Tubescreamer clone problem...

[sengo]

Hey everyone,

I've been trying to debug this tube screamer for a while now, but I can't  figure out whats wrong. I used the General Guitar Gadgets PCB layout. When I turn the effect on a get a very high pitched whining noise, similiar to a dentist's drill, or those test pattern sounds you got when television channels use to shut off during the night. The only midifications I made were to use a 3M3 resistor and a .01 uF capacitor on what I think is part of the input buffer, R22 and C1 on the GGG layout. the sound stops when I test the voltage of pin2 on the IC, and I still get no guitar signal. Here are some volage readings...

Q1

E 7.47V
B 7.47V
C 8.16V

These sound like bad results.

IC1

1 4.43V
2 6.84V
3 4.26V
4 4.41V
5 4.41V
6 4.43V
7 4.48V
8 8.16V

Q2

E 7.46V
B 7.47V
C 8.15V

Those sound bad too. By the way, turning the pots varies the sound slightly, if that helps at all. I've checked my wiring numerous times, the LED lights when the effect is on, and I have resoldered everything on the board, as well as checking that everything is in the right place. Any answers you guys can provide would be really helpful.

Thanks,

Nick

[GibsonGM]

Seems to me that you have a part in backwards, or a solder bridge somewhere that shouldn't be.  Have you checked your pinouts?

Q1:
E  3.22
B  3.53
C  8.57
Is more normal (taken from mine, batt=8.57v)  So, one of the resistors there could be to blame...check the power supply first (Vref, the 2  10K resistors between + and - with the 47uF cap. Check the polarity of the cap, too! ).   You should get about 1/2 battery voltage at Vref.

Q2:
E  3.27
B  3.61
C  8.55
Again, start with what's up at Q1...if Vr is off, so will this transistor.

IC:
1  4.26     8  8.55 (battery)
2  4.28     7  4.22
3  4.25     6  4.25
4  0 (gnd)  5  4.23

Is your chip in backwards??   Either there's a mistake, or it is...pin 8 should be ~9v, pin 4 zero.  Again, if Vref is off or the chip's backwards, it would explain the odd values on the input/output pins, plus that oscillation.  The dot on the 4558 is on the end containing pins 1 and 8 (left and right, respectively). 

Check those, and let us know what happens!  Good luck.

~Mike

[lynessmy]

Check all ground connection....
Some solder bridge might have shorted across traces.
Pin4 voltage of the ic indicate that...start from there....

[sengo]

Ive gone through and still cant find any unwanted solder traces jumping from the ground to anywhere. Checked the direction of the caps, trannys, IC, and diodes and all of them match the GGG layout. Perhaps I should also check them with the actual schematic?

I took a voltage reading of the positive terminal of the 47uf cap, 4.5V... hey! I recognize that section from the "cook your own distortion article" I must be learning.

Anyway,

I re-tested the IC's pins 4 and 8 and got the correct voltages, don't  know what I did wrong last time. I'm getting 6.88V for pin 2, is this bad? Should it be closer to 4.5V like all the others?

Thanks for the help guys.

Nick

[lynessmy]

http://www.generalguitargadgets.com/pdf/ggg_its8_instruct.pdf

Download this file if you haven't do so.
The voltage are all there but i remember there is one wrong reading @ Collector of Q1. The right value should be your supply voltage. 
The layout has no problem.

Or make yourself handy with an Audio probe.
It solve most of my un-sounding pedal.

[GibsonGM]

+1 for the audio probe. It will help you isolate the area the problem is in.  Instructions are in the Debugging link up top of Build Your Own Stompbox page. 

The voltages I posted are from an actual working TS-808 that I built, and yours should be close (but not TOO close), Sengo.  Your pin 2 is high.   Sometimes solder bridges are VERY hard to spot, but you'd be surprised where they end up!!!   A magnifying glass helps, as does continuity tests with a DMM.   The audio probe though - no replacement for that tool!! 
You'll find it....

[sengo]

I like this audio probe, very cool... unfortunatly almost every componant on the board is making that horrible whining noise, some things are louder, like particular terminals on the pots. I can only hear the guitar input up to R1 on the shematic, so I thought it may be that, but it wasn't. I'm starting to think maybe the problem is something major towards output ?

Nick

[GibsonGM]

What I would do, sengo, is remove the stomp switch.  Hook jacks up to the input and output...tie the grounds together with a jumper wire with clips.  Just use a 9v battery to V+ and ground...no adapters as they can also cause problems.   This simplifies things.  If it doesn't oscillate then, you know it's in the switching.  I try every circuit this way before I add the switching, to assure myself that it works and also to isolate later problems to switching or squashed wiring in my enclosure.

Getting it 'stripped down' like that will make debugging easier.   Then listen with the probe again, this may change things.   If it still oscillates, I would change the opamp, and maybe unsolder and replace the 2 transistors (something may have been heat damaged during soldering).  Check all the resistor values {again}...setting the components for too high of a gain can cause oscillation.   All parts that should go to Vref should be checked, too.

