Axis Face - Need help getting Q2 biased -

[Twhjelmgren28]

 I've successfully breadboarded the Fuzz Central Axis Face. Everything worked and sounded great. I recently transferred it to Perfboard but am having issues now. Here is the schematic I'm referencing:




For some reason, I can't get my Q2 to bias. When I adjust the 10k trimmer, the voltage on the collector of Q2 changes from 9.67 to 9.82 - that is literally the entire range of the trimmer. Obviously, when I breadboarded the circuit, there was a much larger range and I set it right on 4.5v...

When I test voltage throughout the circuit, I seem to be getting close to 9.84 (other than around the transistors) until I get to the point where the .01uF cap meets the 3rd lug of the "level" pot. At that point, I don't have any voltage...could that be my issue. As far as I can tell, I have everything wired correctly. I've gone over it several times, double-checked that I used the proper value components, and have gone back over several solder joints (with that being said, I'm terrible at soldering but learning).

I have the 100pF cap and collector of Q2 hitting the 1 lug of the trimmer.  I then have the 1 lug of the trimmer jumped to the two lug...

Any thoughts?

[smallbearelec]

Hi--

There is surely a wiring mistake; you just have not found it yet. What layout drawing did you use? Was it a drawing you did, or a vetted layout from somewhere else? If it was your own, consider that you might have made an error in the drawing and that's been transferred to the board.

Here is a soldered build of the silicon FF that includes some information that might help you:

http://diy.smallbearelec.com/Projects/FurFace/FurFace.htm

The layout drawing, Fig. 14, has been vetted, and you are welcome to use it.

Make  clean copies of the schem and your layout. Set your meter to continuity. Go through the schem point-by-point to make sure that everything that the schem says is connected Really Is. Mark off connections with a highlighter as you work.

Work patiently and you'll smoke out your problem(s). Let us know how it goes.

[Twhjelmgren28]

Hey, thanks for the response!  I actually didn't even do a drawing or use one - maybe that was my mistake - I wired directly from the schematic. 

I'll go back through everything and also double check the points that are shown on your diagram that can be crossed to the axis face schematic (as the axis face seems very close to that).

....Side note: if this is the Smallbear Electric, I've got a shipment arriving today from you, thanks!  And thanks for the help! 

[Cozybuilder]

If you use perf, the following wiring diagram is what I used for the 2 I built:
Both use Q1= 2N2369 and Q2=BD139

[Twhjelmgren28]

Got it!  Thanks for all of the help.  I went back over everything and I couldn't find any issues - so I started checking voltages again and everything really seemed right until I got to the 10k trimmer.  I desoldered the trimmer, soldered it up a different way.  Still nothing.  Desoldered again and put everything back as the schematic showed and it worked!  I guess maybe I had a bad solder joint on either the trimmer or the collector of Q2 - or something was connected wrong and I didn't realize (); not sure. 

I will definitely be making a layout prior to soldering in the future.  Thanks for the advice.

By the way Cozybuilder, the inside of that pedal is beautiful.  My perfboard looks completely amateur compared to yours.

Looking forward to getting that package from Smallbear today so I can start getting it into the enclosure!

[Twhjelmgren28]

Unfortunately, I'm bumping this up again - I had the pedal working great and I'm now having issues again.

I was getting the voltage at the 2nd collector correct and the pedal sounded great.

I went back over everything to resolder a few joints better and now I'm back to the same story as I was before - the collector of Q2 is barely changing when I adjust the trim pot (I wonder if I nicked something). 

1)  Should I notice a voltage change at any part of the trimmer itself when adjusting it?  I have the 1 lug of the trimmer connected to the collector of Q2.  Lug 1 of the trimmer is also jumped to lug 2 and I have a 100pF cap jumped b/w base and collector of Q2...Everything is as shown in the schematic.  When I change the trimmer, lug 1 is changing the exact same as the collector of Q2 (which makes sense as they are connected) but that change is very minimal - it trims from about 9.74 volts to 9.84 volts.  Could the 100pF cap be bad?   

2) In what instance would the 10k trimmer not change voltage more than 0.1 volts?  I'm just wondering if the trimmer could be bad.  I did try a second BD139 transistor in place of the other one and it's doing the same thing...

3) I guess I'm planning on starting back over from scratch which annoys me as I had the pedal working prior to messing with it again.  Is there anything I should be aware of as far as common practice for placing/soldering certain components before others.  I essentially wired each node point, almost literally as shown in the schematic...Is there some trick I'm missing?

4) the only other thing I did was start fitting it into the enclosure itself and add a 3pdt switch,  could i have run into something there?


Thanks again for any help and sorry for all the newbie questions.

-Tommy

[robthequiet]

One thing to test:

Is the pin of the trimmer going to the top of the transistor well-connected? If the trimmer voltage is not changing, it might mean that the current through the transistor is not making it into the pot. You might have some issue with that connection or with the pot itself.

[Twhjelmgren28]

I'm beyond frustrated with this!  After checking the trimmer and Q2 again, and resoldering a few other points, I'm not even getting a signal.  At least before I had a farting noise - thinking of scrapping and starting over.  This is frustrating!

For reference, here my voltages:

Q1
b=0.23
c=0.30
e=0

Q2
b=0.30
c=9.74
e=0.22

I did not vary from the schematic at all...

If nothing else, this is good troubleshooting experience.  Prior to scrapping the project and starting over, I'm going to try an audio probe to see if I can find any problems.  Then I'm going to try an 8k2 resistor in place of the 10k trimmer to see if perhaps the trimmer is acting up.  If all fails, I guess I'll start over; I'll do a layout this time (following cozybuilders as a reference) and then post it here before I do any wiring.  Thanks!

