Question about Wah mod problem

[Govmnt_Lacky]

I received a Guitar Research wah in a trade with another fellow forumite. I proceeded to use as much of the original parts as possible to transform it into a JH-1. Here is what I did in order:

For reference:



1) I replaced the inductor with a 500mH 235ohm inductor (actually was able to fit it into the casing of the inductor that the wah came with)
2) Replaced the 100K resistor that was on the GR board with an 82K (R10)
3) Replaced the stomp switch with a good SPDT (not looking for true bypass
4) Replaced all of the transistors (originally had 2 x sc1815s stacked for Q3 and sc1815s each for Q1 and Q2) I put an MPSA14 for Q3 and MPSA18s for Q1 and Q2. The schematic calls for an MPSA13 for Q3 but all I had was an A14.

I then tried out the wah and it sounded pretty good. It was not perfect but at least the circuit resonated and was "wah-ing."

Next I did the following:

1) Added a 0.022uF poly cap between the wiper of the pot and ground (Lug 1). I put the cap right on the pot. (C9)
2) Added a 0.01uF "greenie" cap (C10) in parallel with the 33K inductor resistor (R9). I drilled holes in the existing board and just soldered the cap to the same points as the 33K.

The 0.01uF sweep cap was already in the circuit. (C3)

Now, my circuit acts like the old volume/treble pedal. It does not resonate at all.

Could it be that the two caps that I added caused the circuit to stop resonating? I HIGHLY DOUBT that the inductor fried as I never even touched it after my initial test when it worked.

[joegagan]

my  first reaction is that you may have knocked something loose while doing these additions. the things you added don't seem like they could cause it to stop wahing on its own. the most common culprit when a wah doesn't wah is the 4.7uf cap to ground at inductor connection, or related to that area.

don't know the specs of the mpsa14 but i assume it is a suitable replacement.

a couple thoughts.

in 99. when your schem was drawn, i am pretty sure dunlop was installing 470k pots in the JH1. don't know why their schem doesn't reflect this. i have an unproven theory that the 470k was to 'mellow' the wah, make the modern circ sound more vintage, which raising the pot value does. but, what it also does is add noise vs the 100k pot. so i suspect the .o22  to ground at the pot was either to cut some hiss; or further reduce highs for the vintage JH tone, or both. what  pot are you using?

not sure what the cap paralleling the inductor is supposed to do.

your 235 ohm reading on the inductor. , in most cases will not sound very lively in this circuit. someone with better math skills than me might be able to tune the components to work with this value, but generally this circ sounds much better if the 500mh inductor is 35-50 ohm ( stack of dimes style), but others have liked as low as 18ohm ( filmcan, some 70s tdk brown).

i would go back to the config that was working and go from there. if you have a more 'normal' inductor, try subbing it in to see how it sounds in comparison.

[Govmnt_Lacky]

so i suspect the .o22  to ground at the pot was either to cut some hiss; or further reduce highs for the vintage JH tone, or both. what  pot are you using?

I am using the stock GR 100K pot. My research has told me that the newer JH wahs use the 100K HotPotz.

your 235 ohm reading on the inductor. , in most cases will not sound very lively in this circuit. someone with better math skills than me might be able to tune the components to work with this value, but generally this circ sounds much better if the 500mh inductor is 35-50 ohm ( stack of dimes style), but others have liked as low as 18ohm ( filmcan, some 70s tdk brown).

i would go back to the config that was working and go from there. if you have a more 'normal' inductor, try subbing it in to see how it sounds in comparison.

I consulted the forum about these inductors before and was told that the "higher" resistance should be negligible.

http://www.diystompboxes.com/smfforum/index.php?topic=92055.0

As for the future debug... I will probably pull out the C9 and C10 caps and retest. If all is OK then reinstall one at a time and move on from there.

EDIT: Joe... did you get me email response? Never heard back 

[pinkjimiphoton]

usually if you're getting that weird volume/treble thing, it's cuz the 4mf cap is bad in my experience. check your solder joints, and re-heat, it may save ya some debugging.

[joegagan]

yes, the later JH wahs use a 100k. i am very anxious to find out if they dropped the .01 paralleling the inductor and the cap to ground off the pot. my hunch is that the new ones will not have those caps.
got your email-will get back soon.

[Govmnt_Lacky]

OK... let me start by saying:

SLAPS FOREHEAD PROFUSELY!!!     

It turns out that AFTER pulling out the 0.022uF and the 0.01uF caps (across the pot and across the inductor) and retesting, the wah STILL did not wah!

Then it hit me.... I had adjusted the pot travel by about 2 gear teeth to try to get rid of some scratchiness. Apparently, this wah will only wah when the pot is as far CCW as possible in the toe-down position!!!!

After adjusting the gear teeth to the point where a micrometer further would cause the pot to break when pushing down to switch on/off.... it snapped back to life. I honestly did not think that the pot would have to be at such an extreme to work properly.

Oh well.... lesson learned! 

Next.... putting the caps back in (one at a time of course) and testing then a pot change to get rid of the scratching. 

[joegagan]

not sure if it makes sense mechanically, might be a minor geometry difference ( the straightgear is not perpendicular to the travel), but:

seems the switch travel problem is improved when the pot is seated at the lowest ( wah upside down) position in the cradle. not as good for tightening, as the casting has more thickness at bottom than sides = uneven mounting surface.

remember me talking about the tapering being the same but the actual start and finish points within the rotation being different in different pots? that is what might be coming into play here. you may have one of the hot pot types that starts its rise very late, so the treble part of the sweep is at the far end where you need play to make the switch actuate.

[Govmnt_Lacky]

Joe,

All I know is this:

When I "adjust" the pot to anything other than the absolute MINIMUM, which is to say when I loosen the treadle clamp and manually rotate the pot above it's FULL CCW position, all I get is a slight treble boost. When the pot is MAXIMUM CCW during switch depression, then the wah works as advertised.

Now, whether this has to do with taper is beyond me. I just found it hard to believe that the pot had to be set almost to the point of breaking the shaft when actuating the SPDT.

Like I said, lesson learned. I am in the process of investigating "other avenues" of pot tapers that you already know about. Stay tuned for findings. It may be a while (1-2 months) but I will get this sorted out. Until then, I need to get a hold of another pot for this. Preferrably a CHEAP, non-scratchy one 

[joegagan]

i will send you some pots pot. same return address as the peds you sent me?

you can try out two: arteffect icar, + hotpotz1?

that pot you have is a great candidate for a touchless wah switch setup heh heh. what is the date code on it?

[Govmnt_Lacky]

i will send you some pots pot. same return address as the peds you sent me?

you can try out two: arteffect icar, + hotpotz1?

that pot you have is a great candidate for a touchless wah switch setup heh heh. what is the date code on it?

You are a gentleman and a scholar Joe 

Same address. And it is greatly appreciated.

As for the original pot. It is scratchy like I said but you are more than welcome to it (least I can do!) Let me know if you want the info from it otherwise I can just ship it to you after I swap in your pot!

Either way, PM me if you have any Qs

[joegagan]

oh, i forgot you said it was scratchy. i now say it is a candidate for the nearest trashcan, after stripping the gear/nuts etc off of it..

[Govmnt_Lacky]

..after stripping the gear/nuts etc off of it..

Why but of course!