vox v847 problem!half sound

[gabael]

Helo, i have changed the pot of a v847 vox wah usa, with a standard audio log 100k pot, searching more sweept and range like an early grey vox wah, but  the wah only sounds half of the pot travel, could it be a bad conection or maybe the inductor dont cover all frequencies?.Any sugerences welcome.Thanks.

[Transmogrifox]

With a log pot, the gradient goes from one terminal to the other.  If you hooked it up backwards, then you would make the problem you were trying to fix even worse.

This is how I will label the pot
A
W
B

A = "A" end connection
W = Wiper
B = "B" end connection

Swap A and B and see if that helps.

If that doesn't help, then make sure you didn't land the wiper in the wrong terminal on the PCB.  That would make it seem you lost most of your sweep range.

Log pot will change the way the pedal travels through the range, but it won't make "more swept range".  The high frequency (full to toe) is set by the feedback capacitor and in ductor.  The lowest frequency (full to heel) is set by the gain on the first BJT stage.  Resistor is usually 200 to 600 ohms in most variants of this circuit.  I think the old Vox wahs use 550 ohms -- so reducing to 330 will significantly expand the range.

I am supposing what you mean is you're swapping in a log pot so that the sweep from heel to toe gives you more control over the range instead of being really "toe-heavy" (meaning you rock your foot forward 25% and it feels like you've run out of wah).  The log pot definitely will do that for you.

So...if you get the log pot installed correctly and you know it's right (will sound at least as good as it did before), and you still need range then there are more things you can tweak to get there.

[gabael]

Ah Thank so much, i instaled the wires like in the other pot but maybe the terminals are diferent...ill made the swap and see if it works.
What i understand with a log pot is that you can change faster in frequencies during the foot travel, and so that you gain acces to more range because less travel is required ,but depending on the value o the log, isnt it?

[Transmogrifox]

The obvious thing I forgot to mention is if by reversing the A and B terminals, the sweep is right you will need to buy a reverse log pot because reversing the terminals will also make it so heel is sweep up, and toe is sweep down.

The value of the pot in this circuit is relatively irrelevant as long as it's between 100k and 1Meg.

It's the change in resistance between the terminals vs the wiper that effects how you sweep through the frequency range.  If the log function is backward from what you need, then it will make the perceived problem worse than a linear pot. 

The more I think about this, the more I think you may have got a bad solder joint or something leaving 1/2 the pot disconnected, which would result in some potentially weird behavior in terms of the sweep range.

Either way, recheck solder job, make sure you haven't shorted wiper to one of the legs or just have a bad solder connection that isn't making continuity where it needs to go.

All in all this is a pretty safe and easy mod so it won't be hard to figure it out by checking your work and trying a few different combinations of wiring order.