wah woes (Vox V847)

[alange5]

A friend gave me a Vox V847 to tinker with.  I've never owned a wah, so I gladly accepted.  The original pot was scratchy, so I replaced it with a Hot Potz II.  I went through the usual mods, and settled on values I'm happy with (as far as overall gain and vocal character).  The problem is the sweep is pretty dark sounding.  Most folks complain that the Vox is overly bright, and jump at the sweep cap to increase its value.  I found myself lowering it, always ending up with a pretty dark sweep.

Today I pulled out the meter and measured the pot, only to find that the max resistance (toe down) is only 80k ohms (supposed to be 100k).  Would this account for the dark sweep and lack of treble in the toe-down position? 

Schematic can be found here:
http://www.electrosmash.com/vox-v847-analysis

My modifications:
True bypass, power supply, and LED
"Vocal mod" R7 changed from 33K to 47K
"midrange" R2 changed from 1.5K to 2K
"gain" R4 changed from 510 ohm to 390 ohm
sweep cap currently stock (a low-measuring .01 - actually about .009.  Various caps substituted, toe-down always a little muddy)
Transistors changed to BC337 - about 400 hfe each
"foxrox" buffer added to output

It's worth noting that these modifications were not done all at once, and my problem (toe-down mud) has been consistent throughout the process.

Is my pot the problem?

[GibsonGM]

Have you loosened the treadle "rack", and hand-adjusted the pot to a nice toe-down treble tone, then put it back together?  1 screw inside the case.

Sorry if you know about this, but some ppl don't!  You adjust the pot to where you want it, THEN attach the 'rack and pinion'....it's how you set the 'max treble'!     

[PRR]

> max resistance (toe down) is only 80k ohms (supposed to be 100k).

That is OK.

The pot is used as a voltage divider. Wiper to top, 100% signal. Wiper to bottom, 0% signal. As Mike says, check that it goes to an extreme.

The pot value is not critical. It should be somewhat more than the thing driving it (R3 22K), somewhat less than the thing it drives (mostly R5 470K; the full analysis is messy but similar). 80K is as good as 100K.

And it seems to be common for pots to run 20% under nominal value. They are specced +/-20% so any wise designer allows for that, as we see here.

Dark? I don't see what would cut treble (or boost bass) other than the intended filtering action. So I dunno.

[alange5]

thanks for the info.

It's very possible it sounds like it should.  I've never owned or played through a wah before, but I'm imagining a more "in-your-face" sound in the toe position.  As it stands, it doesn't "cut" very much.

I loosened the rack, fully depressed the pedal, then attached the rack back to the gear, tightened the screw, and released, so that the fully depressed "button latch" toe position is at the maximum pot rotation.

I'll keep digging.  My primary intended use is wah before a Fuzz Face (fulltone 69 clone), hence the buffer on the output.  It sounds better than it did before installing the buffer.  Before, there was no "wah" when placed before the Fuzz Face, just a weird little notch in the middle of the sweep.  Now it quacks, but it's still a pretty abrupt transition.

I'll keep poking around.

[GibsonGM]

I loosened the rack, fully depressed the pedal, then attached the rack back to the gear, tightened the screw, and released, so that the fully depressed "button latch" toe position is at the maximum pot rotation.

Sorry to come back with this, I'm sure you have done this but it has to be covered for clarity - you've hand-turned the pot all the way to its most treble position, then?

[alange5]

I loosened the rack, fully depressed the pedal, then attached the rack back to the gear, tightened the screw, and released, so that the fully depressed "button latch" toe position is at the maximum pot rotation.

Sorry to come back with this, I'm sure you have done this but it has to be covered for clarity - you've hand-turned the pot all the way to its most treble position, then?

Yup

[alange5]

Think I got it sorted...

Found a bridge on my buffer daughterboard.  After fixing, overall output and high end increased dramatically.  Also changed the transistors to BC109's.

Back in business.  Thanks for the help.