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Dunlop wah doesn't wah, advice?

Discussion in 'Effects [BG]' started by rojo412, Feb 10, 2021.


  1. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    While cleaning up my basement, I found an old Dunlop GCB95 wah pedal that was given to me many years ago. I remembered why it was free when I tried it out... it doesn't wah.

    - The switch seems to work. When it's on, there's a different noise than off. Almost like it's stuck mid-sweep somewhere.
    - I was turning the pot inside in both modes and there's no change in sound. When the switch is turned to on, sound doesn't change, no matter the pot position. And this is with the "pedal gear tooth strap" or whatever, disconnected, running the full sweep of the pot by hand.

    Looking around, most of the fixes in how-to videos seemed to be replacing switches, but the switch is functioning. And I don't want to replace the pot if that's not the issue.

    Does this sound like something obvious to those who've fixed these before? If so, what should I be doing to fix it?
    I appreciate any help and advice in advance.
     
  2. Just because you hear a difference doesn't necessarily mean the switch is working right.

    That said, I'm not able to give any better advice without more detail.
     
  3. Do your part to spur the economy.
    Buy a new pedal. Sell the old for a parts pedal.
     
    friend33 likes this.
  4. Wasnex

    Wasnex

    Dec 25, 2011
    Here's a good place to start:
    ElectroSmash - Dunlop Crybaby GCB-95 Circuit Analysis.

    I believe there is at least one typo: I believe, "The output of the Q2 is connected to the 100K", should read "The output of the Q1 is connected to the 100K."

    The very first component I would check is the pot. If it's not changing resistance value, the circuit will not produce the desire WAH. Also since the circuit is passing audio, I suspect most if not all of the transistors are passing signal.

    I am not 100% certain Q2 is working since it's role is to route a signal back to Q1. So my "Spidey sense" tells me to the problem is in the vicinity of VR1, C4, Q2, R9, R10, C2. I could be wrong.

    This pedal sells new for $79, so it's only worth fixing if you have the skills to do it yourself.
     
    rojo412 likes this.
  5. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    @Wasnex Thank you for the helpful information here, it's much appreciated.
    I checked the pot with my multimeter and it does sweep from 0.0 to about 93.4 or so, which leads me to believe that it's functioning. It's a $21 part + shipping (or $9 for the non-Dunlop version of the pot), so I'd definitely buy that if it's the problem part here, but wouldn't want to spend that if it's not the issue, especially considering it's 1/4 of the cost of a new one.
     
    Wasnex likes this.
  6. Wasnex

    Wasnex

    Dec 25, 2011
    I assume you mean 92.4K, since it's supposed to be a 100K pot.

    If you have a multi-meter, you can turn the pedal on and measure the operating voltages on the transistors. Spec for the VBSA18 says it's a silicon NPN transistor and voltage from base to emitter should be 6.5V.

    Also, most of the current flows in a transistor from emitter to collector or collector to emitter depending on the polarity (NPN VS PNP). So you can measure the voltage drop across resistors R9 and R10, for example, to see if Q2 is operating.

    This is called DC troubleshooting. To be honest I usually had a lot better luck with an oscilloscope and AC troubleshooting, but you work with the tools you have.

    If all of the transistors are biased and working. I would look at the feedback loop between Q2 and Q1. My take is the resistors R6, R7, R8 are probably essential to the DC bias of Q1. C2, L1 are the reactive components that create the Wah sound. I am not sure if C3 has any role in the Wah sound or not. If it does, I would guess it's pretty minimal. If C2 or L1 or bad the Wah effect will probably be totally dead. Inspect the circuit card for cracked solder connections and broken leads.
     
  7. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Okay, I'm gonna level with ya and give some real talk about myself: I am terrible at following schematics. I know, I really need to get that into my head, because if I plan to do anything major for amp or electronics work, it's vital. But like sheet music, I see it, my brain scrambles, and I am friggin' useless.
    I try... but I fail.

    With that said, I'm fine with layouts or diagrams. Show me how it's supposed to go and I can assemble it. And yeah, that's like the "tab" of electronics work, but it works for me.

    So aside from all that, I think I may have had somewhat of a revelation here. When I was moving stuff around, I hadn't noticed that the bottom of the pedal had something taped to it...

    IMG_8873.JPG
    IMG_8874.jpg

    Not sure why I never put 2 and 2 together on this, but this has been there the whole time. Just thought it was something that got stuck where it was stored. But nope, that's a capacitor (right?).

    Looked on the board and saw this:

    IMG_8871.jpg
    IMG_8872.jpg

    I don't know a ton... but that doesn't seem right.
    Am I onto something here?

