a little troubleshooting help

[charriman]

In this thread from a couple of years ago:
http://www.diystompboxes.com/smfforum/index.php?topic=98608.msg862665#msg862665

I posted that i was having issues with a GGG tonebender build.
well, i decided to do a little cleanup and do away with the tranny sockets and solder the silicon trannies to the board.
Did that and now i seem to have messed it up.

with the probe, i've figured out that the board is passing signal all the way up to Q3 so i checked my voltages.  All of the voltages check out on Q1 and Q2
but here they are anyway
Q1
C 6.8
B 1.9
E 0.5

Q2
C 2.39
B 1.31
E 0

Q3
C 1.76
B 2.4
E 1.74

Would a bad solder joint on Q3 cause it to not pass signal through Q3 or is the tranny toast?  I'm still pretty new to all of this with only a few builds under my belt.

here is the schematic:   http://www.generalguitargadgets.com/pdf/ggg_tb_m2p_sc_ns.pdf

it looks to me like the guitar signal is split off at the base of Q2 and it goes to R4 and then out to the 3rd lug of the attack pot.
Since it doesnt appear that that the guitar signal goes through Q3, what does it do?
Any help would be appreciated.
Thanks!

[blackieNYC]

R4 is feedback.  You have the same voltage at Q2C and Q3B, that's good.  Does Q3C voltage change with the R6 20k pot?  Ohm out the connections from Q3 to ground and +9.  Is R5 the right value?  The attack pot should change the Q3E voltage a bit.  Maybe check Q3 with the diode checker, that's red on base, black on E or C,  - .5v or so, but it looks like you've got that.

[charriman]

yes the trimmer does raise and lower the voltage to Q3c.  I was able to get the voltage up to about 6 volts when the trimmer was maxed out.  I've set it to just under 5 volts now.

i guess one question i have is "does the audio signal pass through Q3?"  Looking at the schematic, i can't see how the audio would.  If not, what exactly does Q3 do.
i can tell that the audio leaves the circuit board and goes to the volume control then to the output jack.  I think i might have to check that area to see if i have a broken connection there.

[PRR]

> The attack pot should change the Q3E voltage a bit.

If C4 blocks DC (as it should!), then the Attack pot R7 1K should *not* change DC voltage.

"R6T" should change Q3 collector voltage from near 9V to under 1V (and then rise again). For first-try get it to 4V or 5V. Final setting by ear, listening to the different distortion flavors.

> "does the audio signal pass through Q3?"

Output comes off of R5. Left end of R5 is at Battery (and big C5), so no signal there. Right end of R5 goes through output cap and volume pot.

So how can a signal get onto R5?

It *must* be current coming up through Q3 and R6T.

In my way of thinking/seeing, R5 should be turned to be in-line with R6T. Then it is clear that R5+R6T are Q3's load, and signal is tapped-off part-way (about 1/10th) along the load.

Q2 and Q3 are THE FUZZ FACE circuit. Direct-coupled pair. Not exact FF values (I don't recall R12 R13 in basic FFs), but near-enough.

(Q1 is a simple 1-transistor booster, to give the FF-like stage "MORE".)

[charriman]

Thanks! I think that makes sense. So there isn't necessarily guitar signal through Q3 but it is boosting the signal from Q2. The signal actually goes through R5 and out of the board to the volume. So I need to probe from R5 to the output of the board and from the board to the volume pot.  Is that correct?

[PRR]

> So there isn't necessarily guitar signal through Q3 but it is boosting the signal from Q2.

I'm not sure what you are saying here.

The "signal" SHOULD be your guitar (plus distortion). Q1 boosts it, Q2 boosts it, and Q3 boosts it.... BIG! Banging against the limits (0 and +9V)!! Clobbered and distorted.

All three stages amplify the guitar.

The Q2 stage may have little *voltage* gain, because Q3 B loads it heavily. However Q2 does give current gain so poor Q1 does not have to yank Q3 B directly. The implication is that you may not find much signal voltage on Q2 Base; but even a small signal means Q3 is getting signal.

[PRR]

But look. Your stand-out wrong voltage is Q2 B 1.3V while Q2 E is zero V.

You can not have more than about 0.6V across a base-emitter junction.

Therefore Q2 has a problem. Maybe the transistor is toast. MUCH more likely Q2 E is not really connected to R12. Have a good look around there.

[charriman]

I'll check around Q2. The reason why I asked that is because when doing the audio probe testing. I heard signal at the bases of Q1 and Q2 and also at R4. I'm getting nothing at Q3 at all.  Looking at the schematic I see that R4 should be getting the guitar signal but I was confused as to where the guitar should enter Q3.

[PRR]

> signal at the bases of Q1 and Q2

Signal at Q2 base is half the story. Q2 Emitter has to go somewhere (ground via a small resistor) or Q2 does not "feel" the signal, and has no bias to amplify the signal.

[charriman]

another noob question.
So if i'm understanding this correctly, the guitar signal enters the transistor through the base.  The power from the battery comes into the tranny through the Collector.  The guitar signal is kind of the control voltage that lets the DC from the Collector flow through to the emitter.

So does that mean that the amplified guitar signal exits the tranny via the emitter?  If not, what would the signal flow be?

[Kipper4]

signal path c1 to base of Q1 tranny, out through c2 to base of Q2 (also to r4) out through collector of Q2 (under r3), to base of Q3,  out through c3 to volume
If i have it right.

[charriman]

so this?  I highlighted what i think should be the signal path.

[Kipper4]

Yep I think that's it. Are you probing it?

[charriman]

i did, but not since you confirmed my path. I'll have to carve out some time to do that..  Thanks!  i'll let you know what i discover.

[Kipper4]

But look. Your stand-out wrong voltage is Q2 B 1.3V while Q2 E is zero V.

You can not have more than about 0.6V across a base-emitter junction.

Therefore Q2 has a problem. Maybe the transistor is toast. MUCH more likely Q2 E is not really connected to R12. Have a good look around there.

Did you double check this?
If base is 1.3v then to my mind emitter should be 0.7v
I could be wrong. I'm just assuming this since the diode (in the transistor) drop should be around 0.5-0.7v

[charriman]

I figured it out. I figured it was one of two things. Either I burned one of the trannies up or my soldering sucked. Well my soldering indeed sucked. I had a bad solder joint on the base of Q2. Redid that joint and all of the other joints I touched and it fired back up. Set the bias on Q3 to around 5v since I guess that bad joint messed with the bias of all the trannies   

Thanks for all of your help.

[antonis]

So if i'm understanding this correctly, the guitar signal enters the transistor through the base.  The power from the battery comes into the tranny through the Collector.  The guitar signal is kind of the control voltage that lets the DC from the Collector flow through to the emitter.
So does that mean that the amplified guitar signal exits the tranny via the emitter?  If not, what would the signal flow be?

(Late) Correction..

Guitar signal "disturbs" Collector DC flow..

The amount of "disturbance" is proportional to circuit's gain..
(which roughly is the ratio of R10/R1 because of Negative Feedback loop..)

Guitar signal is "lost" via Emmiter to GND and we never see it again - we dont need it anymore as long as it served as a "disturbator" (sic!)