Troubleshooting Tone Bender MK-II

[aron]

>It's a PCB purchased from General Guitar Gadgets, and I'm using their layout for the PNP positive ground version.

Oh commercial product. Hmmm, have you contacted JD?

Aron

[mrsage]

Yeah. He said that for $20 (plus any parts) I can send it to him and he'll get it working...then send me a summary of what went wrong.

I'd rather fix it myself, but that might be my best option at this point...

[Fret Wire]

It may be something simple you've overlooked both times around. Sometimes when debugging, you can read the layout the same way incorrectly through several re-builds. It can be fustrating like all hell. That's why people get advised to make sure the board is correct before adding the off board components and wiring. It makes later debugging easier. A bad switch, jack, or wiring with an internal break,  can drive you mad as you keep rebuilding over and over again. Then one day, you cannibalize those same parts for another build, that also doesn't work, and it hits you. It can really be dis-heartening.

A fuzz type ckt can add even more problems to the usual list of things that can go wrong with builds. Gain matching, bias, and leakage with Ge trannys can cause their own problems as well as mask other pcb or wiring problems. Add to the fact that a Ge tranny usually won't survive the soldering from several rebuilds (sockets prevent this), which will again lead the person to believe something else was wrong.

We all get that pedal that absolutely defies our best efforts to run. When it's the first or second pedal you've build, it's a pisser. But... it will come together, no doubt.

[mrsage]

I'm sure it will come together.

It's just frustrating because I've put quite a chunk of money into this project (parts, enclosure, etc). And I'm not in it for the money or anything, but it's frustrating to know that I could have spent just a bit more for a pedal that works!

Any other suggestions? Or tips on things I could try testing to figure out where my problem lies? I could take a picture of the PCB if that would help. It's kind of messy at this point (lots of scratches where I was making sure there were no traces jumped and whatnot), but if that would help doublecheck that my wiring isn't messed up...

[sigh]

Thanks again for the help, by the way. I'll bet you guys are hoping I get this sussed out as much as I am at this point!

[mrsage]

Well, I let this sit for a while, then I broke down and bought all new parts for the board. I hadn't used sockets before, so I wanted to make sure I hadn't fried anything. The resistors all tested okay, and the caps were cheap...the only expensive part was the trannies.

They came in from Small Bear today, and I tried socketing them.

Nothing new. The collector on Q3 is still way too high. So I tried putting in a 25k trimpot for R6. When I turned it up to 25k, I noticed that the value on Q3 collector went down...a bit....

It was still at 8+ volts even at 25k.

So I tried a 100k trimmer.

Same deal. I got the value of Q3 all the way down to 7 volts, but that was with 100k on R6!!!

What's going on here? Should I just keep increasing the resistor value until I get around 4.5 volts? Because the voltage is decreasing...just not as fast as you guys make it sound like it should...

And if the value is decreasing, that means there aren't bridged traces, right? Because if there were the value wouldn't change at all, right? Because voltage would be running free?

So the only things "stopping" the full voltage from reaching the Q3 collector are R5 and R6...and I can't change R5 because that connects to C3. So should I just increase R6 indefinitely?

[mrsage]

Maybe this is a good time for a quick electronics lesson, too...

I was playing around with just a 9v battery, some resistors/trimpots, and a multimeter.

I connected a 1M resistor to the battery and measured the voltage from one terminal to the end of the resistor, and it didn't go down that much. From 9.3 volts to maybe 8.5 or so.

What am I missing?

Is the elusive "I" in V=IR?

Because I'm wondering what other factors could keep my Q3 voltage from dropping as it's supposed to. Does it matter that I have the battery connected directly to the 9V(-) input and Ground, with no offboard connections?

Argh. This is frustrating.

[captntasty]

mrsage - I feel your pain.  I have built a number of effects and gotten them to work after much frustration by stumbling over this and that, replacing this and that.  I am by no means all that savvy when it comes to the theory of electronics, but will offer this.  I just built the 3 knob TB version off GGG and it doesn't work period -  :evil:   but that will be another post...

