Tonebender mkii (2) clone: Just crackling noise instead of sound

[Error!]

Hello,
I am new to this forum and basically this entire DIY pedal thing, so if something of the following seems quite stupid to you that's the reason.

I had been thinking about building a guitar pedal myself for a long time and then at some point decided to start with a clone of a Tonebender MkII (2) as my first build.
Why a Tonebender? I have always liked to have one, but being a student I just cannot afford one of those pricy replicas out there; after a quick look at the schematic it didn't seem to be very complicated to build one and I ordered all the necessary components.
Putting it all together was pretty easy as well, there's just one problem:
I can't get it to work properly...

All I get out the effect is some crackling noise, if I hit the strings on my guitar extremely hard – way harder than you ever would when you're playing.
Turning the pots doesn't influence this noise just as the often recommended bias pot (20k), which can be set to literally any position without any effect.
I already found out – using google - that the transistors are the most crucial part of this circuit and that a relatively high leakage is more important than the HFE.

Here is some more information:
The schematic I used is the following:


In combination with this layout (just as a reference):


And these are the transistors I have ordered and measured:


Right now I'm totally frustrated that I can't get it to work, as the circuit looks quite simple and I checked my wiring literally a douzends time and just can't find the mistake...

I hope someone here on the forum has got any idea what I could try.

Any help would be highly appreciated!
Thanks in advance!

Error!

[duck_arse]

hello, error!

I think you are going to have problems with that schem and that layout, because the circuit shows grounded collectors, while the layout shows grounded emitters. it also shows one transistor needs legs bending. so, as we always like to say, please post some photos of what you have built, and we can pick it apart for you. maybe even help you put it back together.

[Error!]

Hello duck_arse,

Thanks for such a fast response!

You are right, I should have given two more details:
1st) The transistor symbol in the schematic shows a modern NPN transistor. I know that this is wrong, but frtizing (the software I used for the schematic) has no symbol for PNP transistors. So just imagine them being PNP type.

2nd) Here are some photos:
The board with pots in enclosure


And a kind of close-up of the veroboard


NOTE: As you can probably see there are no transistors inserted into the sockets right now.

Are the transistors I've got ok, or could they actually be the problem?


Again, thank you very much for your help!

Error!

[robthequiet]

Hello, welcome to the melee. Might we have a look at the solder side of the PCB, as well? Also, if possible, would you be able to measure the voltages of each leg of the transistors when the circuit is powered? It could be a biasing issue.

Just out of curiosity, which transistors did you select for the build, based on the Hfe and leakage?

[Error!]

Hello robthequiet,
Thanks for your help!

Here is a (not too good) photo of the solder side of the board:


I have tried multiple combinations of transistors, so there is not THE ONE configuration I want to use.
Do you see a problem regarding the transistors on the list posted above or can you suggest any combinations?

I assume you mean the voltages between each transistor leg and ground aka 0V, right?
Voltages will follow later on (probably within the next 2 hours)...


Error!

[Error!]

Here are the measurements:

Q1: #5 (leakage: 365µA, Hfe: 83)
Emitter: 9,20V
Base: 9,16V
Collector: 8,39V

Q2: #10 (leakage: 400µA, Hfe: 88)
Emitter: 9,19V
Base: 9,10V
Collector: 9,12V

Q3: #3 (leakage: 410µA, Hfe: 106)
Emitter: 8,39V
Base: 9,12V
Collector: 0,04V

NOTES: Battery: 9,25V, bias pot set to approximately 8k (like fixed resistor)

Just in case there is a problem regarding biasing: How can you bias this circuit? Add more trimmpots?


Error!

[induction]

The transistor symbol in the schematic shows a modern NPN transistor. I know that this is wrong, but frtizing (the software I used for the schematic) has no symbol for PNP transistors. So just imagine them being PNP type.

Actually the transistors in the schematic are PNP, and the polarity of the layout matches the schematic pretty well, with one exception: the +9V at the input jack ring is actually ground. There is no +9V in this circuit.