    Looking at the pic from electrosmash, what I'm seeing (though not EXACTLY the same look of the board) is that this is amiss:

    Screen Shot 2021-02-15 at 10.32.38 AM.png
    If I had to guess, someone in the past had replaced the .22J63 cap there, but may have removed the cap next to it in the process? And then forgot to replace that?
    The one that is taped to the bottom cover is the same as the .22J63, but with no wire leads on it. It certainly doesn't look like the electrolytic (?) cap that is in the pic and appears to be missing from this unit, so I don't think it was there, but just not replaced.

    Is any of this possible or making sense? Or am I just chasing my tail?
    Oh, and if anyone can kinda show me how this relates to the schematic, it may help me learn, so that is definitely appreciated.
     
    Last edited: Feb 15, 2021
  8. charlie monroe

    charlie monroe Gold Supporting Member

    Feb 14, 2011
    Buffalo, NY
    I would totally buy into that, but I probably make you look like the Jaco of electronics.
     
    rojo412 likes this.
  9. Wasnex

    Wasnex

    Dec 25, 2011
    It hard to know the meaning of the extra .22uf cap. Note that there are two of these on the schematic and also two in the image you provided. The number between the Schematic and board do not match, so definitely different versions of the pedal.

    Someone may have bought the capacitor as a replacement. Normally if you replace a part, you throw the bad part away. Or they may have replaced the part and kept the old part. Is there any evidence of solder on the leads? If no, it's probably a new unused part. Maybe somebody had a guess, or maybe they did some trouble shooting. I think C2 is a more likely source of the problem, but I could be wrong.

    As you have noted, it appears that the electrolytic capacitor for filter section is missing. I don't think this would have any impact on operation if you were running on a 9v battery. The electrolytic is intended to filter out noise (hum). If the pedal is passing signal through the transistors, the power supply section is working. It's a good idea to add the electrolytic back to the circuit IMHO if you get the pedal working though.

    Note if you pedal has a true bypass switch, it will pass signal without any circuit function at all.
     
  10. Alternatively, if you can't get this board to work, there are lots of drop-in replacements or for the adventuresome several other esoteric treadle-based designs looking for a good home (enclosure)... all manner of wah, flangers, phasers, and other wackywoo...

    Most basic last resort, convert to volume pedal.


    Learning a lot from Wasnex, thanks.
     
  11. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    I would definitely consider an alternative!
    Volume pedal/expression pedal is one and seemingly easy, but I'd be game to try something else. Do you happen to have a link or keywords for finding such kits?
     
  12. That electro-smash's description of the circuit is that the magic is at the c2 feedback cap, but there's a big hole on both his photo and your pedal at that location.

    I think their schematic is from a single version, and while it probably does describe the circuit generally, it's not specific to your version. There's no resistors in the high teens on his schematic, for instance.
     
  13. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Maybe this helps?

    IMG_8879.jpg
    IMG_8880.jpg

    And as for the cap taped to the bottom, it has no legs on it, so my assumption is, it was a bad one that was replaced and the tech just taped the old part to the bottom.
     
  14. Dead End FX
    SCOTTY — Chris MacGregor Resonator Wah
    COSMO KING — Royal Cosmo Wah
    LARRY — Mutron Flanger (alas, not with SAD1024, but purportedly a good work-around for that chip)
    TALKALIZER — EHX Talking Pedal fuzz section with Colorsound Vocalizer filter
    TRESONATOR — Frostwave's Resonator
    MR MULTI — Korg Mr Multi combination phaser-wah


    mbp.shop
    HARBINGER TWO — Univibe in a Wah shell - includes bi-pin lamp, DPDT slide switch and BS250p
    WEENER WAH — Wah


    Home - GuitarPCB
    MoWah


    Wah Archives - PedalPCB.com
    PARASITE — Tycobrahe Parapedal Wah
    SHAMWAH! — Vertex Axis Wah aka the BBE BenWah
    TEARJERKER — Tweakable Wah Pedal


    Check out Musikding Onlineshop für Musikelektronik, they carry a number of DIY brands including a few listed above and sell complete kits, not just PCBs.

    I don't know where all the wah is, I keep discovering new small-production bootikinook builders/suppliers all the time, but you can surf to see what wah may be out there beyond the aforementioned...

