I currently have a Tonebender MkII on the breadboard which sounds good except that it barely reaches units gain. I noticed that the battery is very low so I tried a brand new one measuring 9V. When using the new battery the circuit doesn't work. I tried the second new battery from the two pack with the same results. I tried the battery again and it work as before, without quite reaching units gain.
This seem very strange. Does anyone have any ideas where to start looking for a solution?
Thanks
Paul
Sounds like a bias problem. It is biased for a low batt but no for fresh ones.
Tonebender and Fuzz Face have the same problem, low output level. The solution is to increase the 470 resistor to 1k or so.
Do you have the transistors voltages for both batts?
mac
Doesn't sound strange really but does sound like problem.
DC voltage measurements between Ground and transistor pins can reveal more.
I am using a 1K in place of the 470. Below are the negative voltage measurements with respect to positive ground.
Q1
c 8.89V
b 0.72V
e 0V
Q2
c 0.64V
b 0.01V
e 0V
Q3
c 4.27V
b 0.63V
e 0.5V
Q1 collector is to high. Best values are within 8.5v - 8.0v. Try a 1M from base to collector. If the transistor has very low leakage it won't bias, as a silicon. But the base at 0.72v is strange, like a silicon.
Q2 collector is ok, but the base should be around 0.1v - 0.2v for a Ge.
Q3 is ok.
mac
I tried a 1M from base to collector on Q1 which brought the collector voltage down, but still no sound with a fresh battery. I then tried swapping Q1 with a new transistor. This brought Q1 collector to 8.6V, base to 0.007V, and Q2 base to 0.95V, but still no sound with a fesh battery (sound with the older battery).
QuoteThis brought Q1 collector to 8.6V, base to 0.007V, and Q2 base to 0.95V
Q2 voltage should be independent of Q1. Check the cap betwen Q1 and Q2.
What resistor are you using for Q2 collector? If the transistor is leaky a 100k is too much, maybe a 47k.
mac
Q1
c 8.89V
b 0.72V
e 0V
Guessing battery voltage @9v, with the collector at almost 9v...something's pulling it up toward 9v/
Q2
c 0.64V er...not sure what's up with this, hardly 1 diode drop above Q1E = Gnd. .. try smaller resistor?
b 0.01V Kinda low for some reason, should be a diode drop above E
e 0V Connects to ground = 0.0v = ok
Seems like it could be problem transistor if everything else measures and looks right.
Q3
c 4.27V Guessin battery @9V or so, not a bad bias point for a Fuzz Face Q2 collector
b .63v
E .5v
It sort of looks like the second gain stage [a Fuzz Face] is ~near a workable bias, perhaps try injecting any signal and seeing if this stage [Q2 and Q3] isn't amplifying.
Hard to say exactly what's up with Q1, why it's not biased, other than something is causeing the voltage of it's collector to be pulled up very near the top rail.
What lay out? Strip board?
The layout I'm using is the one that came with the transistor set from Small Bear. I think I already tried swapping the cap between Q1 & Q2, but I'll try again along with all of the other suggestion when I get home this after noon.
Btw, thanks for all of your help.
Paul
Seems like DC should be easy enough to detect across a cap, like if there's a different DC potential on one side than the other...chances would seem alot better it's not leaking .
I found my notes on the TB.
q1: With very low leaky 2sa52, almost like silicon, I had to use a resistor of about 1M from B to C to bring C down to 8v. Leaky 2sd352 worked ok without the feedback resistor.
q2: With 2sd352 I had to reduce the 100k to 47k for the second stage to bias. With 2sa52 the 100k worked ok.
q3: Leakage is not a problem, nor gain. Just use a pot/trimmer to set C to 4.5v. The higher the leakage at Q2 the higher the value, but remember to increase the 470.
http://www.diystompboxes.com/biascalc
mac
I tried some other transistors, and used a 1m between bas and collector of Q1. This didn't bring the voltage down enough so I used a 4.7M. I also swapped the cap between Q1e and Q2b, and used a 47k for Q2c. Below are the voltages I now get. It now produces a very faint sound with the fresh battery.
Q1
b 0.08
c 7.87
e 0
Q2
b 0.09
c 0.56
e 0
Q3
b 0.56
c 4.60
e 0.44
Readings are Ok now. Do you have an audio probe to test if there is signal out of the transistors?
If it is on the breadboard you could put a 0.1uf cap in all collectors and then to the amp.
mac
Quote from: mac on March 08, 2008, 09:02:02 PM
Readings are Ok now. Do you have an audio probe to test if there is signal out of the transistors?
If it is on the breadboard you could put a 0.1uf cap in all collectors and then to the amp.
mac
The signal seem fine from the first two stages.
Success!
Seems that I had a bad connection on the level pot. The gain and volume now are mind blowing. I'm very pleased.
Thanks for all you assistance.
Paul
Cool!
mac