broken GCB-95 wah...

[shanter]

Ok so i bought this wah off of ebay for 25 bucks. figured i'd try to fix her up and mod her up a bit. if i fail, then ill prbly build another wah and pop it in here. I  e-mailed the previous owner of the pedal, and he said it came broken right out of the box. so there was no misuse that caused the malfunction. But heres whats goin on.

Signal goes throgh the pedal when its off.
When the pedal is on, it sounds like the wah is stuck in the middle of its 'throw'. kinda like its about to wah, but it doesn't.

here are the tranny values. running a 9v adaptor.
Q1- buffer
E 3.75   B 2.44  C 8.81  < something here is fishy

Q2
E .06  B .66   C 4.81

Q3
E3.93  B 3.13  C 8.4

i read a few posts on this pedal and it seems that the inductor is a piece of poop. so could that be the culprit?

[no one ever]

from what i've read, if the inductor is open, it would act more as a volume pedal than a wah pedal, thus not explaining your lack of wah. I would check the pot / see if a known good one works in its place, check traces for cleanliness, and try removing the buffer/building a new one if you're suspicious of the current one.

try the buffer at fuzzcentral for the clyde wah.

[petemoore]

  When the pedal is on, it sounds like the wah is stuck in the middle of its 'throw'. kinda like its about to wah, but it doesn't.
  I'm not real clear, it sounds like the pot is doing a little something, you could try testing the pot by slowly turning it with the dmm first attached to wiper and outside lug, then wiper and other outside lug, looking for 'dead spots' [0 resistance...bad] and sweep between 0k and a little less than 100k on both sides [good].
  Looking at your pin voltages, there is at least a problem with
 
   Q1- buffer
E 3.75   B 2.44  C 8.81  < something here is fishy
  The emitter shows higher Voltage than the base. it is not biased

Q2
E .06  B .66   C 4.81
  Can't tell what this ones purpose is, my wahs have 2 transistors, but the base is sitting very near ground...I think it needs bias help also

Q3
E3.93  B 3.13  C 8.4
  Here again E is above B in voltage, not biased
  I'd try working on the board first, looking for non connect, connect that shouldn't be, just applying the DMM every way you can, measuring resistors in circuit...smaller actual resistance readings than color coded resistor value marks could mean alternate path of current through the board, I'd get the schematic and give the board one serious fine tooth combing, starting with a close in bright light and a magnifying glass, then moving to the DMM applications.

[Paul Marossy]

Maybe there's some kind of defect with the PCB. Or a transistor(s) in there are defective, maybe. What are the transistors? A pair of MPSA18s and an MPSA13?

Double check all the resistor values against the schematic. Maybe some resistors with the wrong values were somehow installed in there. Look closely at the ones associated with the transistors.

About the pot, does the resistance change as the pedal is moved up and down? Maybe it's wired incorrectly...

I really doubt that it's the inductor, but it's also a possibility...

That's all I can think of at the moment.

[petemoore]

  The 47k parallel to the inductor probably doesn't factor in, to matter, and the inductor should have some small resistance across it...maybe you can find a data sheet on something like that, I'd think well under 5k...?

[shanter]

ok im yet to check the resistors again, but i did notice somehthing.

edit
yes its a pair of MPSA18s and a 13
/edit


the pot doesnt seem to change resistance. im not sure which lugs are what but they all read exactly the same when the pedal is in the heel or toe position. So im thinkin its the pot. 

Blue to Black = 103k - both heel and toe
Blue to Yellow = 0k - both
Black to Yellow = 103k - both



also the PCB is very clean, all nice solders.

[Paul Marossy]

Hmm... sounds like the pot may be miswired. Yellow should be connected to the terminal marked "SL" on the pot. Black would be connected to the terminal marked "CW" and blue would be connected to the terminal marked "CCW".

Is that how yours is wired?

[shanter]

i think uve got it!

what i have is Blue on CW and Black on CCW

edit

i jsut looked around on a few schematics, and it seems like the wiring i have is correct. Blue is supposed to go to CW

http://happybob.com/marc/images/HotPotzIIPinout.gif
and
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/dunlopgcb95.gif

edit

[Paul Marossy]

I thought we were onto something there for a minute, but I compared my GCB-95 to your picture, and it looks the same (sorry, I got the blue and black connections mixed up). Did you disconnect the wiring harness from the PCB when you did your measurements? Maybe that affects the readings?

You should measure 100K between the blue & black wires on the pot, but the other readings don't make any sense - they should change when you move the foot pedal. And the wiring harness itself looks to be correct. Strange.

Well then, I would then verify that all the resistors are correct by actually mesauring them with a DMM. If they are correct, then I would have to conclude that maybe there was a bad batch of transistors made and your wah has some in it. I've never heard of that happening before, but anything is possible... 

[shanter]

ok GREAT! news. 

i decided to take a stab at it and see if i could fix this.

i did the true bypass which got rid of some fishy resistors. (2.2M was reading at only 1M - dont know if thats normal, but common sense said it wasnt) then i wanted to rule out the pot and make sure that wasnt the problem. i took it out and hooked up a 50k pot i had sitting around. that worked! so i figured that the original pot was bad. i did some more tests on the pot and saw that it wasnt giving me the constant 103k reading anymore. it was actually responding to the turns. (or maybe my DMM skills suck.) so i went ahead and quickly resoldered the pot back on and it WOOOOOORKED! soo happy.

but i still like my 535Q more. so this one is goin to my buddy.

and.

THANK YOU EVRYONE FOR YOUR HELP! THIS IS TRULY AMAZING!!   

[Paul Marossy]

Cool! I still don't understand what the heck was wrong with it, but hey you fixed it - and that's what counts. 

[shanter]

hehe i dont know either. i guess it was jsut a bad solder. cause i resolderied it exactly the way it was.

[Paul Marossy]

hehe i dont know either. i guess it was jsut a bad solder. cause i resolderied it exactly the way it was.

Must have been!