Tonebender Mk-lll problems

[polifemo]

A while ago I bought a Tonebender Mk-lll kit and today I built it.

I does make sound but I can't bias it even though I turn the 22k trimmer (instead of the 18k resistor) down to "min"... (with the 18k resistor the bias is about 6,6V )

I get these readings:

- Q1 

Collector = 5,73V
Base = 7,37V
Emitter = 7,48V

- Q2

Collector = 5,74V
Base = 7,48V
Emitter = 7,68V

- Q3

Collector = 5,72V
Base = 8,88V
Emitter = 7,99V

I'm new to this, but something is really wrong, right?



The schematic is this:



PS It does oscillate at the highest gain settings with the mine toggle switch in the "fat mode" Ds

[LucifersTrip]

how good is the sound you have at 6.6v? do you have correct voltages of a working one that you are shooting for?
I don't know what the correct voltages are for this one, but some of the Tonebenders Q3 collectors hit 7-8v+


Edit:
I did some quick searching and whether it's correct or not,  it seems most are shooting for 4 - 4.5v on Q3's collector.
here's one I found:
Q1: C= 3.611, B=1.55, E=1.452
Q2: C= 3.611, B=1.453, E=1.329
Q3: C= 4.29, B=50mv, E=0

so, you may consider troubleshooting the first stage with the darlingtons (Q1/2) before you work on Q3

[azrael]

any idea how much leakage you have for Q3?

[polifemo]

any idea how much leakage you have for Q3?

0,39mA (HFE 156)

[polifemo]

how good is the sound you have at 6.6v? do you have correct voltages of a working one that you are shooting for?
I don't know what the correct voltages are for this one, but some of the Tonebenders Q3 collectors hit 7-8v+


Edit:
I did some quick searching and whether it's correct or not,  it seems most are shooting for 4 - 4.5v on Q3's collector.
here's one I found:
Q1: C= 3.611, B=1.55, E=1.452
Q2: C= 3.611, B=1.453, E=1.329
Q3: C= 4.29, B=50mv, E=0

so, you may consider troubleshooting the first stage with the darlingtons (Q1/2) before you work on Q3

I´ll continue tomorrow.
The weird thing is that I get such strange readings...

[mac]

0.39ma*18K=7v!!!
q3 is too leaky, you need a low leakage germ here, less that 0.1ma... or a very leaky diode to shunt some of the germ leakage to gnd.
do a search about the tonebender, i wrote a lot about biasing q3.

q1,2 should bias a little below 4.5v as noted before.

mac

[polifemo]

0.39ma*18K=7v!!!
q3 is too leaky, you need a low leakage germ here, less that 0.1ma... or a very leaky diode to shunt some of the germ leakage to gnd.
do a search about the tonebender, i wrote a lot about biasing q3.

q1,2 should bias a little below 4.5v as noted before.

mac

OK.

I've got some trannies coming in next week, so let's hope that I'm lucky enough to find a good one!

The pedal itself actually sounds really good!?...
I'm comparing it to a Skinpimp Mklll right now, and my build sounds really close but due to the high bias it has a bit less "Fuzz character" and more of a distortion tone. But actually a really good one :-)

I suppose that I'll leave it as it is for now, but as I'm learning about fuzz design I'd love to get it to bias properly.

[petemoore]

- Q1 

Collector = 5,73V
Base = 7,37V
Emitter = 7,48V

- Q2

Collector = 5,74V
Base = 7,48V
Emitter = 7,68V
  Is there a trace [that shouldn't be there] which causes the B/E of Q2 to be = bias voltage ?
- Q3

Collector = 5,72V
Base = 8,88V
Emitter = 7,99V
   This is a PNP neg gnd. schematic, so there are added complications compared to NPN/Neg gnd. or PNP/Pos gnd.
  If it works that's one thing, if it doesn't work...a specialist in PNP/neg. gnd. power-inversion circuits may be able to better assist you...quite a few good experiences [including my own] have been had by using one of the above italicized versions, ie converting to a known to always work circuit.
  I'm turned around with the PNP/neg. gnd., and the transistor voltages seem 'backwards' to read, generally the emitter is closest' to ground [whether this be + or - is inconsequential to the notation] and the base is a larger bias voltage number, then the collector [of a gain stage] is somewhere above both...around the 'middle' of the supply [around 1/2v or so].

