Help me debug my Tonepad tube screamer

[rosssurf]

I have built the tonepad tubescreamer seen here    http://www.tonepad.com/getFile.asp?id=81     It has an LED and a 3PDT switch with NO battery!, and two switchcraft enclosed MONO jacks. I used a TLO72 IC and MPSA18's as substitutions for the 4558, and 2N3904's respectively I did not use ANY tantalum caps. All were panasonic ECBQ's except for the 47uf and 10uf electro's, as well as the tiny 51pf ceramic.

When the effect is bypassed sound passes through as it should, When i engage the effect the LED lights up but I get NO sound, reguardless of where i set the 3 pots. I have looked at aton of posts about this and i simply can not find what might be wrong. Do the following readings tell anyone where my problem might be. This is my first debug post with all of the various reading forgive me if i have left out any other pertinent info and feel free to inform me of what other info,readings, or parts may be helpfull.
Thanks in advance



Q1
C = 9.12
B = 4.56
E=  3.97

Q2
C= 9.12
B= 4.08
E= 3.65

IC1 (or U1)  SUBSTITUTION  I use a TLO72
P1 = 4.57
P2 = 4.58
P3 = 4.55
P4 =   .1mV
P5 = 4.56
P6 = 4.57
P7 = 4.57
P8 = 9.12
.
.
SUBSTITUTION ..I used 1N4148
D1
A (anode, the non-band end) = 4.59
K (cathode, the banded end) =  4.57

D2
A = 4.57
K = 4.59

Z1 (zener)
A =
K =

[petemoore]

  Set DMM to Beep, clip to a known ground, Test pin 4 [should beep from ground, actually it should be 0.0v from ground when voltage testing, meter anomoly ?].
  Anyway with a lead going to a known ground, test all grounds, then test signal path points and see if there isn't a ground getting in there somehow.

[Projectile]

Voltages look about right from a quick glance. Check the MPSA18 datasheet to make sure your transistors are installed correctly and the pinouts match up. Sometimes very similar transistors can have different pinouts. Also, check the electrolytic caps to make sure they are oriented correctly. Next, I would build a simple audio probe. I'm sure you will find where the problem is very quickly once you use an audio probe. It is an essential tool for debugging.

Edit: I just checked the datasheet for the MPSA18 and it looks like it should have the same pinout as used on the tonepad layout, so that should be okay. There should be nothing wrong with your substitutions. I'm not fond of the TL072 in that circuit, but it doesn't make a huge difference.

[R.G.]

solder bridge or wiring.

[MikeH]

Build yourself and audio probe.  Start at the input and trace your signal path until you lose the signal.  There's probably something improperly gounded.  If you have no sound at input, check your switch wiring.

[rosssurf]

Thanks you all for your help. I have had very little time to try everyones suggestions. I really appreciate your input folks. I will build an audio probe. . i admit i have not searched yet,but can anyone steer me to a link that tells me how to use a probe and what to listen for?
Thanks in advance

[mdh]

See Debugging in the wiki.