One problem I see is that the base of Q1 should be connected to ground by a 100k resistor. It's  correct in the schematic, but it's only 10k your build and in the layout.

[induction]

The next problem I see is that you seem to have left out the 470R between the negative rail and the junction of the trimmer and the output cap (10n).

Nevermind, I can see it from the back of the board. It must be hidden by the output cap.

[Error!]

One problem I see is that the base of Q1 should be connected to ground by a 100k resistor. It's  correct in the schematic, but it's only 10k your build and in the layout.

Originally there was a 100k resistor, which didn't work either. I replaced it with a 10k resistor, because the AC125s I'm using are listed as a replacement for OC75s and the OC75 version of the Tonebender MkII had a 10k resistor in that place - at least that is what I found on this site: http://www.diystompboxes.com/smfforum/index.php?topic=91000.0

Any other ideas?

UPDATE: Right now I'm not even getting the crackling noise I got before. As soon as I hit the bypass switch there is just silence.
Looks like I have built the most complicated mute switch of all time... 

[induction]

The solder connection for your input wire (second row from top on the layout, all the way on the left) looks a bit dodgy. Same for the 100k in the middle of the board.

[Error!]

I know, I used my multimeter in continuity mode to test them and all connections are fine.

Thanks anyway!

[Electric Warrior]

One problem I see is that the base of Q1 should be connected to ground by a 100k resistor. It's  correct in the schematic, but it's only 10k your build and in the layout.

No, the schematic is wrong. The MKII had 10k on Q1's base and 47k on Q2's collector (OC75 version) or 100k in both positions (OC81D version). Never a 100k and a 47k or a 10k and a 100k from what I've seen. The bigger resistors in the OC81D version make up for their lower leakage. Judging by the voltage readings, they biased them to the same voltages as OC75s.

There is at least one OC81D pedal out there that has the 10k/47k resistor setup. They must have had a couple of leaky OC81Ds, the voltages were not out of the ordinary.

Just in case there is a problem regarding biasing: How can you bias this circuit? Add more trimmpots?

Trim pots on Q1's base and Q2's collector. Q2's collector will bias both, Q2 and Q3. No need for a trim pot on Q3's collector. You probably won't need any trim pots though. You probably won't have a problem finding transistors that bias correctly.

Here are the measurements:

Q1: #5 (leakage: 365µA, Hfe: 83)
Emitter: 9,20V
Base: 9,16V
Collector: 8,39V

Q2: #10 (leakage: 400µA, Hfe: 88)
Emitter: 9,19V
Base: 9,10V
Collector: 9,12V

Q3: #3 (leakage: 410µA, Hfe: 106)
Emitter: 8,39V
Base: 9,12V
Collector: 0,04V

You have some kind of grounding problem or you're not measuring correctly.  Q1's and Q2's emitters must measure 0V.

Here are some voltages from my vintage unit, they might be useful:

Battery: 9.67V
Q1 C -9.02V B -0.03V E 0V
Q2 C -0.17V B -0.08V E 0V
Q3 C -8.44V B -0.17V E -0.11V

[smallbearelec]

I used my multimeter in continuity mode to test them and all connections are fine.

If the voltage measurements you took are correct, something is not connected properly; we just don't know what. On Q2, for one example, you can't have C, B, and E at nearly the same voltage--that can't work. What I would do is make a clean copy of the schem with the Emitter symbols where they belong to avoid confusion. Remove the stomp switch and power connections; we'll restore those later once the board is working.

