V1 muff trouble shooting "update"....need some guidance please

[plexi12000]

Well....got my auduio probe in action (thank you Aron)  I think i can safely say my problem is in the 2nd clipping stage.

(*just to refresh....i had no ouptut at all. noticed collecter lead of Q3 was just over 9v)  Replaced collector resistor and still nothing.

Today, using the probe, i got what seems to be a nice strong signal from both leads of the 2nd clip stage, input .1uf coupling cap. (which is C7 on the GGG made PCB.)

also good signal at both ends of the 8.2k resistor as well. But input to tone stack, .004 cap, is pretty much nothing...getting a very weak audio signal. So figgered the
prob is in the 2nd clip stage?

Got audio signal from emitter and base of Q3. Nothing at collector.

So all that is left....is the diodes, 500 feedback cap and resistor.....?  I think! -lol   So probed them.  here's what i "heard" at each of those components...one lead had

strong signal....other lead was very weak.    I don't know what else to do now, really.

I could just go bonzai and just replace all the components.....but i'd really like to learn how to properly diagnose instead.  i'm annoyed now and don't want to give up!  lol

Any suggestions what I can do next to nail this prick down?   (i tried other Q's with no success)

[GibsonGM]

Good diagnosing so far, Plexi - you are doing it right

My best guess - you COULD have something boned up in the feedback loop at Q3, your instinct there is correct.  What is the voltage at the collector side of R17 (15K)?   We combine techniques - audio probe w/meter.   

I ask for that voltage (in fact, all of Q3's voltages) because if something down stream (C10, R19...) is messed up, it could really drag the signal down so much you won't hear it at Q3's collector.   A sure way to know is to break the circuit at left side of C10 and try probing again - but voltage reading there first!   The tone stack is expected to load the stage, and decrease the volume - what happens farther down the line, at C13?    Gotta try to eliminate the obvious, first  

Check your R values at Q3, be sure they are what they're supposed to be, and post voltage at Q3 collector...you are going about it the right way, no need to tear it apart yet.

[plexi12000]

thanks GibsonGM....at least thats a positive vibe-   

yeah ok-- i follow....i think!   q3 emitter and base was right on the money---from the V chart from GGG.  Collector was like 9.1 or 9.2v.

the thing that sucks... i can't audio probe or use my MM in half the tone stack.....because two component bodies (box type capacitors) are flat down on the dang

board.

I'm gunna measue that resistor now. have to look at my schematic and figger out which lead is to Q3 collector.   -I shall return!  lol

[plexi12000]

alrighty........ i'm getting 9.08vdc at the collector-15k.   Same reading at Q3 collector lead...9.08v

Base: .8v
Emitter: .2v

checked my V chart...maybe emitter volts slightly high?  chart says .02 volts.  I have no idea what sort of range is "acceptable".

Hey...another kinda related question on this....you mentioned how a stage gets "loaded"? that means the signal "weakens"?  so...like in a tube amp...say analog reverb...you need a "recovery" tube to boost the signal back up.  same type of thing happens in these small circuits too?

[aron]

This is completely a guess but I think the problem is in the wiring of the tone control pot. The reason you hear nothing at the collector of Q3 is that somehow your tone control is grounding the output - it's a short to ground right at the tone control. Check your wiring there.

[plexi12000]

man!!....thats funny you said that aron.  i actually suspected that the other day.   i was doing some continuity checks with my MM.  my leads to ground were ok.

but i couldn't get continuity when checking directly on the tone pot solder lugs.   i got continuity on the other pots--  wiper lug to their ground lugs.   couldn't get anything

on those lugs of the tone pot.    Thank you....i'll recheck my lead work.

[plexi12000]

this seems a little odd.  I checked the resistence of my tone pot -100k (still wired into the circuit) ...set at max resistance. one test lead on input lug, other on wiper.

it starts at about 50k and slowly creeps up to about 70k tops, after a couple of seconds.

when i do this same thing with the other pots...i get their max reading immediately.  huh?  swapped in another pot.  same "creeping" resistance.

i dunno !  -lol

[antonis]

Something wrong with tonestack/pot wiring...

Your pot behavior looks like there is a cap between your test points..

[GibsonGM]

Is the effect out of its enclosure, where the pot can't touch any case to ground it?
 
They're right, above - that creeping V thing = capacitance charging.  So, you have to trace the wiring back into the stack and see what's up.  It's easy to connect to the wrong end of a cap or something!  A SMALL amount of this would be expected given the caps around the pot, but not for a 'long time', like an observable R-C time constant...

Are C12 and C11 the right values? 

Anyway - Q4 is the 'recovery stage' to boost your signal back up after the tone stack has its way. It's a very greedy tone stack, the insertion loss is pretty big.   So it's not unexpected to be 'quiet' in there...the previous stages get your distortion, and tone control...the last one brings it back up.    But it does sound like you're having a trouble...

[plexi12000]

dang it......man, this one is a real ball buster. hahaha!  i had it out of the enclosure for close up inspection...used the 'ol magnifying glass too.   couldnt find any stray solder, etc.

maybe i'll just 'rebuild' the damn tone stack and be done with it.  i even remelted all the solder joints.

[duck_arse]

alrighty........ i'm getting 9.08vdc at the collector-15k.   Same reading at Q3 collector lead...9.08v

this is no good! you want about 4V5 at the collector, or something less than V+, to indicate that the transistor is drawing some current.

[GibsonGM]

^  This

[plexi12000]

well.....IT FINALLY WORKS-   And it's rather embarrassing.  as usual, it's the idiot holding the soldering iron! hahahaha

i got frustrated and just stripped out the tone stack components....cleaned the board thoroughly and started over.  eff it!

all those components checked out ok with the MM.

i had socketed a few of the 100n coup. caps when i first made the board so i could play around with values.   i removed a couple of them to give myself room to work....i

just set them aside.  later, i realized one of those 100n's....was actually a 10n.  so, i don't know where it actually was in the circuit....but i think it was at C4.

so..the damn thing works!!!   i suppose it was the value of that cap?? 


best of all.....i don't like it!  LOL-    i think it's pretty "thin" sounding with all the .1ufs.   my previous V1 build, i used 1uf electrolytics.....and it's fkin awesome.  big and

chunky.    might try some .47ufs.   but whatever, thats what makes it all fun. 

[aron]

Congratulations!!!!! Well at least it works!

[plexi12000]

thanks brother man- thats all that matters, right? haha   -wish i could have really figured out EXACTLY what is was.  but, oh well....i hope i get better at trouble shooting

with time and experience.   thanks again for the help everyone!

[GibsonGM]

Sometimes you will just never know what the problem was, ha ha    I'd guess NOT the cap (altho post-filter, a signal can be very quiet for sure!), but there's no way of knowing if you porked the cap somehow (radical over heating?).   Chaulk it up to experience!

I hate "tilt controls" myself, Plexi - I don't find them really useful for the music I play.  They are one-trick ponies, IMO - early 90s stuff, you can really get that BIGGGG fuzz with a scoop....but that's all they do.   Just an opinion.   I like BMT, or at least BT, or T cut...I also don't like what you get for giving away so many dB!!!      

Good detective work.