DOD FX-17 Wah/Vol De-Bug Help

[emilyandmiles]

I've a DOD FX-17 that works fine apart from having no sweep.
Mine has a vertical scratch on the far right, through the plastic sheet exposing the copper on the PCB and i'm guessing that's what the problem is.
Does anyone have any suggestions on how to repair this / what material to use to replaces the torn plastic sheet with?
Cheers guys, and MERRY CHRISTMAS!

[Steve Newton]

From these sources...

Hand drawn schem -
http://www.diystompboxes.com/cgi-bin/webbbs_scripts/webbbs_config.pl?read=201

PDF -
http://www.diyguitarist.com/Schematics/FX17-Schem.pdf

Paul Marossy's page on the FX-17 -
http://www.diyguitarist.com/DIYStompboxes/FX-17.htm

Looks like the rip in the plastic sheet could be the problem, the metal "cam" could be shorting through it rather than acting as a variable capacitor. Or the scratch has changed the capacitance to a range that's useless for the rest of the circuit.

How thin is that plastic sheet? have you got anything else lying around that's thin enough and non conductive?

[emilyandmiles]

Yeah...

I've tried changing the plastic sheeting with various things (although nothing exactly the same.

I'm guessing it's the scratch that's messing things up, as the sweep is locked at the top end (if the resistance was too great surely it would be at the base of the sweep).

Can anyone think of anything to cover the scratch with? (non conductive pen, glue, thicker plastic etc)

[Steve Newton]

Have you tried sellotape behind the plastic where the metal bracket thing can't catch it and drag it off? You never know the simplest solution might work.

Also what about all the trim pots shown on the schem?Can they be adjusted to compensate electrically for the altered mechanical changes? Tweaking them is mentioned here   - http://www.diyguitarist.com/DIYStompboxes/FX17Mods.htm

It's such a weird set up that you're probably going to be blazing the trail in learning how to fix these things 

[GFR]

Looking at the schematic...

Measure the output of the 0-5V jack with a multimeter and see if it follows pedal position.

There's a trimpot (P1), did you try tweaking it?

With the pedal disassembled, try approaching the plate with your hand or a grounded plate and see if it sweeps.

Else... do you have a scope?

[Paul Marossy]

Hard to say on this one. I think the whole piece of plastic needs to be replaced. It looks like a piece of mylar - kind of like the mylar sheets I used to draft on 15 years ago, only about twice as thick.

[emilyandmiles]

HA!

So after trying everything I could think of and not getting anywhere, turned out a trim pot leg had snapped

Ended up swapping the plastic for some parcel tape. Seems to do the job.

What a cool pedal.

Phew! 

[Paul Marossy]

Cool, glad you got it fixed. I think you want something a little more durable than parcel tape, though. That should buy you some time to find something that'll work for a long time to come. I really like it as a volume pedal, it works very well, no scratchy pots - ever. 

[Steve Newton]

Glad that it's fixed. I have to agree with Paul though, a more durable replacement for the "dielectric sheet" should probably be found. Though the parcel tape shows that the material isn't critical, any thin insulator will probably do. You just want the toughest you can get.

[Paul Marossy]

You just want the toughest you can get.

My point exactly. 

[emilyandmiles]

Yeah, I've taken to carrying the original plastic sheet around in my pocket, constantly on the search for the perfect substitute (believe me, it's harder than it looks)

By the way, cheers for the info on yr site Paul, that proved very useful.

I'll be swapping the trims for offboard pots soon, will post some pics when of sound enough mind to get round to it (still much left-over booze from Christmas, and then there's New Year!)

Should I lubricate the U shaped metal earthing part?

[Paul Marossy]

Yeah, I've taken to carrying the original plastic sheet around in my pocket, constantly on the search for the perfect substitute (believe me, it's harder than it looks)

I know, that's not an easy thing to find. 

By the way, cheers for the info on yr site Paul, that proved very useful.

You're welcome. It was worth making the page if it helped at least one person. 

Should I lubricate the U shaped metal earthing part?

I wouldn't. It's not really necessary, and might actually make it do funny things... 

[emilyandmiles]

I know I'm going off on a tangent here, but does this pedal have any similarities to the way Zacharys' "Probe" series pedals work?

[Paul Marossy]

I know I'm going off on a tangent here, but does this pedal have any similarities to the way Zacharys' "Probe" series pedals work?

Simple answer is, no. 

[emilyandmiles]

Simple answer is, no.

   

[hoerni]

Just picked up one of these things the other night at the local GC.  Didn't sweep at all but my dielectric sheet was perfectly intact.  Figured it would be a fun debug.

Started probing around the pedal with the scope and noticed the signal from the ring oscillator (first 4007 chip in the schematic near the fixed plate) wasn't getting far.   Thought it might be a bad MPS4124, but then I noticed one of the legs to this had a broken solder joint.   5 seconds with the soldering iron and it was working again. 

A little disappointed that it was too easy of a fix, but happy to have another wah.  This has a cool sweep that I think would work for bass synth sounds.

[hoerni]

Simple answer is, no.

   

Actually I think it does.  From what I can tell looking at the schematic and briefly playing with mine, it uses a variable freq oscillator whos oscillating frequency is determined by the variable capacitor created by the pedal part.   The frequency is then filtered and integrated to give a control voltage out.  The red LED (and possilby the green one too) vary in intensity with the sweep of the pedal.

From what I understand of Z Vex's design, he uses a similar method.  His works more like a theremin though so your foot doesn't need a ground wire attached to it

[Paul Marossy]

His works more like a theremin though so your foot doesn't need a ground wire attached to it

That's the part I was getting at.