Tonebender Mklll problem

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Tonebender Mklll problem

Beitragvon polifemo » Do 05 Jan, 2012 17:25

Sorry for writing in English, but my German is quite rusty nowadays... *

A couple of days ago I built the Tonebender Mklll kit and unfortunately I´m experiencing some problems with it.

With the mini toggle switch in the position with the most low end (68nF cap) the pedal starts motorboating/oscillating with the drive past 4 o´clock.
It seems like the circuit can´t handle the low end somehow.
I reduced the 68nF cap to 22nF, but the problem remains.

Has anyone else experienced the same problems?

I don´t need to use the at full gain, but the oscillation indicates that something is wrong.

I´d also appreciate if someone that has built this pedal could check the bias voltage of the three transistors as I´m a bit suspicious of the reading that I get!

* Ich habe Deutch in die schule studiert, aber ich habe fast alles vergessen...
polifemo
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Beitragvon Uwe K. » Do 05 Jan, 2012 18:16

Hi Peter,
you are sure, that you have the toggle switch right wired??. If not the input hang up in the air.

regards Uwe
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Beitragvon polifemo » Do 05 Jan, 2012 18:58

Uwe K. hat geschrieben:Hi Peter,
you are sure, that you have the toggle switch right wired??. If not the input hang up in the air.

regards Uwe


From what I can tell, yes.

The wire from the middle lug of the switch goes to the inner hole of the pcb at the 6,8nF cap.
The wire from an outer lug of the switch goes to the outer hole of the pcb at the 68nF cap.

Bild

Correct, right?

The pedal works OK when I´m going thru the 6,8nF input cap, it´s only when the larger cap in beeing switched in that the motorboating occurs.
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Beitragvon polifemo » Fr 06 Jan, 2012 11:59

In a way I´ve located the problem but then another one occurred...

I had a 22k trimmer in place of the 18k resistor on the collector of Q3 in an attempt to get the bias right.
I was aiming for 4,5v (collector/Q3) but I was never able to get the bias any lower than about 6v.

Today I removed the trimmer and soldered the 18k resistor in and the motorboating did go away BUT Q3 now reads about 9 v (!) and the sound is a bit gated and I have lost volume/output...

Could anyone take readings of the 18k and 10 resistors in circuit?


Bild

I have a feeling that there´s something wrong in the bias circuit.
polifemo
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Beitragvon polifemo » Fr 06 Jan, 2012 15:14

Another question: Is the 10k resistor (marked in a red ) on the schematic the same as the "marked in red" resistor on the layout?

Bild
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Beitragvon Uwe K. » Sa 07 Jan, 2012 04:08

Hi,
I have the restor marked.
Bild

The toggle switch put only the 68nF parallel to the 6,8nF cap. the original Bender had a input cap of 0.1µF = 100nF.
And have you right the transisors soldered (E-B-C)?
For exp. you can desoldered the wires from the T-switch and soldering the wire from the input (switch) directly to the 68nF cap.
Another way, you can soldering a bridge in the two holes by the caps. Then is the input cap approx 75nF. What makes the circuit then?
If you have no sucess, i will build a MK3 to measure the voltage for you.

regards Uwe
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Beitragvon polifemo » Sa 07 Jan, 2012 08:56

Hi,

I´ve been going over the soldering/component placement for hours trying to get this pedal right, but I can´t find any mistakes from my side.

With 18k on Q3´s collector the motorboating goes away BUT the sound is now "the sound of a misbiased fuzz" (sputtery, grainy, harsh...) and the collector of Q3 now reads almost 9v!!!???...

Unfortunately there are no markings of the 10k transistor that I asked for on the layout/schematic in your reply, but I did tweak the value of the 10k resistor that I marked in my previous post, and I found that by lowering it´s value the bias of Q3 goes down. The pedal now sounds much better BUT the motorboating (from 4 o'clock on the drive) comes back...

Every attempt at trying to bias Q3 by changing values on resistors seem to put the circuit of balance, creating this motorboating and hum...

Someone, on another forum, suggested that it´s the diode that biases Q3, and maybe it´s the diode that is bad?
Or maybe I got the wrong one?
Is it the 1N60 GE that is doing the bias of Q3?

In this pic the 1N60 GE diode is quite large

Bild

The one that came with my kit was as small as the 1N4148 in the lower right hand corner.

I´m really frustrated by all this.
I´m quite experienced at soldering both pedals and amps - so I´m not a newbie - but right now I can´t get this pedal to work and sound right.

I must chose between a pedal that does not hum or go into motorboating but that doesn´t sound very good or a pedal that sounds great but has a lot of hum and where I can´t use the full range of the drive knob.
polifemo
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Beitragvon Uwe K. » Sa 07 Jan, 2012 09:15

hi Peter,
I have ypu send a mail with voltage measurements. The one Diode is a 1N60 the other for the Bypass switching 1N4148. They shown equal.
9V on the collector is devinitively to high. I measured with the 18K resistor 6.35V.
In the pic was a 1N34. if you have changed the 2 diodes, the base of Q3 is 0.3V higher. GE = 0.3V Si= 0.6V

regards Uwe
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Beitragvon polifemo » Sa 07 Jan, 2012 09:24

Thanks Uwe,

Here are my voltage readings:

Q1
E= 7.51V
B= 7.39V
C= 5.60V

Q2
E= 7.62V
B= 7.51V
C= 5.62V

Q3
E= 8.99V
B= 8.89V
C= 8.76V

They seem OK until we reach Q3´s collector.

What can be the problem...

Is the 1N60 diode any part of the voltage on Q3?
Seems like I´ve checked everything else now.
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Beitragvon polifemo » Sa 07 Jan, 2012 09:49

I´m starting to get tired of this pedal :x

Two more things:

- Can the problem be that the transistor for Q3 that came with this kit had a really high leakage (0,39mA)?
I´ve been told that the transistor in Q3 in this circuit should have a leakage of less than 0,10mA.

- I´m checking the voltage on the 1N60 diode and on one side I get 9v and on the other 8.9v.

Is that normal?
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Beitragvon polifemo » Sa 07 Jan, 2012 13:57

polifemo hat geschrieben:- I´m checking the voltage on the 1N60 diode and on one side I get 9v and on the other 8.9v.

Is that normal?


I feel that this is the cause to all my problems: The diode must be faulty...

There should be a 0,3v voltage drop instead the 0,1v that I get.

Are there other diodes that can be used instead of the 1N60 if I can´t get hold of another one?
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Beitragvon polifemo » Sa 07 Jan, 2012 17:26

I´ve been contacted by Uwe, who has offered to help me out in the most generous way, and now it seems that everything will be ok 8)

UK-Electronic is highly recommended!
polifemo
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Beitragvon polifemo » Mo 23 Jan, 2012 10:38

So, thanks to Uwe my Mk-lll is now working perfectly :-D

It´s one hell of a pedal that does everything from great overdriven tones to fuzz heaven 8)
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Re: Tonebender Mklll problem

Beitragvon polifemo » So 24 Nov, 2013 16:30

Haven't been using this pedal much as I found the mid content to be a bit overwhelming...

Today I decided to try 2.2nF - which was the original Tonebender III value - in the tone control section instead of the 15nF in this version, and I like it much better.
The pedal will probably get more use now :-)
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