[duck_arse]

remove power. switch your meter to ohms. measure from the supply line to the collector of Q2 AT THE TRANSISTOR LEG. then measure from the Q2 E to ground line. what values do you get? does the collector leg R value change w/ trimpot? [] do the same measures for Q1, for completenesses.]

[Twhjelmgren28]

measure from the supply line to the collector of Q2 AT THE TRANSISTOR LEG. then measure from the Q2 E to ground line. what values do you get? does the collector leg R value change w/ trimpot? [] do the same measures for Q1, for completenesses.]

Got it.  Just for clarity, when you say supply line, that is where power would be coming in if it was connected, not the signal coming in, correct?

I will get those values posted here after work today.  Thanks for the reply!

[duck_arse]

where the (+) and the 100uF (+) and the 1N4002 Kathode and the 33k and the 1k2 all connect - that is "supply" in this inst. signal is signal, tou never want to connect that to "supply".

[Twhjelmgren28]

Cool, thanks! 

I'll get those measurements posted later.

[Twhjelmgren28]

Alright, here are my resistance measurements:

Q1
b=33.6k
c=33.3k
e=34.5k

Q2
b=33.6k
c=1.24 to 11.17k (the resistance did change when adjusting the trimmer)
e=134k


Q2 emitter to ground line = 100.9k
Q1 emitter to ground line = 0.00k

[Twhjelmgren28]

Major Victory:  Q2 biased!  I don't know what exactly it was but I didn't have the trimmer wired correctly.  I ended up trying a different method of hooking up the trimmer (shown in a Wampler book) and that allowed me to trim it perfectly!  From what I read, I basically wired it as a voltage divider rather than regulator - hopefully that is okay.  So I grounded lug 3, put lug 1 to the supply at the 1k2/.033uF, and put lug 2 to the collector of Q2...

Only issue now=No output!!! There was an output yesterday when my biasing woes started - a farting noise at that point.  After screwing around for a while, the farting noise left completely.  I have since been working out the biasing bug.  Now I have an output bug.  If you have any suggestions, I'm all ears!!!

May try an audio probe?

[duck_arse]

ahhh, first - do your resistance readings end with a "k", or are they flat out ohms, less than 100? we do need to be pedantic and accurate about/with our multipliers.

if ending with a k ....
your resistance measures - Q1 collector looks fine. unfortunately, it seems you may have shorted all three pins together, not so fine.
Q2 base looks good, cause it's hard to the Q1 collector. Q2 C looks sane, following the trimmer. and Q2 E might be looking at the shorted Q1 via the 100k, totalling 134k-ish. Q2 emitter is connected to ground via the 1k of the pot. it can't be 100R. it might be 100k if there is something wrong with the pot connection.

your trimmer - you don't want a voltage divider connection, you don't want one end connected to ground. you want a rheostat connection, so it looks like a variable resistor. short the centre leg to one of the outer legs, then take the two outer legs as connections and you'll be sweet.

what is the Q1 E to ground measure? have we seen photos of what you have built yet?

more pedantism:

.... put lug 1 to the supply at the 1k2/.033uF, .....

the 1k2//33nF connect to the supply. the trimpot connects to the 1k2//33nF, not supply.

[Twhjelmgren28]

ahhh, first - do your resistance readings end with a "k", or are they flat out ohms, less than 100? we do need to be pedantic and accurate about/with our multipliers.

Sorry - yeah, those are all k measurements (i'm pretty sure - I'll double check tonight)

So, I need to do the following:

1. Fix the shorted Q1 pins and maintain the resistance at the collector
2. Fix the 1k pot connection(s) b/c Q2 E resistance is too high
3. Fix the trimmer so it's a rheostat connection

I can get pics up soon - I'm pretty embarrassed.  Nothing is very neat and I have resoldered multiple points multiple times...

I really appreciate your help - I'll definitely work on being more accurate in the details. 

[Twhjelmgren28]

Okay, so I confirmed:  all of the resistances I had listed are k.

Q1 E to ground is 0.00k

1. I looked at the solder joints of the Q1 pins - those points appear fine and don't run over to the other pins - I assume the short is coming from something other than the solder joints themselves
2. I completely redid the wiring from the 1k pot to the perfboard.  I'm still getting the exact same resistance measurements
3. I changed the trimmer back to the rheostat connection - the measurements are still the same at Q2

I did take an audio probe and followed the input signal.  Noise didn't cut out until I got to the base of Q1 meeting the .22uF cap and the 100k resistor...Any ideas?  Thinking about resetting and soldering that point.  Could it potentially be a bad part?

[duck_arse]

Q2 emitter is connected to ground via the 1k pot, so it can't be 0R. put your meter on ohms, (I don't need to remind you, POWER OFF for ohms meterings .....) and test the resistance between the Q1 pins. do black on base, red to E then C, then do red on base, black to E then C.


photos?

[Twhjelmgren28]

Sorry, that was Q1.  Q1 E to ground is 0.00k.
Q2 E to ground was still 100.9k

I don't know if these will help much b/c the bottom of my board is pretty bad - Also, I moved Q2 to the bottom when I tried replacing that transistor - just so I could see it a bit better:

[Twhjelmgren28]

Here are the Q1 resistance measurements:

Black on Base:
E=0.11k
C=0.11k

Red on Base
E= 0.11k
C=0.11k

That's obviously not right is it...That's what I got and I double checked.  Bad transistor?