Troubleshooting Tone Bender MK-II

[mrsage]

I just finished building a ToneBender MKII with parts from SmallBear and a PCB from GGG...


Well...it works...but I don't know if this is how a ToneBender is supposed to sound:

http://www.voxblues.com/guitar/mk2.mp3

As you can hear, the fuzz fades out as the signal gets quieter. It's almost like there's a threshold that activates the fuzz...above that you get distortion, below that you get a clean guitar. I recorded direct, so the ringing guitar sound you hear between fuzz intervals is actually a clean signal passing through unaffected...the fuzz has cut out entirely, but the signal continues to pass.

Any suggestions? I've been mostly plug-n-play up to this point (a couple of paint-by-numbers DIY kits and some True-bypass conversions), so treat me like a newbie. I just got a multimeter, though, so I'm ready to dive in and start learning how to debug...

Thanks in advance...

[Samuel]

Definitely read through the FAQs and tutorials on debugging, that sounds very much not right. In particular, take some voltage readings from the legs of the transistors and post them here (if you're doing a PNP circuit clip the black (-) lead of the meter to the leg you want to test and the red (+) lead to ground)

[RDV]

It sounds badly, badly mis-biased. Did Steve send recommended resistor values and did you use them. Also, are you sure you used the transistors in the right sequence, i.e. 1 thru 3 as I know he labels them?

RDV

[mrsage]

Yep, it was all from Steve...I put the trannies in as labeled, and when I placed the order, I told him that it was for the MK2, and that I needed the proper resistor values.

He sent the same resistors that were listed on the GGG layout.

[Jim Jones]

What voltage reading are you getting on Q3's collector?  It usually sounds best to me when it's approximately half the supply voltage.  Replacing the Q3 collector resistor with a 25K trimpot is a common mod that allows you to dial in a good sound.

HTH!

Jim

[mrsage]

Sorry for the newbie question, but how do I measure the voltages on the transistor?

I know I would put one of the test leads on the part in question (like the Q3 collector)....But where does the other lead go?

[Fret Wire]

The other lead goes to ground. Use the ground pad on your board, or if you're sure your wiring is good, use the ground on your jack. Set your DMM for 0-20v. Turn on the pedal, and plug a patch cable into the input jack.

And post the voltages from all your transistor leads. Also, post the voltage of your battery. List them something like this:

Q1: E
     B
     C
Do that for all three transistors. Also, double check your tranny pinout orientation.

[mrsage]

My battery reads 7.48V

Q1
C: .25
B: 7.07
E: 7.15

Q2
C: 5.8
B: 7.01
E: 7.15

Q3
C: 5.99
B: 5.81
E: 6.00

[Arno van der Heijden]

Try a fresh battery!

[mrsage]

It does it even with the 9V power supply plugged in.

[Fret Wire]

Ditto, the battery is too low to gauge your voltages.

[Fret Wire]

It does it even with the 9V power supply plugged in.

Does what? You get the exact same voltages with a power supply? Using a fresh battery helps alot since we don't know how much your power supply is putting out.

How is this wired? pos ground or neg ground? What are the trannys, PNP or NPN?

Did you double check your tranny pinout's?

[mrsage]

It's a PNP, negative ground wiring, per the GGG layout. I triple-checked the pinouts on the trannies.

The effect (or lack thereof) is the same using the power supply. Here are the voltages with it plugged into a wallwart (I can't find my fresh batteries):

Q1
C: 0.30
B: 9.42
E: 9.50

Q2
C: 7.80
B: 9.35
E: 9.50

Q3
C: 7.99
B: 7.80
E: 8.00

[Fret Wire]

Those voltages are way off, more than a misbias problem. Time to check for cold solder joints, bridged (touching) traces, and correct component values. Whip out the magnifying glass and double check the traces on the board to make sure none are touching.

Another problem that may come up when all else is checked, is that sometimes (not all the time), a pnp tranny fuzz will not cooperate when wired neg ground. When this happens, the only fix is to go pos ground. Battery life is very good anyways on these.

[petemoore]

Ballpark, once you start getting voltages in the ballpark, use [and post] the supply voltage as a reference to what the other voltages should be %age wise.

[mrsage]

Well, I think I'm in over my head.

I re-wired it to positive ground, per the diagram on GGG, and now I'm getting no LED and very low (or no) voltage readings.

Q1
C: 6.96
B: 0.05
E: 0.00

Q2
C: 1.34
B: .014
E: 0.00

Q3
C: 1.16
B: 1.34
E: 1.15


That's with the same 7.4 V battery.  I probably messed up the capacitors or something, but I don't have any spares to swap out, so I can't be sure.

Thanks for the help...looks like this is getting shelved for a while.

:cry:

[Fret Wire]

You're a lot closer now. The emiters of Q1 & Q2 should be at zero. Q1's collector looks much better. Q3's collector is low, it should be half your battery.

[mrsage]

Hmm...okay....

I'm a little less depressed about it now.

What else should I look for to help the Q3 collector be all he can be?

And why isn't the LED coming on anymore?

[Fret Wire]

Try dropping R6 (8k2 from Q3's collector) down about 2k, to somewhere between 5-6k.
http://www.generalguitargadgets.com/diagrams/tonebender_m2p_lo_pp.gif

[Fret Wire]

And why isn't the LED coming on anymore?

When you rewired it for pos ground did you remember to turn the LED around (neg flat side of LED).

Pos-ground: neg (flat) of LED goes toward resistor
http://www.generalguitargadgets.com/diagrams/tonebender_m2p_lo_pp.gif

neg-ground: neg (flat) side of LED goes toward switch
http://www.generalguitargadgets.com/diagrams/tonebender_m2p_lo_pn.gif