Before that I built the PNP Neg Grnd MKII and it does work!  Not to insult your intelligence, but the layout your looking at from GGG is the right one, right?  4 different versions - 2xPNP and 2xNPN.

Now for something that might be helpful.  From my tonebender thats working, here are the readings off the trannies:

Battery=9.42

Q1
C=0.61
B=9.13
E=9.25

Q2
C=8.06
B=9.08
E=9.24

Q3
C=1.04
B=8.05
E=8.19

That is with the trim pot in a range that works (a 250K because that's all I had.  How do you have that hooked in there?)- if I adjust it to the point it drops out, voltage on Q3-C jumps to 8+ volts.  So this is how a functioning MKII Q3 behaves.  What it all means?  Well, I'm not  technically savvy enough to explain it intelligently, but I hope this info helps by giving you some comparison - and if anybody with more knowledge is willing to elaborate - fantastic!

Good luck and I'll keep an eye on this thread...

[mrsage]

Those voltages seem different from what other people have said...and different from what the Fuzz Central project has listed:

Vin
9.65V

Q1 Vc (hfe=70)
8.11V

Q2 Vc (hfe=70)
1.10V

Q3 Vc (hfe=100)
4.5V


Everyone has been telling me that the voltage on the Q3 collector should be about half of the battery voltage. I just can't get it that low!!!

[RDV]

If someone has already said this to you, then please forgive me, but you've obviously made a wiring or parts placement mistake. There's no other explanation. It's not that hard of a circuit to get going.

RDV

[mrsage]

If it were a parts placement error, wouldn't it have to be either the R5 or R6 resistor, though?

Because there's nothing else between the 9v (-) and the Q3 collector!

And I've been over it a thousand times to make sure the values and placement are correct...Is the layout for the PNP positive ground MK-II on GGG correct?

Because I followed that to a T.

[brian wenz]

Hello Hello-
     I don't know if anybody else made this suggestion, but I've had similar problems and went through the circuit and re-soldered all my connections.  This solved the problem for me.
Just a quick thought..
Brian.

[Doug_H]

d;lkj

[nightingale]

if you could post some decent pics of the board~
i think it might help.
just a suggestioin..

[RDV]

I have been where you're at. It was the transistor Obsidian. I wired it up on perf during a football(american) game, and had probably not paid complete attention to either. It didn't work, and the problem elluded me for more than a month. I had a cold solder joint.

RDV

[mrsage]

:roll:

Somewhere around page 2, someone suggested removing all the offboard wiring and just hooking it up directly to a battery.

So I was testing it without any of the other offboard connections after that point...no pots, switch or jacks.

Well, I just now decided to hook everything back up and give it another whirl.

I got about 6 volts on Q3. That was my closest yet, so I tweaked the trimpot, and sure enough: I got it to 4.5 volts pretty easily.

The LED still didn't work, but upon closer inspection, the General Guitar Gadgets layout has both ends of it going to ground (through the switch, of course). I wired up one end to the 9V connection and it worked.

Then I hooked it up to my amp and everything was all good.




So....

Can someone explain why attaching the offboard connections made such a difference? Is it because there is significant current flow through the circuit, and I was getting too much current flowing directly to the Q3 collector? So that wiring everything else up lowered the current, which lowered the voltage?

If so, why would you want to test without the offboard connections?!

[mrsage]

By the way, thanks for everyone's help.

I appreciate your hanging with me through this.

[brian wenz]

Hello Hello--
     More connections  =   more chance for mistakes  [cold solder joint?]
Brian.

