Very strange bias issue with Fuzz Face C on Q2

[vanessa]

I'm building a Fuzz Face (with ge's) and I'm using the original values except using a trimpot on Q2's collector to bias it. I've built a few of these before and never ran into this.

When I measure Q2's collector I get 9.6V no matter where the position of the pot (10K and I have tried a 25k) is. Taking a measurement right before the pot I get 9.7V. On Q1's collector I get 0V. I've pulled the trimpot and found it to be working perfect. What's strange is once the pot is pulled I can take a reading before (where the pot would have been) and I read 9.7V and after reads 0V. Why I find that strange is it almost looked like the pot was shorting somewhere with that reading of 9.6V after. I stuck a 8.2k ohm resistor in its place and I got the same readings before and after. I've checked all my traces and leads and it’s all wired up right.

What could be giving me this type of reading? It acts like the resistor is not even there (short), but it's not shorted. You take it out and it's open.

[formerMember1]

yeah that's odd, the same thing happened to me, i had the reading of the power supply on the transistor in my wah, unfortunatly i don't know what did it, because i fussed with it and it eventually went away, I think maybe i had the power jack miswired,

I don't think Q1's collector is supposed to read 0volts, although i could be wrong,...

when you get it working, try the below, you will get way closer to jimi hendrix this way,..

add a 1k resistor in series with the ground lug of the 1k fuzzpot.
then set the trimpot  for 8.2k(remove it and measure to make sure you get it right on 8.2k) and instead, of adjusting that pot for biasing, adjust the 33k resistor up or down, until you get q2 collector at 4.50volts.  It makes the fuzz sound a little gated, but not too much, really close to JIMI's purple haze, voodoo child foxey lady etc,..(especially let the good times roll)  Fretwire taught me this,...mainly what you are doing is biasing using the 33k instead of the 8.2k,... you get way better fuzzface tone!!!  I love it this way, complete night and day fuzzface tone,....  with my transistors i ended up changing the 33k to a 28k resistor,(you may need to socket it, and solder different values resistors in series to get the right value)(or just temporarily use a trim pot) 

the fuzzface also sounds better with a wah this way too!!

try this and tell me how you like it, 

PS: read the recent thread on "biasing a fuzzface" for more info(or something like that)

[Rob Strand]

Sounds like a dead transistor, board short or soldering problem. 

Once Q1-collector goes to 0V that's the end of the it, the rest of the ckt is dead.

You could pull Q1 and see if the collector voltage of Q2 falls or goes to zero.  After that if the bass of Q2 is still at 0V then there's something wrong with the 33k resistor, Q2 or the board/soldering.

An alternate plan of action would be to get it running with (cheap) silicons then when that works plug in your germaniums one at a time.  If the circuit goes bad after you put one in then you know there may be a problem the the transistor.

[vanessa]

I was thinking that it may be the transistors. I've tested them and they checked out great before I put them in. Like you said maybe there was an issue with the soldering prior before this happened? The layout is good; I’ve used this one in the past. I’ve checked all the traces with a magnifying glass no less. I’m thinking it may have been a cold solder, or maybe a light jumped solder point in the beginning. Then after a good scrub, or the light dab of the iron it was gone but my transistors were as well  .

I thought about putting silicon’s in there last night before I went to sleep. I should have done that from the beginning instead of farting around possibly destroying my germaniums (which I'm sure are by now).

I also thought that the 33k may be an issue, but I pulled it and it checked out okay. It's the first time I'm using carbon comps for that real "mojo" (LOL), I've never had issues with films or metal films.

[vanessa]

Well I wanted to let you know how this issue was solved. I always get bummed that few people follow up with their resolution.

As you know I was getting a voltage reading before and after the trimpot. As I said I looked over the board with a magnifying glass, but when things just were not checking out after I put silicon transistors I felt it could only be an issue with the 33k (I had already replaced it so that ruled that out) or a short on the traces.

I ran the magnifying glass over some select points that I thought could be causing the issue. The number one being the power rail. Then you have to look where it could be shorting after the trimpot (or if not using, the 8.2k). There was a (smaller than) hair thin bit of copper trace between the power rail and the Q1's base. I used a wire pick and removed it and it fired up perfect.

So if it helps, to you that may be starting out or even those who been around the block but today are feeling a little light headed from all those solder fumes, you always have remember current flow when trouble shooting. Symptoms of shorts and opens directly affect current flow. Just like a river, a circuit directs the flow of the current. Open circuits are like a dam in the river blocking water flow (current). Shorts are like aqueduct redirecting water (current) from the river elsewhere (circuit).

Thanks all!