[mac]

Ideal leakage for q3 is 50-100ma. This values will put the collector at near 3.5v-2.5v if the diode is not leaky, and will not be so sensible to temperature changes... unless under direct sunlight
If the diode is leaky it will act as a resistor sending leakage to gnd and making the collector go up.
If you want to know how much leaky is the diode in reverse way, put a series 1k resistor with it and check the current through the 1k.

q1,2 leakage should be also low, no more than 100ma to put both collectors at 4v or so. But this bias network has a resistor draining leakage to gnd, the 10k going from q1 emiter to gnd, which makes the pseudo-darlintong stable.

mac

[polifemo]

The pedal is now neatly wired using shielded cable for input/output of the pedal and in/out, to/from the circuit board but the motorboating remains from 4 o'clock and up on the drive knob.
I can live with this as there's plenty of gain at tap within the usable drive knob range and when having the toggle switch in the 6,8nF-position I can use max gain.

The pedal sounds awesome, but as I'm a tonefreak/tweaker nerd I'm a bit disturbed by the fact that I can't control the bias with my trimmer knob (by the way, would a trimmer on Q1´s 47k have been a better idea instead of my trimmer instead of Q3´s 18k?) and that this motorboating won't go away  :x

But as I said, the pedal kicks ass 

[polifemo]

Hi,

Thank´s for your help and suggestions  

The following has made me a bit puzzled:

0.39ma*18K=7v!!!
q3 is too leaky, you need a low leakage germ here, less that 0.1ma...
mac

Ideal leakage for q3 is 50-100ma.
mac

Do I need a "low leakage germ here, less that 0.1ma" here or is "the Ideal leakage for q3 50-100ma" ?

Sorry but I´m new to GE transistors.

[mac]

Do I need a "low leakage germ here, less that 0.1ma" here or is "the Ideal leakage for q3 50-100ma" ?

myst

Sorry I mean between 50-100 ua, that is 0.05-0.1ma.  

mac

[polifemo]

OK, thanks!

I did try a low leakage transistor but not much did change...

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!!!???...

I tried to tweak the value of the 2:nd 10k resistor on the negative power supply, and I found that if 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 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?

I'm really frustrated by all this.
I'm quite experienced at soldering both pedals and amps - so I'm not a total 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...


PS Here are my voltage readings (no mods to the circuit)

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

Q3 collector IS strange...

[LucifersTrip]

I was going to ask if you tried a battery for supply, but the voltages are too far off as noted before.

I'd still guess you're either not using transistors that work (do you have the gains & leakages of all 3 transistors?) in this circuit or there is a miswiring.

According to the voltages I found (above) and what mac noted, Q1 & Q2 C should be < 4.5v.
In addition, E & B for Q1/2 in my example are ~ 1.5v

Can someone confirm that 1.5v is close to what they should be?

[polifemo]

Q1: HFE 92/leakage 0,15mA
Q2: HFE 63/leakage 0,10mA
Q3: HFE 156/ leakage 0,39mA

[LucifersTrip]

it would seem Q3 leaks a bit too much and since you have a reasonable Q1/2  and your E/B voltages are way off (7.5 instead of 1.5), I'd check wiring/shorts in that area first

[polifemo]

The voltage readings of this particular circuit seem to be a bit different compared to regular a PNP/positive ground Tone Bender Mklll.

Hera are the values of another unit made (not by me)  from the same kit:

Q1
E= 7.72V
B= 7.61V
C= 6.42V

Q2
E= 7.82V
B= 7.71V
C= 6.42V

Q3
E= 9.38V
B= 9.28V
C= 6.35V

Quite like mine except for Q3´s collector.

[mac]

What's your battery voltage?
Since this is a PNP positive gnd, your readings are taken placing the red probe on the positive side, gnd, and the black one on the collector, right?

mac

[polifemo]

I´ve got 9,1v from the battery and regarding where to place the red/black probe it doesn´t really matter as the difference is a "minus reading" in one way and a "plus reading" on the other.

Either:

"8,76v"

or

"- 8,76v"

[LucifersTrip]

Since this is a PNP positive gnd

according to the schematic, it's actually PNP negative gnd

========

@polifemo

can you link to the thread where you got the voltages from....maybe a discussion where I can read about this version.


Edit:  just out of curiosity, did you try to wire it as pnp pos gnd as the original, without that extra circuitry (bottom left on schematic?) to make it neg gnd ?
http://fuzzcentral.ssguitar.com/3knob.php