Tonebender mkII - troubleshooting

[crookedtodd]

Hello Everyone!
This is my first post, but i have spent many hours nosing around the pages here. Very enjoyable and informative site!

Anyway, My skills and knowledge are novice...at best. I have attempted my very first build. I decided to build a Tonebender mkII circuit. I have done this project on perfboard. The schematic and info I used can be found at http://fuzzcentral.ssguitar.com/mkII.php

To my surprise/luck, the pedal DID work on the first attempt. It does make a nice sounding fuzz, but i am having a few minor issues.
1) the "Attack" control seems to have no effect. The pedal seems to have all out fuzz at all times.
2) the signal is "gated". When picking normally the effect works fine. If i pick lightly with fingers or back off the guitar volume, I lose signal. It does make sound at this point, but it "farts" out and gates off quickly.

a few other notes:
- I substituted OC75 transistors for the OC81D transistors which the original schematic called for.
- I subbed a 1k resistor for the 470ohm resistor called for on the schematic.
- I added a 1M resistor to the input to prevent the switch from "popping"
- I added a 1N4001 diode to protect against accidental reverse polarity
- the pedal is battery powered ONLY. No ground conversion has been done to use the unit with a power supply.
- the following measurements were taken from the transformers (bare with me if this doesn't make sense, I'm here to learn)
  Q1   C=-.106 B=-.058 E=0
  Q2   C=-.029 B=-.022 E=0
  Q3   C=-.667 B=-.024 E=-.009

Any assistance would be greatly appreciated!
Thank You

[LucifersTrip]

Hello Everyone!
This is my first post, but i have spent many hours nosing around the pages here. Very enjoyable and informative site!

welcome

Anyway, My skills and knowledge are novice...at best. I have attempted my very first build. I decided to build a Tonebender mkII circuit. I have done this project on perfboard. The schematic and info I used can be found at http://fuzzcentral.ssguitar.com/mkII.php

This one?  
http://fuzzcentral.ssguitar.com/mkII/voxmkIIschematic.gif
You can embed the img by simply putting img and /img (each in brackets) before and after the url, so it can be viewed quicker

To my surprise/luck, the pedal DID work on the first attempt. It does make a nice sounding fuzz, but i am having a few minor issues.
1) the "Attack" control seems to have no effect. The pedal seems to have all out fuzz at all times.
2) the signal is "gated". When picking normally the effect works fine. If i pick lightly with fingers or back off the guitar volume, I lose signal. It does make sound at this point, but it "farts" out and gates off quickly.

a few other notes:
- I substituted OC75 transistors for the OC81D transistors which the original schematic called for.
- I subbed a 1k resistor for the 470ohm resistor called for on the schematic.

Unless you wanted 1K (it will give more volume), you can get 470 with two 1K's in parallel

- the following measurements were taken from the transformers (bare with me if this doesn't make sense, I'm here to learn)
 Q1   C=-.106 B=-.058 E=0
 Q2   C=-.029 B=-.022 E=0
 Q3   C=-.667 B=-.024 E=-.009

transformers...transistors

Q3C should be close to 4.5v, so no use worrying about 1) and 2) above until you have that sorted. I would sub a 20K pot for the 8K2 and see if you can get closer, but I believe you will have to go lower than 8K2. If you can't get close to 4.5v, then I'd consider a different set/combination of transistors...but definitely triple-check the wiring of the attack pot, anyway.

Also, I believe the voltages on Q1C and Q2C are too low. Wait for someone else to post the correct voltages...

good luck

[crookedtodd]

Thanks for the response!
This is the schematic i started with -

Unless you wanted 1K (it will give more volume), you can get 470 with two 1K's in parallel

I went with the 1K to achieve more volume. I did try a 470 at first, but it seemed a little weak in volume.

Quote
- the following measurements were taken from the transformers (bare with me if this doesn't make sense, I'm here to learn)
  Q1   C=-.106 B=-.058 E=0
  Q2   C=-.029 B=-.022 E=0
  Q3   C=-.667 B=-.024 E=-.009

transformers...transistors

Q3C should be close to 4.5v, so no use worrying about 1) and 2) above until you have that sorted. I would sub a 20K pot for the 8K2 and see if you can get closer, but I believe you will have to go lower than 8K2. If you can't get close to 4.5v, then I'd consider a different set/combination of transistors...but definitely triple-check the wiring of the attack pot, anyway.

Also, I believe the voltages on Q1C and Q2C are too low. Wait for someone else to post the correct voltages...

ooops, i meant to say transistors...I will definitly be rechecking all connections, and will probably give the 20K pot a try soon. THANKS!

If anyone has correct voltages and/or any other thoughts, please chime in! I'll keep you posted on my progress.

[Govmnt_Lacky]

+1 on LucifersTrip comments 

You need to adjust the resistor that is directly between your supply voltage and the Collector of Q3. That low voltage on your collector (0.667) is what is causing your "sputtering." This voltage should be about 1/2 of your supply voltage OR about 4.5V

Definitely check you pot wiring as this may be pulling down the circuit in some way. Would also be a good idea to double check ALL component values and placements (resistors, caps, etc.) and then double check your traces for bridges.

Good Luck 

[crookedtodd]

  break through!
After some probing and re-checking, I found the 1N4001 diode I added to protect against reverse polarity was pulling down the battery voltage to under 1 volt. I checked the polarity and it seemed to be correct. I'll investigate more this weekend.
I removed it from the circuit and the pedal came to life...ATTACK control worked, and no more sputtering/gating!
The voltage at the collector of Q3 is now at -6.7
Now I have something to work with!
There does seem to be quite a bit of extra hum/noise in the circuit. I'll be checking into this when i have more time this weekend. Any ideas or thoughts on trouble shooting hum/noise?

