I built a TB MKII using the board and layout from GGG.I got the transistors from SmallBear.It was sounding awesome and all of a sudden it started howling.When I turn the gain down it drops the pitch.Below 3:00 the noise goes away but the pedal still sounds weird.I've checked the transistors and they seem fine.Any ideas on what it could be?
Thanks,
Billy
Check the filter cap across the power supply.
mac
Is it set up with PNP transistors and negative ground?
Quote from: R.G. on March 13, 2008, 10:43:51 AM
Is it set up with PNP transistors and negative ground?
You should make that your "signature", R.G. :D
It might save you from having to ask all the time !
I should, but putting it in a signature is placing it where it will certainly be ignored. :icon_lol:
Quote from: mac on March 13, 2008, 06:29:04 AM
Check the filter cap across the power supply.
mac
I found that the solder broke free from the board on the power supply cap.I fixed it but it's still howling.Hmmmmmmmm
Quote from: R.G. on March 13, 2008, 10:43:51 AM
Is it set up with PNP transistors and negative ground?
No it's PNP positive ground.
Any ideas on what it could be?
Yes, it is a Tonebender which doesn't function correctly and needs debugged.
See the debugging thread, or at least post battery and circuit point voltages [from Gnd. across supply, then to each transistor pin].
When I turn the gain down it drops the pitch.
= Oscillating
Below 3:00 the noise goes away
pitch and oscillation are gain dependant
If
but the pedal still sounds weird
Q2 and Q3 of Tonebender is a Fuzz Face [so all fuzz face related debugging info applies]
Q1 of tonebender is the extra gain stage tacked on the FF's front end.
Quote from: R.G. on March 13, 2008, 10:55:13 AM
I should, but putting it in a signature is placing it where it will certainly be ignored. :icon_lol:
;D
I'm such a newbie ;D
I'll I took some reading and here's what I got
Voltage at the circuit board end of the black battery lead = 9.62
Here's the Schematic http://www.generalguitargadgets.com/diagrams/tonebender_m2p_sc_pp.gif
Q1)
C= 7.05
B= 0.09
E = 0.00
Q2)
C= 0.40
B= 0.11
E=0.00
Q3)
C= 0.45
B= 0.40
E= 5.30
Do the readings look okay?
DId you socket the transistors?
Find the Tonebender 'ballpark voltages at ROG, Homewrecker or GGG [I forget where I see those], or use just the FF to debug the FF portion of the TB..
and start with Q1, treat it like the booster stage it is.
Q1)
C= 7.05 Collector is near top rail, what does it's bias resistor measure?
B= 0.09 Base needs to be >1 diode drop above emitter, check any resistors *connected to it
E = 0.00 Seems 'near 'nuf' to ground being at exactly ground potential.
*When measuring these resistors, choose 'distant point' [farthest probe point] to also test as many connections as possible at the same time
Q2)
C= 0.40 {Low, apply DMM to measure bias resistor,
B= 0.11 [Should be higher, probably comes up when collector does,
E=0.00 = Gnd.
Q3)
C= 0.45 {Measure bias resistor,
B= 0.40 [nothing but Q2 collector on this one ;)
E= 5.30 {Oh my, much higher than collector, which is wierd, collector should be the one connected nearest the voltage farthest from ground...I suspect typo, mis-pinout or...hmm...check everything including resistance to ground here, 100k to Q2base etc. something funny going on here.
Use the search function, keywords : Tonebender voltages
Q1: if the transistor is leaky then collector voltage goes down. If leakage is very small, like a Si, then you need to add an extra resistor of about 1M from C to B.
I'd like Vc1=8.0v or so.
mac
I really appreciate the help guys.I ordered another board from GGG.I'm going to debug this one first.I have a lot to learn and this place is full of valuable info.I'll use the search function next time ;)
Thanks,
Billy
If you plan to build more pedals I suggest you to buy a breadboard. It will save you a lot of trouble.
mac