[petemoore]

:roll:

Somewhere around page 2, someone suggested removing all the offboard wiring and just hooking it up directly to a battery.
 >>>I always use the test jig, [a stack of two boxes, one with 2 mono 1/4'' jacks, the othr just forms an insulated tray for the circuit to sit on. 3 wires with alligator clips are also part of it, they connect to:
 RED: input jack tip
 BLUE: output jack tip
 BLACK: ground on both jacks [sleeves]
 Not much more to it than that, I flop the circuit to the top box 'tray' clip to it red blue and black...plug the guitar in the input jack and amp to the output jack...Im ready to test the board, battery clip, pots, basically everything that's on the 'bare bones' schematic is on my boards, nothing else until the second testing stage which doesn't begin until the circuit passes the first testing stage..which includes
 "decent Voltages at certain points like OA and  Q  pins from ground, and V supply points from ground.
 'normal' Sound comes from output when input signal is present
 The knobs work 1/2 right...seem to do some of what the function of their label specifies or illudes to.
 So I was testing it without any of the other offboard connections after that point...no pots, switch or jacks.
 >>>Most of these circuits need their pots or something where the pots go 'hooked up'' to operate, it's easier to use pots than to figure out how not to.
 Well, I just now decided to hook everything back up and give it another whirl.
 I got about 6 volts on Q3. That was my closest yet, so I tweaked the trimpot, and sure enough: I got it to 4.5 volts pretty easily.

The LED still didn't work, but upon closer inspection, the General Guitar Gadgets layout has both ends of it going to ground (through the switch, of course). I wired up one end to the 9V connection and it worked.

Then I hooked it up to my amp and everything was all good.




So....

Can someone explain why attaching the offboard connections made such a difference? Is it because there is significant current flow through the circuit, and I was getting too much current flowing directly to the Q3 collector? So that wiring everything else up lowered the current, which lowered the voltage?
 First of all I can't picture the circumstance well enough, second...no make this first...you've got the signal path part of the circuit working?
If so, why would you want to test without the offboard connections?!

I find it's just easier to eliminate 'all that garbage' till you need it. By veryifying that the actual 'circuit' [ I wish there were a name for a bare' bones' circuit other than 'bare bones' circuit...the effect part is always different] the offboard wiring is nearly always following a similar pattern...depending on the options"
 battery switched off at I jack
 Jacks [instead of wires coming out saw an old 'old in this case actually means 'old' factory unit with wire/plug that comes out and wrapped around it]
 LED indicator
 LED indicators
 All this stuff adds more to figure out. having a circuit that works makes it easier to figure out what else isn't working. Remember it only takes 1 thing...having a buncha unverified garbage adds confusion.
 Debugging a circuit that has all of the above, or is even already in the box is just plain silly, and the liklihood that ANother problem breaks out as a cold solder that got pulled on, a wire coming loose from being pulled on, wires touching and burning stuff up because it has turned into a gangly mess...
 Unelss I'm missing something here, testing circuits, plugs, jacks, switches, switching arrangements, LED indicator, etc etc etc. is easier when each section hopefully already works. Without initial testing of sections, it is impossible to know this, not that I ever made a mistake.
 Mistakes seem much easier to find and adress one at a time.
 If I have two sections of a circuit, neither pass signal, I also have a combination lock...I may then spend days finding the combination.
 The Tonebender alone being a combination lock. All three transistors must be biased and everything must be right for it to work.
 Testing your box's bypass and on mode should be easy enough. N/P to do with a wire that has an alligator clip at either end and a DMM. I can type more about this if you like.
 Testing the LED should be N/P with a battery and a resistor. The switching can be debugged for this, the Tonebender doesn't need it to function. I can Type about it later too.
 If You're planning on doing some circuits, the 5$ test jig, and the 20 minutes to set it up is worth the effort in time saved. I used a RACO large for the bottom box cause it's heavy, and any cardboard to make the top tray. Test it by plugging a cable to input and test for continuity from other end cable tip to input alligator clip, repeat for output jack, then test that both jacks are grounded to the ground wire clip. mark them at jacks and where wire comes through cardboard or...