Troubleshooting tube screamer issue

[c4am95]

I built an 808 a while ago, and it recently lost all clipping abilities. It seems as though the tone / volume knobs are working, but the drive knob provides a marginal increase in volume when turned but no distortion. I replaced the op-amp to no avail and I checked all the pots to make sure they weren't broken. Any intuition as to what the problem might be? I assume that the issue lies around the diodes / op-amp, but it'd be nice to get some insight from someone with more experience.

[petemoore]

Any intuition as to what the problem might be?
  Somewhere within the realm of the unknown.
   For us to find out more and locate the problem, you'll have to read/apply the info in the debugging sticky.

[c4am95]

Finally got around to doing this. I'm using this schematic from Tonepad:

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

I did the TS-808 build with the symmetrical diode configuration.

The pedal turns on and carries the guitar signal all the way through. The volume and tone controls seem to be working, but there is no drive whatsoever. The drive knob is affecting the sound: clockwise, it produces a subtle increase in volume. However, no clipping at all. Kind of makes for a useless pedal as it is.

I'm not using a battery, just a 9v power supply. I measured it at 9.47v. Here's a list of the component voltages:

Q1:
c = 9.47
b = 3.48
e = 2.95

Q2:
c = 9.47
b = 3.57
e = 3.1

IC1 (4558):
p1 = 4.7
p2 = 4.8
p3 = 4.7
p4 = 0
p5 = 4.7
p6 = 4.7
p7 = 4.7
p8 = 9.47

D1:
a = 4.8
k = 4.7

D2:
a = 4.7
k = 4.8

[R.G.]

Just off hand, I'd say that your clipping diodes, smoother cap across the clipping diodes, or drive pot is shorted.

[c4am95]

I checked the diodes / cap, nothing is shorted. The pot is fine too. It's not shorted, and it's definitely working normally.

[c4am95]

ahhhhh! does anyone have any other thoughts? i've been troubleshooting this pedal for so long. i'm on the verge of just rebuilding it.

[Quackzed]

maybee reflow the solder on the diode/cap/drivepot/ traces, clean up the board flux goo dirt etc...
the drive pot should be having more of an impact than just giving a subtle volume increase...
i'd recommend taking a long second look for a short or open/broken trace around those parts.
and a short can be just some oil or other nearly invisible speck, so i'd maybee use an old toothbrush and magnifying glass and check continuity around those parts, make sure the parts that aren't supposed to be making contact, aren't somehow shorted...

[c4am95]

i don't know what changed, but now having the volume/gain knobs up produces a lot of static. no distortion really, just a lot of static. it's weird...the pedal does sound good even for its subtle gain and tone control.

i've checked everything on pins 1 and 2 of the IC so many times it's sickening. everything near/attached to the clipping section and the transistors is good continuity-wise. is it possible that a part is just broken? any thoughts regarding this static symptom?

[Tony Forestiere]

Audio Probe Time. 

[R.G.]

The advice you have already been given is very good indeed. A broken part is possible. However, it is the least likely thing to be wrong. Could be, but highly unlikely.

One aphorism taught to med students is that if you hear hoofbeats in the hallway, don't go looking for zebras; it's vastly more likely to be a horse.
Bad new parts do happen, but they're so rare as to be the last thing to look for. It is very, very much more likely to be a broken solder joint or wire, or an accidental short. Could be a mechanically broken part (or lead, or solder joint or wire).

What is your level of technical expertise?
Did it ever work correctly?
If so, how long?
If so, what were the circumstances prevailing when it stopped working right?

[johngreene]

If turning the volume and gain knobs up produces static then it is producing gain so it could be a problem with the input itself. The easiest thing to do would be to touch the signal path with a pair of tweezers starting at the input jack and move towards the opamp. You should hear significant hum every time you touch the signal path. If it goes from low hum to high hum then you just found the bad part.