Dunlop JD-F2 Fuzz Face: Help with trimpot biasing?

[sixstringphil]

A friend asked me to help him with his Fuzz Face. It's the red one with the Dallas Arbiter smiley face label. The board has 2 trimmers for of each NKT275. This is the pcb with the large 330uF caps. I set the collector of Q2 at 1/2 the battery voltage. But when I try to adjust the Q1 trimmer,  Q2 voltage changes. I guess what I'm asking is:
What should the collector voltages be for Q1 & Q2,  and how do I set one without messing up the other?

I've read through RGs Technology of FF several times, but don't really see anyting there that applies to this one.
I've set the trimmers by ear, but I'd like to know better how to set the voltages.

I've considered ordering germs from Smallbear. Would there be noted improvement, or would I be better off etching a traditional FF board and starting from scratch?

[George Giblet]

> But when I try to adjust the Q1 trimmer,  Q2 voltage changes.

That's a weird circuit and I'm not entirely sure what the motives were behind it. 

Both pots will affect the  Q2 collector voltage - that's normal.  That's part of what is strange about the circuit.  The circuit has a thermistor on the collector of Q1.  Given that, I suspect part of the motivation for the circuit is to try to make the FF operate consistently over a wider range of temperatures.

Here's how I'd go about it:

- To start set the 100k pot to the mid positiion - that makes some aspects work like the original.
- Adjust the 1M pot so the collector of Q2 is at 4.8V (or whatever you like the sound of)
(- If you can't get Q2's collector voltage to the desired voltage leave the 1M pot in the max'ed out position that gave you the closest voltage, then adjust the 100k pot to give you the desired voltage.)

You can either stop there or, go further:

- Get a hair dryer, and hold it at about 300mm (12 inches) from the board for about a 2 minutes (or more) and observe how the voltage on Q2 changes.  Aim the hair dryer centrally and wave around for even heating.
- wait for the unit to cool down and the Q2 voltage returns to it's original value
- Repeat with the 100k pot set at 25% and 75% positions.
- choose the setting with the least variation
- if no setting is better then the idea behind the circuit is crap

I don't think it's worth while buying new parts.  In the worst case you can just mod that unit to be like a normal FF.

An alternate simpler view occurred to me:  the 100k pot sets the bass response, the 1M pot sets the biasing.  So set the 100k as desired then adjust the 1M to get your desired Q2 collector voltage (if possible)

[sixstringphil]

Thanks George. Very helpful. I got it sounding a little better, but it's still not anything to get excited about. Rather than mod the existing PCB, I'd more likely start from scratch. It's very difficult to follow the nearly invisible traces they use on that board. Thanks for the help!

[petemoore]

the 1M pot sets the biasing.  
  That's strange, 1meg as Q2CR would be hard to set around 6k8 - 12k or whatever gets Q2c biased.
  It would be interesting to find out exactly how a 1meg is used to set bias in this FF.

[George Giblet]

The schematic is here:

http://www.diystompboxes.com/pedals/ff99.gif

The 1M trimpot is on Q1C, even there it will be a little twitchy to adjust.