Creepy fingers doomidrive

[nightendday]

I know that this was based on the unidrive, but what changes were made? the youtube clips of this this thing make me want to build it, really bad.

[nightendday]

Ok so I want to build this, but I have no trimpots, does that trimpot control how much voltage is hitting the transistors?

[Keppy]

That trimpot controls the bias on the base of Q3 and the collector of Q2. You can do without it by replacing the trimpot with a resistor straight from the first or second row to the third, but you'll have to experiment to find the right value. Start with 10k and see what changes as you move the value up or down. Either breadboard the circuit first to get it right, or put a socket in that spot to try different resistors. Alternatively, you could solder in a 20k-50k pot in there to experiment and replace it with a resistor when you find your favorite value.

[artifus]

yes, commonly referred to as bias. if it asks for a trimpot of 20k then it wants a value of somewhere between 0 and 20000 ohms (20k) as a bias resistor, depending on the transistor and voltages used, in that position which you would set and forget, usually at half the supply or some other specified voltage and so could be replaced with a fixed resistor in that position. if it is 4.5v then that is what you would want to measure between q2 collector and ground with your multimeter while adjusting your trimpot. you could then measure the resistance of that trimpot and replace it with a fixed resistor of a value as close as possible to that reading.

so you don't need a trimpot if you have a bunch of fixed resistors between the values of 1k and 20k. just swap out resistors and measure that point until you are in the right ballpark of the required voltage. you wont hurt it by swapping out biasing resistors between 1 and 20k so play away and have fun finding a sound that you like. hope that makes sense - bit of a ramble, sorry.

search the forum for 'transistor bias'

*edit* ignore me - keppy knows his stuff.

[nightendday]

Would either of you have any insight into what changes were made to the doomidrive? or know where I could find some gutshots?

[Keppy]

No idea. Sorry!

[slacker]

Could be similar to this http://www.diystompboxes.com/smfforum/index.php?topic=84608.msg864649#msg864649

[Toney]

Hey I did that layout
This was the thread that inspired it. You can see about the various mods here:

http://www.diystompboxes.com/smfforum/index.php?topic=36273.0

[Toney]

Heres the original schem:
http://www.univox.org/pics/schematics/unidrive.gif

[nightendday]

It's described as lower gain, and mostly a boost, which is not how I would describe the doomidrive. HMPH. Looks like I'll just mod this to death and socket the transistors, see what I come up with. 

[nightendday]

So I have no signal, here's the measurements so far. I used a normal 25k pot. instead of the trim pot, and it doesn control the amount of the readings, a LOT, but never any signal.

Q1: E-0.18
      b- 0.79
      c- 2.54
Q2: E-1.38
      B - 2.54
      C - 1.94

Q3: E-0.21
      B - 0
      C - 1.94

[Keppy]

So I have no signal, here's the measurements so far. I used a normal 25k pot. instead of the trim pot, and it doesn control the amount of the readings, a LOT, but never any signal.

Q1: E-0.18
      b- 0.79
      c- 2.54
Q2: E-1.38
      B - 2.54
      C - 1.94

Q3: E-0.21
      B - 0
      C - 1.94

Q2 is completely saturated since B is higher than C, but that's probably just your pot setting. The real problem is Q3. Q3 B should be the same as Q2 C, so there's a broken connection or a missed pinout there. Also, Q3 C connects straight to 9v, but that's messed up as well. Check pinout and connections on Q3.