Starting at the junction of C5, + side of C4 and the Tip contact of the input jack, use the continuity function of the multimeter to confirm that these points are all connected. Mark Off The Connection With A Highlighter. Proceed from left to right comparing the schem with the physical wiring and find your mistake(s). You may have to look for shorts as well. When you think you have the wiring correct, connect Tip and Sleeve of jacks correctly to input and output and connect a battery. If you don't get the effect, measure voltages again and post here.

it didn't seem to be very complicated to build one

Unfortunately, that's not the case; there are numerous landmines on the way. IMO, the big one that you hit was picking a three-stage circuit; it has too many parts to be good for a first build. In a perfect universe, someone would warn you before you take the leap, but Life is not like that. This Forum is where many people come to figure out what happened and how to recover. My own take on beginner problems is here:

http://diy.smallbearelec.com/HowTos/BeginnerArticle/BeginnerDos.htm

Let us know how it goes.

[Error!]

Thank you very much for all your suggestions so far!

I have tried a couple of things:

No, the schematic is wrong.

-> Changed it to yours (OC75 -> 10k + 47k; without the optional 100Ohm resistor):

Remove the stomp switch and power connections

-> done

use the continuity function of the multimeter to confirm that these points are all connected. Mark Off The Connection With A Highlighter. Proceed from left to right comparing the schem with the physical wiring

-> Also done; couldn't find any mistakes...

If you don't get the effect, measure voltages again and post here.

-> I do get this weird sound again, but here are some new voltages:
Q1: #5 (leakage: 365µA, Hfe: 83)
Emitter: 0V
Base: 0,02V
Collector: 0,8V

Q2: #10 (leakage: 400µA, Hfe: 88)
Emitter: 0V
Base: 0,1V
Collector: 0,08V

Q3: #3 (leakage: 410µA, Hfe: 106)
Emitter: 0,81V
Base: 0,08V
Collector: 9,15V

NOTES: Battery: 9,25V, bias pot set to approximately 8k (like fixed resistor)

Compared to

Battery: 9.67V
Q1 C -9.02V B -0.03V E 0V
Q2 C -0.17V B -0.08V E 0V
Q3 C -8.44V B -0.17V E -0.11V

they aren't too bad or at least not as way off as before - except for the collector of Q1.

So is this just a biasing issue?

Can I simply replace the 10k bias resistor of Q1 (R1) by a 20k trimmpot?


Again, thanks for you help!


Error!

[robthequiet]

Compared to Electric Warriors's measurements, I think there is a pretty big difference on Q1. I wonder if there may be a tiny solder bridge somewhere? What is the voltage drop across the 10K resistor on Q1 collector?

[Electric Warrior]

Right. This is not a bias problem.

[duck_arse]

either - you have the power supply backwards - or - you are not measuring the way we expect - or - you are missing the (-) symbol in your measures.

with the battery positive connected to the jax sleeves, we would have a positive ground circuit. this means that when you put your BLACK probe to the sleeve/ground/case connection, all measures then read negative. it is important you don't leave the sign out when reporting the voltages.

[Error!]

PROBLEM SOLVED! 
I'm so stupid . . . I simply had forgotten the cut under the 47k resistor... 
Now all voltages are fine.

Thank you so much for your help!
This forum is awesome!


Error!


P.S. Is there a way to power this effect with a (isolated) multi power supply?
I'm just curious, because I recently bought a Palmer PWT05MkII and I 'm kind of afraid of destroying it. The standard solution, using a voltage inverter IC seems logic, but doesn't it create a short circuit as the sleeves of the in- and output jacks are connected to ground, which still is positive, while it's negative in all other effects that are connected?

[duck_arse]

if the power supply connected to the positive ground is not also connected to any negative ground pedals, no problem (hah!). the charge pump goes inside the pedal, and provides a negative voltage, hanging below the ground. the outside world provides the pump with positive volts, and this pedal can be daisy chained with normal neg-gnd pedals.

generally, ground is ground is ground.

[Electric Warrior]

PROBLEM SOLVED! 
I'm so stupid . . . I simply had forgotten the cut under the 47k resistor... 
Now all voltages are fine.

Thank you so much for your help!
This forum is awesome!

Nice! Now it's time to swap transistors around until you find the perfect trio.