problem with modded wah?

phintze · March 15, 2011, 04:10:05 PM

I have been in the process of modding my crybaby, and I ran into a problem today. I installed trimpots in place of several resistors, and I discovered that there is a short on the "Q" resistor traces. I have triple checked all of my solder points with a multimeter, and I can't find any shorts. I am at a loss and would be grateful for any advice or assistance.

Govmnt_Lacky · March 15, 2011, 04:15:05 PM

Quote from: phintze on March 15, 2011, 04:10:05 PM
...and I discovered that there is a short on the "Q" resistor traces. I have triple checked all of my solder points with a multimeter, and I can't find any shorts. I am at a loss and would be grateful for any advice or assistance.

I am at a loss too.

You say there is a short on the "Q" resistor traces. Then, in the very next sentence, you say you cannot find any shorts??"

phintze · March 15, 2011, 04:42:00 PM

What I meant to say is that I can't see any obvious solder points where I may have inadvertently created a short.

Govmnt_Lacky · March 15, 2011, 04:50:01 PM

The same method for finding out you had a short on the Q traces SHOULD BE the same method you use to find out where the short is. Just break it into small sections and go one-by-one until you find it.

Good Luck

phintze · March 15, 2011, 11:47:44 PM

I had another old crybaby PCB laying around, so I decided to start experimenting with it to see if I could get this one working. When I started out, there were no shorts in the "Q resistor" section of the PCB. However, the problems started when I removed the buffer components. I removed three components (Q3, R14, and C9) and decided to check for shorts. All of a sudden, there was continuity in that Q section. Is there some known issue with this? Am I forgetting some crucial step here?

Govmnt_Lacky · March 16, 2011, 08:00:18 AM

Sounds a lot like a solder bridge to me. Post some pictures of the build and voltages would not hurt either.

phintze · March 16, 2011, 11:16:26 AM

Here's a pic of the PCB. I know it's ugly - this board is several years old, and I lifted a lot of traces before I figured out how to use desoldering braid. Even though it looks bad, I can't find any solder bridges anywhere.

Here's a full size image: http://dl.dropbox.com/u/20622/wahpcb.jpg

The place where I'm having issues is on the resistor immediately to the right of the inductor. I'm going to try replacing the buffer before I do anything else.

Govmnt_Lacky · March 16, 2011, 11:27:28 AM

Sorry but I can't view dropbox while at work. Gotta try later.

Maybe someone else will chime in

phintze · March 16, 2011, 12:10:31 PM

Cool, man - I appreciate your willingness to help!

Talon5051 · March 16, 2011, 06:36:40 PM

Did you put a trim pot in for the Q resistor? Can you check to see if the pot is bad? You could try cleaning the flux off of the board. I use alcohol and a stiff brush. This seems to work pretty well. Also, what does the other side of the board look like?

phintze · March 16, 2011, 10:17:40 PM

At first I put a trimpot in, but then I replaced it with the original resistor. The weird thing is that the connection is shorted out even when nothing is soldered between the two points. In other words, I removed the trimpot, tested the connection, and my multimeter showed continuity. I know that my soldering isn't clean, but I can't find any solder bridges anywhere.

Talon5051 · March 17, 2011, 07:31:06 PM

Remove the inductor and test then. Check the resistance of the inductor. It is in parallel with the resistor that you are checking and if that is shorted you will read continuity. The inductor itself should read around 15 ohms.

phintze · March 20, 2011, 12:05:41 PM

Where do I check the inductor? I removed it and it is showing continuity between two poles diagonally. Does that mean that the inductor is bad?

Talon5051 · March 20, 2011, 02:40:55 PM

When you say continuity are you checking resistance in ohms? What does the inductor measure in ohms? There will be only 2 poles that will show anything as far as resistance. The other 2 will show an open because they are not connected to anything. Before we say if the inductor is bad, please give a reading in ohms. Also, what does the board show now that the inductor is out of the circuit?

phintze · March 20, 2011, 04:20:59 PM

It reads 18 ohms. I didn't check the board - I'll do that when I get home.

phintze · March 20, 2011, 04:59:59 PM

I just checked, and the board doesn't read continuity when the inductor is out of the circuit. I'm assuming that this means that my inductor needs to be replaced.

Govmnt_Lacky · March 20, 2011, 05:23:47 PM

Quote from: phintze on March 20, 2011, 04:59:59 PM
I'm assuming that this means that my inductor needs to be replaced.

NO. 18 ohms is a GOOD reading for the inductor. Looks like your problem is somewhere else in the circuit.

Good Luck and DONT GIVE UP!

Talon5051 · March 20, 2011, 08:23:39 PM

Some more questions
Did you add the components (trim pots) and then check for continuity or did you try it out and there was no sound and then checked for continuity?
Did you measure resistance when the inductor was in the circuit or did you use something like a diode checker and listen for a beep?
If the inductor reads 18ohms and there is no short when it is out of the circuit I think there they may not be a problem.

phintze · March 20, 2011, 10:14:28 PM

Quote from: Talon5051 on March 20, 2011, 08:23:39 PM
Some more questions
Did you add the components (trim pots) and then check for continuity or did you try it out and there was no sound and then checked for continuity?

Here's the process that I went through:
First - the wah was working; it just didn't sound like I wanted it to sound.
I changed a few things at once after that. I wired trimpots in the place of several resistors, converted the circuit to true bypass, and removed the buffer.
When I finished wiring it, the pedal worked, but the bypassed signal was much louder than the effected signal. Also, there wasn't much quack - the effect was extremely subtle. I figured that I had the Q trimpot set wrong. When I tried to adjust it, I discovered that it was shorted.

To troubleshoot, I removed the trimpots (and put the original resistors back), replaced the buffer, and the short was still present.

QuoteDid you measure resistance when the inductor was in the circuit or did you use something like a diode checker and listen for a beep?

I just used a meter and listened for the beep.

phintze · March 20, 2011, 10:40:53 PM

Quote from: phintze on March 20, 2011, 10:14:28 PM
Quote from: Talon5051 on March 20, 2011, 08:23:39 PM
Some more questions
Did you add the components (trim pots) and then check for continuity or did you try it out and there was no sound and then checked for continuity?

Here's the process that I went through:
First - the wah was working; it just didn't sound like I wanted it to sound.
I changed a few things at once after that. I wired trimpots in the place of several resistors, converted the circuit to true bypass, and removed the buffer.
When I finished wiring it, the pedal worked, but the bypassed signal was much louder than the effected signal. Also, there wasn't much quack - the effect was extremely subtle. I figured that I had the Q trimpot set wrong. When I tried to adjust it, I discovered that it was shorted.

To troubleshoot, I removed the trimpots (and put the original resistors back), replaced the buffer, and the short was still present.

QuoteDid you measure resistance when the inductor was in the circuit or did you use something like a diode checker and listen for a beep?

I just used a meter and listened for the beep. However, I just reinstalled the inductor and when I measured the q resistor, it is showing up as 18 ohms.

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problem with modded wah?