TS808 first attempt

[lazerphea]

Hello all!
I'm trying to buind the legendary TubeScreamer; I'm making some advances but the stuff isn't still working.
So, following the guidelines:

1.What does it do, not do, and sound like?

It doesn't distort; the signal passes throught the circuit, volume pot (R21) works correctly, tone pot (R20) too: it affects the low frequencies blending. The Drive pot (R19) doesn't affect the sound.
I get a nice, full clean tone with maybe a little bit of distortion hardly noticeable.
It seems clear to me that the problem is in the driving stage... isn't it?

2.Name of the circuit:

Ibanez Tube Screamer 808

3.Source of the circuit (URL of schematic or project):

http://www.generalguitargadgets.com/pdf/ggg_its8_sc.pdf

4.Any modifications to the circuit?
5.Any parts substitutions?

Yes: I substituted R22 (1.5M) with two resistor in series, Ra=1M and Rb = 510K; R12 (510K) has been substituted by 2x1M resistor in parallel, in series with a 10K resistor.
- C4 has been substituted with the parallel combination of C4a = 47pG and C4b=3.9pF.
- I ignored Sw1a and Sw1b, directly connecting input's tip to the parallel combination of C1 and R22, and output's tip to the parallel combination of C11 and R13.
- The led part hasn't been taken into consideration, too.
- 4558D substituted with a 4558C.
- D1 and D2 are germanium diodes.

6.Positive ground to negative ground conversion?

I don't know what is this, so I assume I didn't do this

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?

V+ = 8.97V
Vr = 4.32V

8. Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn.

Q1:
E = 3.29V
B = 3.74V
C = 8.98V

Q2:
E = 3.26V
B = 3.72V
C = 8.98V

JRC4558C:
1 = 4.44V
2 = 4.44V
3 = 4.44V
4 = 0.0V
5 = 4.44V
6 = 4.44V
7 = 4.44V
8 = 8.91V

D1:
A = 4.44V
K = 4.44V

D2:
A = 4.44V
K = 4.44V

That's it! I'll try now to rewire the driving stage...

[ayayay!]

First of all, that's an excellent post.  Very clear and good measurements.  Take comfort in knowing your troubleshooting skills are already very good. 

It seems to me your wiring on the drive pot is off somewhere.  How is it wired?  If you're looking at the pot's backside, with the lugs pointing downward, you should have the input on the left lug and the output wired to the center & outside lugs. 

Do you have the 51k in at R7, and is it getting a signal? 

[Zben3129]

If your drive pot is shorted out (or any direct short in that feedback loop for that matter) you will get clean boost. Test the resistance between pins 1 and 2 on the opamp, if it is 47k-ish no matter where the drive pot is set, your pot is shorted. If it is 0 ohms no matter what, then the whole loop is shorted. The voltages are good so the opamp is functioning correctly.

Zach

[George Giblet]

Other possibilities:

I'm thinking there's break in the components around C3 and R6.  Perhaps R6 isn't connected back to Vr?

Another option is C4 is shorted.

[ayayay!]

I'm thinking there's break in the components around C3 and R6.  Perhaps R6 isn't connected back to Vr?

I was just gonna say that.  It seems Vr has connections all over the place so double check all of them. 

[lazerphea]

First of all, that's an excellent post.  Very clear and good measurements.  Take comfort in knowing your troubleshooting skills are already very good. 

Thanks man, I tried to be as clear as possible!

It seems to me your wiring on the drive pot is off somewhere.  How is it wired?  If you're looking at the pot's backside, with the lugs pointing downward, you should have the input on the left lug and the output wired to the center & outside lugs. 

Do you have the 51k in at R7, and is it getting a signal? 

Umh... I read 4.42V on both of R7's pins....
Now I'm almost sure I wired the pot the wrong way: looking at the pot backside, lugs pointing downward, I connected the left wire of the pot to R7, and the central wire to D1, while I left the  right wire unwired... do I have to solder it to the central one??

[lazerphea]

Oohhh... three new posts! I'll now try everything you suggested!

[Bensnap]

In the schematic you have it looks like the third lug on the gain pot looks like its not connected to any thing. Its should be connected to the middle lug.

[lazerphea]

IN the schematic you have it looks like the third lug on the pot is not connected to any thing. Its should be connected to the middle lug.

Sorry: is the third lug the one connected to the right pin of  the pot (looking the pot backward, lugs pointing down)?

[lazerphea]

If your drive pot is shorted out (or any direct short in that feedback loop for that matter) you will get clean boost. Test the resistance between pins 1 and 2 on the opamp, if it is 47k-ish no matter where the drive pot is set, your pot is shorted. If it is 0 ohms no matter what, then the whole loop is shorted. The voltages are good so the opamp is functioning correctly.

Zach

I put the black lead of my meter on pin 1 and the red one on pin 2 of the IC; what I read is:
3.25 Ohm if the 500K pot is in 500K position
2.99 Ohm if the 500K pot is in 0K position

[lazerphea]

Other possibilities:

I'm thinking there's break in the components around C3 and R6.  Perhaps R6 isn't connected back to Vr?

Another option is C4 is shorted.

Alright: R6 goes from C4's pin to Vr directly, so I think this is ok.
How can I say if C4 is shorted? Remember that for me, C4 is C4a=47pF parallel to C4b =3.9pF.

[Bensnap]

Sorry: is the third lug the one connected to the right pin of  the pot (looking the pot backward, lugs pointing down)?

Right. The first lug is on the left connected to R7. Then you want to connect the middle and third lug on the right together. Then out form there.

[Zben3129]

If your drive pot is shorted out (or any direct short in that feedback loop for that matter) you will get clean boost. Test the resistance between pins 1 and 2 on the opamp, if it is 47k-ish no matter where the drive pot is set, your pot is shorted. If it is 0 ohms no matter what, then the whole loop is shorted. The voltages are good so the opamp is functioning correctly.

Zach

I put the black lead of my meter on pin 1 and the red one on pin 2 of the IC; what I read is:
3.25 Ohm if the 500K pot is in 500K position
2.99 Ohm if the 500K pot is in 0K position

Oops, I forgot about something  . The diodes will make the resistance read open across the feedback loop, so the only way to measure the fb loop would be to disconnect the diodes. You may or may not want to do this, if you don't then just triple check the pot and 51k resistor are ok.

Zach

[Bensnap]

Right. The first lug is on the left connected to R7. Then you want to connect the middle and third lug on the right together. Then out form there.

Like this.

[lazerphea]

Ok, I soldered the central wire of the pot to the right one, and took the output from there, but nothing happens... If i remove the pot from the circuit I get exactly the same result.
I'll now try to make a schema of my breadboard layout and then I'll post it!

[Bensnap]

I have a tube screamer bread boarded at home but it will be three hours till i get home. if you haven't worked it out by then i can take a closer look at mine. I had the same problem last night with my gain stage.

[lazerphea]

Right. The first lug is on the left connected to R7. Then you want to connect the middle and third lug on the right together. Then out form there.

Like this.

Yeah, I did exactly this wiring! But no changes...

[lazerphea]

I have a tube screamer bread boarded at home but it will be three hours till i get home. if you haven't worked it out by then i can take a closer look at mine. I had the same problem last night with my gain stage.

You're very kind, man! I'll try and try again in the meantime

[ayayay!]

Hmm okay so we're getting somewhere.  I don't know where but we're getting there...

Are the diodes facing in opposite directions as in the schem? 

[Bensnap]

You are Bread Boarding this right?