debugging diy tubescreamer- only clipped signal

[black_labb]

hi guys, got the urge to make a tubescreamer-like pedal the other day. after debugging the build i now get sound, but it is only the clipped signal coming through. as you strum hard, it gives a clipped signal, which dies away quite quickly. essentially i am getting the clipped signal through the clipping diodes, as well as some high freqeuncy content that comes through the 51pf cap. the drive pot will adjust the amount of signal coming through, as well as limit how long it stays (as the distorted content will reduce as the attack declines).

here is the schematic i am working off
http://www.schematicheaven.com/effec...truebypass.pdf

on mine i am using an LM358 as it was suggested on another schematic. i've checked the pinout, which is the same as on the schematic.
i have used led's for the clipping diodes
i am using 2n3904 for the transistors
changed the input cap (C1) to 0.047uf
changed C3 to 0.1 uf

i am using a board with strips of connections attched, and use wires to connect perpendicularly to the strips. i used a drill to break the strips where it was neccesary for the circuit.


what could the problem be? ive checked that there is not a cold solder joint at pin 1. all controls seem to be working correctly (other than the level pot soldered backwards). the tone control hasnt been able to do much, but i'd say thats bacause the clipped signal still sounds clipped when on.

the way i see it, the problem is between the pin 1 and 2, or within the first op amp. does the op amp amplify the clean signal out from pin 1? or does it flow through the drive pot?

sorry if the questions are a bit silly, i am more of a tube guy, and have very limited knowledge with transistors.

[black_labb]

just posting to say that i picked up an 8 pin socket and swapped a TL072 in there, which didnt fix the problem.

any ideas?

[black_labb]

here are the voltages for the ic.

pin 1: 4.25
pin 2: 4.87 with drive on full, 4.35 with drive on minimum
pin 3: 4.16
pin 4: 0
pin 5: 4.25
pin 6: 4.25
pin 7: 4.25
pin 8: 8.61

[cbarrow7625]

That's a tough one given the info on hand. Based on heavy distortion with the decay you describe, I would guess that one of your ground or VR connections are either disconnected or shorted somewhere they shouldn't be. Either the bias is messed up or the gain is more-or-less open loop causing way to much signal to try to pass through the clipper stage.

Why not try disconnecting the clipping dipdes & see if the stage works well as an amplifier? If you get similar results then you know you have distortion coming from something not related to the clipping diodes. I would even suggest breaking the signal path between stages to isolate each stage. Check the input transistor for proper operation first, then the clipper stage (without, then with the diodes), then the tone stage, etc., etc. Move through the singan path from input to output one stage at a time to verify the operation of each stage so that you can isolate where the problem comes in.

It would be a bitch to spend all of your time looking at the clipper stage if the real problem was a fried input transistor.

Anyway, once you isolate where the FIRST problem is, start checking for opens & shorts, etc. on everything connected to that stage. You will find something. If it doesn't work, something has to be mis-wired. But you won't find it until you isolate the stage in which it is occurring.

Good luck.

[black_labb]

thanks for the help.
i used a testing probe and it seems that the quick decay happens on the output side of C10. (meaning it is fine on one side, and the decay happens on the other). ive replaced the cap, and checked that r12 is correct (measured with an ohmmeter). also measured resistance between the nearby tracks, to check wether they are bridged. everything seems fine, but its not...

[black_labb]

after having a bit of a think about it, i realised that if the level control was not effecting the sound, other than the volume, the decay issue must be before that.

i disconnected the clipping doodes, c4,and the wiper on r19 from pin one. this gives no signal at the output, leading me to believe that the problem is with the clean signal being emitted from pin 1 again.

would this allow the opamp to amplify correctly?, or does the lack of feedback not allow it to emit? should i leave the pot in the circuit, and just remove the clipping diodes and the 51pf cap?

[cbarrow7625]

Leave the feedback pot connected. Otherwise the op amp is running at open loop gain & is probably in mad oscillations, hence giving no output. It could even damage the op amp.

However, that could be a good clue for troubleshooting this. Disconnecting the clipping dipdes is the right thing to do. See if the stage will cleanly amplify the signal. If it can't, your problem is probably in that stage.

If, after putting the pot back in the circuit (leaving out the diodes), the op amp still isn't working correctly I would check the connections for R6, R7 and C3. R6 & C3 are a voltage divider that work with the 500k pot to set the gain of that op amp stage. If R6 & C3 aren't connected properly, you will still be applying too much feedback, choking off the gain of the stage (reudced gain). If turning the pot does not significantly change the output at pin 1, you have a problem somewhere in there.

[black_labb]

thanks again for the help, will do some more testing tomorrow pm (australian time)

[black_labb]

reconnected the pot and left the clipping diodes disconnected. playing gives no sound on any settings of the drive pot (r19).

i shorted across c3, and measured the resistance from pin 1, to Vr, which read 56 with drive on least and ~550 on full. doesnt seem to be an issue there. could a cold solder joint be somewhere where the ohmmeter reads correctly, but not actually work correctly?

not really sure where to go from here. im almost tempted to completely redo it... (using the extra parts). having one working might also help me fix that one, by trying possible things until it does what the one i have now does.

[Eltek]

Hi black labb,

Probably you have the C5 connected the wrong way. It should be placed in series with the R8 and not connected to GND. R16 must then be connected from Vr to PIN5.

Have fun,
Ivan

[Eltek]

Sorry it was a brainfart...

This is the schematic of a TS. Look at the input transistor biasing...

[black_labb]

i put this on a bit of a backbench (sitting on my desk) for a while. been meaning to fix it, but i take on alot of projects and dont have much time. might have another look at it sometime soon.

thanks, i'll update when i have a look.

[el_fela]

Sounds like you have q2 inverted pinout... just a guess, but it happened to me and sounded like you described...

Anyone knows how to remove the clean part of the signal from the circuit? When i put a booster in front of the ts, the clean signal is much more noticeable, and i want distortion only.