    1776
    AION
    AMZ
    ARCADIA
    BYOC
    CARCHARIAS PCBs Archives | Carcharias Effects
    DEAD ASTRONAUT
    DELYK
    DIYEFFECTS http://diyeffects.com/products.htm
    GPCB
    General Guitar Gadgets — sure these guys have at least one
    GRINDCUSTOM — defunct, hope they come back soon.
    JEDSPEDS.co.uk
    JMK
    LECTRIC-FX
    MADBEAN
    MASK AUDIO ELECTRONICS
    MKLEK
    MOODYSOUNDS.com
    PCBGUITARMANIA
    PEDAL PARTS PLUS
    PIGEON FX PCB
    Puzzlesounds.com
    TMK
    VIPFX
    CODTONE
    GEO FX
    RunOffGroove
    PUZZLESOUNDS PCBs
    TonePad
    EffectPedalKits Diy effect pedal kits and PCBs | Effect Pedal Kits
    uk-electronic.de
    Aluminium Enclosures
    Big Dipper Effects
    MusicPCB
    Parasit Studio
    Rullywow
    TH Custom
    VIPFx
    Shop
    OP Electronics


    That ought to keep you busy.

    You could also just stick in a simple fuzz circuit, ala what Kay did back in the day, and rig up the fuzz pot to the treadle.
     
  15. Wasnex

    Wasnex

    Dec 25, 2011
    AFAIK the hole in the circuit board is where a screw is used to mount it to the pedal's case. The hole has a solder trace around it, so I believe this is where the circuit ground bonds to the pedal's case.

    upload_2021-2-16_13-45-48.png

    upload_2021-2-16_13-46-46.png

    If you look closely, it appears that a component belongs here. If you check the other board, there is a 22J63 in this position (the mysterious spare part). Perhaps the capacitor simply broke off, and someone cannibalized the 220uF electrolytic for something else.
     
    cheapbasslovin likes this.
  16. That's a good eye. Those two holes clearly have wire in them on the opposite side.

    20210216_123039.jpg
     
    Wasnex likes this.
  17. Wasnex

    Wasnex

    Dec 25, 2011
    cry-baby-wah-gcb-95-bias.jpg
    If I am right the mystery component is C4. So VR1 is functional, but since C4 is missing from the circuit there is no response.


    Q2 should still passe the DC bias test. The role of C4 is to couple the AC signal from Q1 to the base of Q2. The signal is taken off the emitter of Q2 (the arrow) and sent back through C2.

    The question: Did C4 fall off and cause the problem. In other words was the pedal functional until C4 broke off. Or was there another problem, and C4 was broken off during ham-fisted troubled shooting.

    IMHO the first step is to replace C4 and C6 and see what happens. Maybe you will get lucky.
     
    Last edited: Feb 16, 2021
  18. rojo412

    rojo412 Sit down, Danny... Supporting Member

    Feb 26, 2000
    Cleveland, OH.
    Man! You guys are great!

    First off, thanks to @Feral Feline for a worm hole to climb into. That is awesome.

    Second, it totally seems like you guys are onto the culprit here. If that cap taped to the bottom is what used to have the legs soldered in the spot where you pointed, then it's certainly worth pursuing.
    Sounds like I need a 220uf electrolytic cap and a .22uf cap to get it complete, yes? Or at least that's what's worth trying for the basics...
     
    Wasnex likes this.
  19. Wasnex

    Wasnex

    Dec 25, 2011
    Yes. I would try installing new parts to make this complete.

    The disturbed solder on Q2 has me a bit concerned as well. I suggest doing the DC measurements I wrote about earlier to make sure it is biasing properly. If not order a couple of the transistors as well.

    Not sure where I got the P/N I quoted for the transistor earlier in the thread but it doesn't match the P/N on the webpage :eek::facepalm:.

    The P/N for Q2 is MPSA18 and it costs under $1.00. The datasheet says the VEB is 6.5V but the schematic shows 4.3V on the base, with respect to ground. Measure the voltage on the base and the voltage drop across R9 and R10 to see if Q2 is working. If not, order a couple of spares. It's fairly easy fry a transistor while installing it when you are not experienced.

    If you do not know, the 220uF electrolytic is polarized, meaning it must be installed correctly. The negative side of the capacitor will be marked, usually with a solid band and - signs.

    As I mentioned previously I believe the screw hole is the ground bus. I suggest turning the pedal on and measuring between these points to confirm the polarity is correct. (see image)
    upload_2021-2-16_17-18-51.png

    I believe the connection of the Zener confirms the polarity
    upload_2021-2-16_17-19-29.png
    I.E. the arrow points away from ground.
     
  20. @rojo412 You'll fix this one, and then buy another junky one just to try out the DIY stuff! :laugh:


    Learning a lot from Wasnex in this thread, and to think I almost didn't click on the thread-link.

    Also, lately I've learned a lot from cheapbasslovin, who seems to pop up with solutions in a lot of other threads I've been skulking around.

    Kudos to both of them.
     
    rojo412 and cheapbasslovin like this.
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Feb 28, 2021

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