THANKS again guys for your ideas and suggestions!

[Electric Warrior]

Yeah, select a combination of transistors that gates a little to get rid of the noise.

Also, you might want to go for the original resistor values that were used in OC75 Tone Benders: 10k from Q1's base to ground and 47k on Q2's collector. Might work better with OC75s.



Collector voltages are around -8.5V (Q1),- 0.13V (Q2) and -8.5V (Q3) in vintage units.

This is what they sound like: http://www.youtube.com/watch?v=QRKKNvWuJa0

[crookedtodd]

Thanks for the schematic Electric Warrior!

:icon_biggrin:Ok, now I have something resembling what I was expecting to hear from this pedal.
First of all, when i recieved my OC75's from smallbear, they came with 2 resistors to sub in also. He sent a drawing which subs in a 20k resistor on Q2's collector, and a 110k (instead of 100k) from the base of Q2 to the emitter of Q3. I did not use these in my original attempt.
So, after seeing Electric Warrior's OC75 TB schematic, here's what I ended up doing:
I put the 20k on Q2's collector, and put the 470ohm back in (i had subbed a 1K).
Now my collector voltages read : Q1= -8.48 Q2= -.784 Q3= -3.57

Hum/noise is now at a normal level, compared to other fuzzes i've used! 
I'm still considering putting the 20k pot in as suggested by LucifersTrip, to tune that voltage on Q3. Although it seems that maybe if i pick up a 47k resistor and put it in where i have the 20k on Q2's collector, I can accomplish the same thing?

So after all that I'm pretty happy with the pedal, but I do have a few questions:
1) What effect would there be on the circuit if I subbed in the 110k, for a 100K, from the base of Q2 to the emitter of Q3, as provided by smallbear?
2) On the OC75 TB schematic it shows a 25uf cap on the -9V , instead of a 50uf, on the original. What effect would this change have?
3) Would there be any effect on the tone by changing the value of the 5uf cap on the ATTACK pot?

Thanks for all the help!

[Electric Warrior]

Thanks for the schematic Electric Warrior!

:icon_biggrin:Ok, now I have something resembling what I was expecting to hear from this pedal.
First of all, when i recieved my OC75's from smallbear, they came with 2 resistors to sub in also. He sent a drawing which subs in a 20k resistor on Q2's collector, and a 110k (instead of 100k) from the base of Q2 to the emitter of Q3. I did not use these in my original attempt.
So, after seeing Electric Warrior's OC75 TB schematic, here's what I ended up doing:
I put the 20k on Q2's collector, and put the 470ohm back in (i had subbed a 1K).
Now my collector voltages read : Q1= -8.48 Q2= -.784 Q3= -3.57

Hum/noise is now at a normal level, compared to other fuzzes i've used! 
I'm still considering putting the 20k pot in as suggested by LucifersTrip, to tune that voltage on Q3. Although it seems that maybe if i pick up a 47k resistor and put it in where i have the 20k on Q2's collector, I can accomplish the same thing?

So after all that I'm pretty happy with the pedal, but I do have a few questions:
1) What effect would there be on the circuit if I subbed in the 110k, for a 100K, from the base of Q2 to the emitter of Q3, as provided by smallbear?
2) On the OC75 TB schematic it shows a 25uf cap on the -9V , instead of a 50uf, on the original. What effect would this change have?
3) Would there be any effect on the tone by changing the value of the 5uf cap on the ATTACK pot?

Thanks for all the help!

2) I don't know if it has any affect on the tone. They increased the size of that cap early on by adding a second 25µF cap in parallel to the existing one.. later units have a single 64µF or 50µF cap there. I didn't add this info to the schematic to keep it simple. There is a number
3) I think so, give it a try.

[LucifersTrip]

I put the 20k on Q2's collector, and put the 470ohm back in (i had subbed a 1K).

Hum/noise is now at a normal level, compared to other fuzzes i've used! 

Isn't it funny that the 2 problems you had were caused by additions/changes you made to the original circuit...haha...

A simple way to decrease hum/noise is to put a very small cap 50-200 pf across the base-collector of Q2. If you do that, you will
not have to sacrifice the extra vol you got by subbing the 1K for the 470...Tho, you may lose a little high end...so use the smallest cap that will work.

I'm still considering putting the 20k pot in as suggested by LucifersTrip, to tune that voltage on Q3. Although it seems that maybe if i pick up a 47k resistor and put it in where i have the 20k on Q2's collector, I can accomplish the same thing?

no, that'd probably be way too high...you want to be able to have it adjustable from ~ 2-3K up to ~ 15-20k

[crookedtodd]

Isn't it funny that the 2 problems you had were caused by additions/changes you made to the original circuit...haha...

no doubt, Man...good lesson to learn!

I took it out to the gig this weekend and it performed beautifully!!!
First, my strat through my Twin Reverb...glorious with some reverb added!
Second, tried it with a friends Tele through his tweed Tremolux...tasty!
I think I'll leave it as is for now. We'll see what happens, maybe I'll tweak some more down the road. I'll try and post some sound clips if i get a chance.

Learned lots and I'm already getting the itch to try another project. Probably another fuzz, but maybe something with lower gain, something a little more primitive sounding.

THANKS EVERYBODY!!!

[Govmnt_Lacky]

Learned lots and I'm already getting the itch to try another project. Probably another fuzz, but maybe something with lower gain, something a little more primitive sounding.

Wait until you are REALLY hooked and try some modulation effects. Thats when the REAL fun starts with debugging!