DIY Tubescreamer: Getting some wierd problems, please help.

[JObscure]

Hello,

So I finished my first DIY stompbox project and I have a strange problem. When the pedal is plugged in and on I only get sound if the electric guitar volume is turned down about halfway.
The guitar vol. pot is fine.

Help would be very much appreciated.



Followed the layout for the TS-808, breadboarded the project with a 100% accurate layout. (I get the right sound from it.)
No parts substitutions.
Negative Ground



1.What does it do, not do, and sound like? I can get overdriven sound. BUT! When the pedal is plugged in, and on I only get sound if the electric guitar volume is turned down about halfway.
The guitar vol. pot is fine, I checked.

2.Name of the circuit = Tubescreamer from Tonepad

3.Source of the circuit (URL of schematic or project) = "http://www.tonepad.com/getFile.asp?id=81"

4.Any modifications to the circuit? NO. I Followed the layout for the TS-808, breadboarded the project with a 100% accurate layout. (I get the right sound from it.)

5.Any parts substitutions? If yes, list them. NO

6.Positive ground to negative ground conversion? NO

7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? => 8.2

Voltage at the circuit board end of the red battery lead = 8
Voltage at the circuit board end of the black battery lead = 0 ??


Q1
C =7.8V
B =0V
E =2.2V

Q2
C=7.6V
B=0V
E=2.2V

IC1
P1=3.8V
P2=4V
P3=1.95V
P4=0V
P5=3.6V
P6=3.85V
P7=3.9V
P8=7.8V

D1
A =4V
K =3.95V

D2
A=3.9V
K=4V

[MmmPedals]

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[JObscure]

EDITED Thanks

[R.G.]

7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? => 8.2
Voltage at the circuit board end of the red battery lead = 8
Voltage at the circuit board end of the black battery lead = 0 ??

Your battery is low. That may have a part in what I suspect. Low batteries have higher internal impedance.

Q1
C =7.8V
B =0V
E =2.2V

Q2
C=7.6V
B=0V
E=2.2V

That's odd. Are you sure the voltage on the bases is really 0V?
What kind of meter did you measure with? Some meters can pull down a high impedance and give a false reading.

Perhaps you should re-measure these with the black/- lead on the emitter and the red/+ lead on the base. We expect that to give us about 0.5 to 0.6V in this circuit.

What I really suspect is that the low battery and/or wiring is letting this thing oscillate when the guitar volume is turned up. That's a trickier problem than simply not working. Several things can contribute to this: battery state, wiring, presence or absence of decoupling caps, many things.

I notice that there are no power supply decoupling caps on the layout you're using. After remeasuring the base-emitter voltage, try this:
find a ceramic capacitor of 0.01uF to 0.1uF - the exact value isn't important - and solder its two leads to the power pins of the operational amplifier. Solder one lead to pin 4 and the other lead to pin 8. I would also try a fresh battery.

If you're a decent solderer and careful, you can do this on the bottom/copper side of the PCB. Or - now that I think of it - you say "bread boardedf". Did you use a PCB or construct this yourself on perfboard or something similar?

[JObscure]

Checked again you were right: Both were at about 0.5V (Analog multimeter.)

I'll try to get a fresh battery if you think it will help.

I put a .1uF cap over the chip like you said and it has solved the problem!

The circuit was soldered onto a veroboard and I have checked the joints. (I guess different people use different terms, sorry for the confusion)

[R.G.]

I put a .1uF cap over the chip like you said and it has solved the problem!

That's the definitive answer. I suspected that it was oscillating, and if that capacitor stopped it, that verifies the problem and the fix.

Most circuit diagrams don't show them, but you **always** need something like an electrolytic capacitor and a ceramic capacitor across the power supply. National Semiconductor used to say this explicitly in their application notes. The standard in the industry is at least one 0.01uF or 0.1uF cap (or both) per integrated circuit, and more for big, power hungry chips.

Leave that capacitor there, and add a 22uF to 100uF electrolytic capacitor rated for 16V to 25V across the power and ground on the board. That should complete the fix and I doubt you'll have any more problems with it.

[JObscure]

Thanks for the help!

Now how to fit two jacks, three 24mm pots, switch, battery and board into a Hammond 1590B...