Dunlop Nultiwah acting like a volume pedal **SOLVED**

[AntKnee]

My 535Q multi wah is acting like a volume. I checked the solder joints around it and it looks fine. The pcb is all smd, with the exceptions of a few 220uF caps. I researched it a bit and found that the 4.7 cap needs to be checked, also. There are four of them, and I'm not sure which I should check. Looking at the hand drawn schematic, I think it is C3, but could someone help me out with that? Also I want to test the inductor itself, so how would I do that?





With component numbers:

[jimilee]

What happened before it stopped working properly?


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[AntKnee]

I honestly don't know. I haven't used it in a while and I was surprised to find it acting this way. It may have been bumped or something is all I can think of.

[wgc]

Check the inductor, and the cap connected to it.

You should get a fairly low impedance measurement on the coil, i.e. not infinity. I'm thinking a bad solder joint or connection more than a faulty part.

[somnif]

Are the parts numbers in the hand drawn schematic your own, or based on the pcb? Because I am seeing a bit of confusion between the two. If your hand drawn it would be C3 to check. On the PCB it would be C12 (I think, some of the traces are a bit tricky to track)

[AntKnee]

Are the parts numbers in the hand drawn schematic your own, or based on the pcb? Because I am seeing a bit of confusion between the two. If your hand drawn it would be C3 to check. On the PCB it would be C12 (I think, some of the traces are a bit tricky to track)

Yeah, I am seeing the same thing. I don't think the hand drawn one is accurate. I did not draw it. Some values do not match the Dunlop schematic. I also could not even find a C3 on my pcb.

I was poking around on the pcb and discovered that the inductor does have some play in it if I wiggle it. It has a slight snap to it. It appears that it is shifting slightly, like the plastic casing is moving up and down the legs. I have desoldered and resoldered it already, and I don't think the joints are moving, I think the inductor is moving on the legs. Would that indicate a structural failure?

[somnif]

Not necessarily. The plastic is just a cover, not structurally connected to the coil inside. Check this blown up pic:

https://www.tubesandmore.com/sites/default/files/uc_products/p-ecb-fi-02.png

The actual connections are those tiny copper wires that connect to the legs on the bottom of the white plastic. So a bit of play of the plastic elements may be nothing, or if the legs have actually come loose from the white plastic that wire is pretty fragile and may break off. Did you happen to check the resistance of the inductor when you had it desoldered? (If you are an absolute EEE101 wizard you could probably math out what the resistance should be in situ, but that makes my brain twitch to think of)

[AntKnee]

Not necessarily. The plastic is just a cover, not structurally connected to the coil inside. Check this blown up pic:

https://www.tubesandmore.com/sites/default/files/uc_products/p-ecb-fi-02.png

The actual connections are those tiny copper wires that connect to the legs on the bottom of the white plastic. So a bit of play of the plastic elements may be nothing, or if the legs have actually come loose from the white plastic that wire is pretty fragile and may break off. Did you happen to check the resistance of the inductor when you had it desoldered? (If you are an absolute EEE101 wizard you could probably math out what the resistance should be in situ, but that makes my brain twitch to think of)

I pulled the inductor back off, and checked the resistance. I measured 15.3 ohms. I also took a good look at it with a magnifier and the copper wires appear to be intact. I was afraid of exacty what you described, but that appears not to be the case.

[somnif]

That sounds about right.  Ok, check the power connection,  if it's gone wrong that can cause oddness.  Also make sure there are no tiny trace breaks.

[AntKnee]

I inspected the metal trace around and inside the inductor holes, and i appears that one of them may be missing some metal inside the hole. However, the circular pad on both sides are intact. It may not be making a good connection if solder is not reaching both sides. Tomorrow I am going to try and solder it on top and bottom and see if that fixes it.

[AntKnee]

Yes, this is exactly what is happening. I do not have continuity though the hole  for the leg connecting to R7 (bottom hole). I am going to solder the top and bottom side and that should fix it. If it doesn't solve the problem, I'll have to jumper the inductor to the proper connection to fix it. Either way, I know what the problem is.

I'll post up when I get it working again.

[AntKnee]

Solved!

I had to solder on some extentions for the inductor legs to enable me to solder both sides to the board. This solved the problem and I now have a working wah!

Thank you for your help, somnif!

Here's what I did:

[somnif]

Awesome! I'm still dreading the day I have to kludge a fix on a SMD board (shaky hands will make that an interesting experience) but its good to know that sometimes a simple fix is all it takes. Hell, my first "paying" gig in the effects world was diagnosing and repairing a buddies Zoom multi effect pedal that boiled down to the input jack solder joint cracking. (I got a bowl of orange chicken and chow mein for that job, it was awesome)

Glad its working again, and happy to help!