Generally speaking, you're looking for something that is feeding back and causing this oscillation. So, signal is being sent into a device again, at too high of a level, I think.  Unfortunately it is difficult to find out why sometimes, but it's there somewhere! 
Keep trying, you will get it. 
Think of all the experience you are getting with debugging!  )

[GibsonGM]

Oh, and if you lift the 1k resistor (on my schem it's 1K, anyway) at the output of the first opamp section, pin 1 of the 4558, you can isolate the input buffer and 1st opamp section from the rest of the circuit.  Then test the parts in front...if it's not oscillating, they are ok.  If it does oscillate, the problem is earlier!   You should have amplified sound at pin 1, with some clipping....

[ambulancevoice]

record that dentist drill sound and send it to me!!!
please!!! i could use that for something

[sengo]

record that dentist drill sound and send it to me!!!
please!!! i could use that for something

I would, but unfortunately I have no means of recording it, sorry.

Think of all the experience you are getting with debugging!  Shocked)

Yeah! I'm really enjoying this actually, it's not like I need this pedal in a hurry, so I really don't mind debugging it. I removed the switching, and it still makes the noise, however, I removed that 1K resistor you were speaking of and pin one of the IC sounds fine!

Thanks for all your help by the way, I think I'm starting to get the hang of this stuff, hehe.

Nick

[GibsonGM]

OK, sengo...if pin 1, output of the 1st opamp of your 4558 sounds ok, then the trouble likely lies farther down the line!  We learned something, but there is more to do.  I suspect a problem with the 2nd opamp section but can't be sure...when you use the drive control, and listen to pin 1, you should get some distortion. If not, check that feedback loop w/the diodes...

I'd check the 10K resistor at the 2nd opamp input and be sure the voltage = Vr.  Check all solder connections around the 2nd opamp, and all points labeled Vr.  Be sure the values of the feedback resistors in 2nd opamp loop are correct and that they didn't get accidentally get exchanged - that could cause the oscillation!

There are 10 ways you could test this thing....apply a CD player signal at the 1st opamp input before the 1uF cap....listen to the output of the 2nd opamp section.  If it still oscillates, pull the 1k resistor to the output transistor and disconnect the input buffer transistor.  This would tell you if the problem is in the opamps or transistors.   If it still does it, suspect the feedback loop with diodes (is the 4.7k connected to Vr or ground?).

You could apply a signal to the input and listen to the 1st transistor after the 1uF cap to the 1st opamp.    Could apply signal to the recovery transistor before the .1uF cap and listen at the emitter.  Use your probe with the cap in it to protect your amp from DC.  The thing is to isolate the section you are checking, and apply a signal to it.  It can be a pain, but gets quicker and easier if you do it often enough! 
)

[sengo]

I haven't had time yet to setup a device to supply me with an input signal, but I did do the things in your post that I could easily do without it. I don't know if this is useful or not, but I relocated the 4K7 resistor so that instead of connecting to Vr it is now grounded as per the schematic, and all the humming stopped. I still got an output signal on pin one of the first opamp. When I connected the entire effect to the amp some peculiar things happened. I can hear the the guitar, but it's greatly muted, and very trebley. I can keep strumming for a while, but eventually something sort of gives way and I get a very loud car horn type sound. and when I rotate the drive knob I get bird chirping sounds that occur at regular intervals.

Not sure if any of this helps, I'll probably just put the resistor back to Vr as shown in the PCB layout, and continue running the other tests you proposed.

Nick   

[GibsonGM]

Hmm....the 4.7K resistor goes to Vr...that's where it's supposed to go.  Grounding it probably just reduces some feedback component or something, which is why you got some output. 

Have you seriously checked everything in that feedback loop, Sengo?  The caps, resistors and diodes? Those resistors set the gain...if they are reversed, they could be generating so much gain that you have that feedback....also check which way the wiper is tied...be sure it's tied to the output side of the opamp.
Sooner or later, you'll find the problem!

[sengo]

I've checked everything in the feedback loop for both halves of the opamp, everything seems fine.

Edit: Does removing the wire going to the second lug on the level pot isolate the output buffer from the rest of the effect? I removed it and listened to lug two through the audio probe and still got the oscillation, does this mean the op-amp, or something I missed in the feedback loop, is definatly the cause of the noise?

[GibsonGM]

Yes, nick, removing the wiper from the 100KA volume pot will isolate the final transistor stage, which you can then check.  That is the output of the opamp section.  Listening to pin 7 of the IC is the same thing.  I suggested disconnecting the 1st and 2nd transistor so they don't affect the IC operation.   If you do this and get oscillation, there's your problem.  Bad or wrong opamp can do this, bad part(s), etc etc etc. 

I am just very, very certain there's some kind of wiring/soldering error there...this is phsyics - if it's wired correctly and given the correct power supply, it WILL work!!!!  It has to.   So, there must be a problem with power, component values or the wrong part went in the wrong hole.   The only way to get to the bottom of it is to check EACH part (what a pain!) for value, orientation, and solder bridges.   

My gut says if you get a mildly amplified, clean tone out of opamp1, the problem is in the second stage...or an interaction between first & 2nd stage.  That's why I'm all about checking those parts so much. A REALLY common error is to cross legs of the opamp socket with solder by mistake...sometimes you need a magnifying glass to see it, or use a DMM/continuity tester. 

[jophus]

Did you ever get this sorted?

I have a similar problem, with the 'dentist drill' sound you describe- it seems to get worse when you turn the drive and level pots...
Incidently, I can't get hold of 1uf np's so had to rig up 2 polarised instead- not sure if this is working properly...

cheers

Joe