Interesting Results with Tube Screamer TS-808 build

[studiostud]

Hello,

I'm having some "interesting" results with the "what I thought was complete" tube screamer build from tonepad.com.  I purchase the PCB board from tonepad and everything else from Smallbear Elec.  First off, technically, the pedal works.  The bypass works fine.  I'm getting correct voltage at the output jack.  What is odd is that the output signal, even with all 3 pots turned full is pretty weak.  Also, the level pot does nothing to the signal, the tone pot seems to raise the gain, although there's no real distortion, and the gain pot seem to raise the level, although the gain and the tone pot could be role reversed.  All the solder joints look clean and none of the offboard wiring looks out of place.  Are there any ideas of things I can try?

[jlullo]

hey!

check out the debugging thread
here:
http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

post your results, and then we can start narrowing down the problem!

jonathan

[Dave Z]

"gain and the tone pot could be role reversed" yeah could well be, and since the tone is usually a much smaller resistance, you would not get much gain out of it even when maxxed out. Should be easy enough to trace that!

[studiostud]

I of course posted my problems BEFORE I read the dubugging page...  sorry bout that.

Here's the info I gathered for starters...

Name of Circuit: Tube Screamer (TS-808 version, not TS-9)

Sources:
http://www.tonepad.com/getFile.asp?id=81  (Project Diagram)
http://www.tonepad.com/getFile.asp?id=76  (Offboard Wiring Diagram)


What does it do?
* The LED on/off indicator does not light
* The true bypass works
* The pedal does work, technically.  The output signal is very low, even when all pots are turned to full.  The gain pot seems to do nothing.  The level pot and the tone pot both control a degree of the output volume.  Even when all pots turned to full, the output signal does not sound distorted.

Modifications to the circuit?  Yes
I made (3) small modifications from the original schematic: 
1. I swapped the 2PDT stomp switch with a 3PDT and wired it for true bypass using diagram #5 from the Offboard Wiring Diagram file.  The bypass works fine.
2. I installed a DPDT toggle switch connected to the clipping diodes.  D1 & D2 are both set normal.  On D3, the non-cathode side slot has a jumper wire going to the center pin of the DPDT switch.  One side of the switch has another jumper wire and the other has a 3rd diode with the non-cathode side connected to the switch.  These two are wired in parallel and the single wire then goes to the cathode side of the PCB board diagram.  The idea was that the switch would toggle between symmetrical and assymetrical clipping.
3. I installed a 2nd DPDT toggle switch connected to the .047uF cap position on the PCB diagram.  One side is connected to the center pin on the switch.  One side of the switch has a .1K poly film cap and the other side has a 1uF electrolytic cap.  These two caps are wired in parallel and the single wire then goes to the other side of the cap position on the PCB diagram.  The idea here was to create a toggle switch that allows for switching between poly and electro caps for different tones. 

Parts substitutions:
* the (2) 510K resistors were each replaced with a 470K and a 39K in series
* the 51K resistor replaced with 47K
* the (3) clipping diodes and the two TS-808 specific resistors are utilized within sockets

Positive/Negative Ground:  negative ground circuit

Out of circuit battery voltage: 8.97V

With negative lead from dig multimeter on Ring tab of Input Jack (where black lead from battery connects) -
* Voltage at circuit board end of red battery lead:  8.97V
* Voltage at circuit board end of black battery lead: 8.80V

Transistor voltages:
Q1 - 2N3904
C = 8.97
B = 5.95
E = 8.72

Q2 - 2N3904
C = 8.97
B = 5.95
E = 8.72

IC1 - 4558D
1 = 8.79
2 = 8.79
3 = 8.80
4 = 8.32
5 = 8.80
6 = 8.62
7 = 8.61
8 = 8.97

Diode Voltages: all diodes are 1N914
D1
A = 8.50
K = 8.79

D2
A = 8.79
K = 8.50

D3 (L = diode side of switch, R = jumper side)
A = 8.43
KL = 8.49
KR = 8.43

So far what I've noticed for collecting all the voltages is that I read that on 3-pin NPN transistors like the ones I'm using, the base voltage should be more positive than the emitter voltage and on both of mine, they are not.

[studiostud]

I found out some other things too...

THINGS I FOUND ODD

* on cap DPDT toggle switch. 
When on electro side: electro voltage   + = 8.42   - = 8.84
When on poly side: electro voltage   + = 4.51  - = 8.85

* 10uF has 8.72V on both sides

* on diode DPDT toggle
When on jumper side - all (3) pins measure 8.43V
When on diode side - jumper = 8.5   center = 8.43    diode - Non-Cathode = .01   Cathode = 8.5

* when I have the negative side of the multimeter clipped to the ring of the input jack where the negative lead from the battery connects, and I touch the sleeve tab where the ground wires are, I get a positive voltage 8.80V.  This is turn gives me the same 8.80V positive on both the sleeve of the output jack and the negative tab of the 9V power jack.  Just a note: I have not used a 9V wallwart yet.  Only battery.


