloaded by too much signal and clamp down.  I'm completely at a loss with this one.

[beneharris]

what is your IC doing when it happens?

[cajone5]

Not sure -- haven't put the voltage meter to it (and now sure what to look for if I did).

I have now replaced the pot, transistor, IC and nothing fixes the issue.  It's as if I am shorting the signal to ground at the very end of the level pot taper.  I'm starting to think giving up and rebuilding the thing is a better plan at this point...

[beneharris]

Well my thought, and it may not be a good one, is that you've got a solder bridge, or an improper connection someplace, that when the pot gets too high is causing an issue with the IC? Check the voltages on all the pins when its working, and see if those change when the issue is happening. That's where I'd go.

[cajone5]

Finally getting around to measuring voltages.

Op amp (LM386L) voltage measurements using this numbering...



Transistor (2N3819) with this pin out... [D - G - S]
http://dpnc.unige.ch/tp/elect/spec-sheets/2N3819.pdf

Power Supply - 9.5 V

Level @ 0%,50% and 100% [Gain and tone at 50%]
Pin 1 - 9.1
Pin 2 - 9.5
Pin 3 - 9.5
Pin 4 - 9.5
Pin 5 - 9.1
Pin 6 - 9.5
Pin 7 - 9.1
Pin 8 - 9.1

D - 9.5
G - 6.5
S - 9.5

I get the same readings no matter the level. 

[beneharris]

Have you tried a different LM386? I haven't ever purchased one with an L suffix on it before. All of mine have always been like N3. The datasheet says the output power and everything matches between the two but maybe there is an issue with that? Boy, sounds like a doozie.

The datasheet says
"When  using  the  LM386  with  higher  gains  (bypassing  the 1.35kΩ resistor between pins 1 and 8 ) it is necessary to by-pass the unused input, preventing degradation of gain and possible instabilities. This is done with a 0.1 μF capacitor or a short to ground depending on the dc source resistance on the driven input"

It looks like from a schematic I found pin 2 would be the unused bypass, and its only got a resistor going to ground. In fact looking at the layout, it seems 2 is just bypassed, and 4 goes to ground. is that right? That could be an instability that its mentioning.

[cajone5]

I did try a "N3" as well with the same result. I didn't measure voltages with that one in though. I did have someone mention rebiasing to 4.65v. I'll try that next unless someone has a better idea

[beneharris]

Try running pin 2 to ground, or put a cap to ground. Unless I'm totally off base and I'm misreading the layout?