OTHER THINGS I FOUND WITH FURTHER INSPECTION

* I diagnosed the problem with the LED indicator.  It is wired incorrectly with the resistor on the positive side between the LED and the stomp switch.  Could that be the whole problem or would that only affect the LED?

[johngreene]

Looks like you are making all your measurements with nothing plugged into the input jack. Therefore you have no connection between battery ground and the circuit ground.

--john

[GibsonGM]

What does your voltage bias network read??  Those 8's should be closer to 4's at the IC, I think...here are my 4558 voltages from an 808 (slightly low battery tho):
1 4.26    8 8.55
2 4.28    7 4.22
3 4.25    6 4.25
4 0         5 4.23

and transistors:
Q1
E 3.22
B 3.53
C 8.57
Q2
E  3.27
B  3.61
C 8.55

You could have a short to ground somewhere, something supposed to go to Vref going to V+ instead, shorts between Q or IC pins...or other issues like that...if you used a PCB, the problem shouldn't be too hard to spot ;o)

[studiostud]

Thanks John.  That would be correct.  My bad.  I now have a a tone generator plugged into the input jack and the transistor voltages are closer to what Gibson was listing.

1 - 2N3904
C = 8.93
B = 3.58
E = 4.36

Q2 - 2N3904
C = 8.93
B = 3.40
E = 4.08


IC seems a bit high though still and problem persists...

IC1 - 4558D
1 = 8.23
2 = 7.43
3 = 7.43
4 = 7.83
5 = 8.04
6 = 8.31
7 = 8.33
8 = 8.92

I inspected every solder point on the PCB and everything looks clean without any false connections.  btw, I'm using one of the pre-made PCBs from Tonepad.

[studiostud]

Any new thoughts on my IC values?  They seem pretty high and I don't have a pin with a 0 voltage.  I have checked the polarity of all the polarized caps and the transistor values are pretty close now.  Any thoughts?

Jeff

[MikeH]

Yeah, your IC is not grounded.  Check for solder bridges.

[johngreene]

The base voltage on the transistor should not be lower than the emitter. Either you mixed those two pins up when you measured them or something else is wrong. Pin 4 of the IC should be directly connected to ground. Bad solder joint maybe?

--john

[studiostud]

Alright.  I did find a cold solder joint where the PCB ground wire connects.  I'll fix that up and see what happens.  that could very well fix a lot of the problems that I may have not even found yet... 

[studiostud]

Well my Tube Screamer was working great for about two weeks and then just died.  I know it's not a dead battery cause it acts the same way with a 9V power supply that I know works fine.  I thought I had killed the cap that is positioned just before the output wire on the PCB so I swapped that with a new one but no luck.  I re-checked my voltages in the trannys and IC and the trannies are fine but the 4558 seems a little high on most of the pins...

1  8.14
2  7.85
3  5.46
4  0
5  7.89
6  8.25
7  8.22
8  8.85

Any ideas of what could cause the change in these values?  TIA for any help.

Jeff

[R.G.]

Did you do any mods or changes to the thing before it died, or did it just die?

What led you to suspect that you had killed the cap before the output wire?

[studiostud]

I didn't do any mods before death.  It was working fine one day and I took it out the next day to show it to a buddy and nothing.  It was a sad day.  I originally suspected the output cap because I was measuring about 3-4V on the positive side and 0V on the negative side which in turn put 0V on everything after it.  But the more I thought about it, the more I thought that that is just the behavior of a cap that doesn't have power to store up.  Does that logic sound plausible? 

[studiostud]

I've swapped out some more components just for kicks and still no luck.  since the voltages on the IC are off, this leads me to believe it's something on the PCB and not the offboard wiring.  any other ideas?

[b00kworm]

I think this may help:
1. pull 4558 chip out of socet
2.connect a battery to board (without a rc4558)
3. read votlage on 47u cap  its always close to 1/2 of supply votlage (ex 4.5V if supply is 9)
4. get voltage of  pin 3 (4558 empty socket)
5. get voltage of pin 5 (4558 empty socket)

all they should be a 1/2 VCC.

if its not try to fix a 10k 10k 47u circuit a voltage divider

Lucas

[studiostud]

Well, it looks like I got it figured out.  Turns out I fried the 4558 cause after I put in a new one, the voltages were correct and it played no problem.  The LED that wasn't working before it even working now.  Thanks for all the help.

[Barcode80]

most likely you at some point grounded the power supply accidentally or accidentally reversed polarity of the ps somehow. does nasty things to an IC

[jlullo]

most likely you at some point grounded the power supply accidentally or accidentally reversed polarity of the ps somehow. does nasty things to an IC

yes... and diodes and trannys as well