Help with attenutaor box

[DeepEnd]

I posted this in "DIY" but some of you amp gurus can probably help. I built a box to go in the "Pre Out/Power In" loop on my Fender FM65R amp. I wanted to be able to turn down the amp's volume while preserving a strong signal to go to a DI box. The box has three 1/4" jacks--Input, Attenuated Output, and Bypass--and a 5K audio pot. The Input jack is wired to input of the pot and to the Bypass jack. The Attenuated Output jack is wired to the output of the pot.

But there's no signal to the Attenuated Output jack at all and the pot affects the volume to the Bypass jack but in reverse; turning the pot clockwise reduces the volume. I've checked the connections and everything seems okay. A continuity tester indicates that the connection between the Input and Bypass jacks is good. I thought it would be like a simple "Y" connection going to the Bypass jack and the pot, so both simultaneously or either by itself should have a signal, and only the Attenuated Output jack should be affected by the pot, which shouldn't be exhibiting backwards behavior. What's going on?

Do I need to isolate the two outputs? If so, how? Should I simply rewire it so there are two attenuated outputs? Thanks in advance. Here's the design as is. The pot is seen from "below" (i.e., from the side opposite the shaft):

 

[onelife]

If the jack you are using for the Atten Output presents as a short circuit then your box will exhibit the the symptoms you describe. Perhaps the jack is faulty or is a shorting jack like the ones used for amplifier inputs.

 

Use your continuity tester to check between the centre of the pot and ground.

[DeepEnd]

It's definitely not a shorting or "switched" jack. I made sure of that. I'll have to test to see if the jack is shorted somehow and get back to you. Thanks for your help.

[DeepEnd]

I don't know exactly what you mean by "between the centre of the pot and ground." Do you mean the back of the pot and the ground lug? The center (out) lug on the pot and the ground lug? I also tested the two lugs on the jack and got continuity for all three tests. I inspected the jack and I can't see anything wrong. Now what? Replace the jack on principle? Replace the pot? Give up and start over from scratch? Thanks and please be patient.

[onelife]

I should have (and meant to) write "the centre lug of the pot and ground" - I apologize.

 

From what you describe, it seems the centre lug is being pulled to ground for some reason. As you turn the pot up, it pulls the Bypass jack closer to ground resulting in what you described as "the pot affects the volume to the Bypass jack but in reverse; turning the pot clockwise reduces the volume." If you do make the measurement, do so at few different settings of the pot and see what the differences are.

 

My first thought was that the Attenuated Output jack was shorted. You can test for this by removing it from the circuit then checking for output at the Bypass jack as well as measuring from the centre lug to ground again.

 

If removing the Attenuated Output jack has no effect, then I would look for a short circuit at the pot - perhaps a stray blob of solder or a loose wire - or simply replace the pot.

 

Maybe you could post a picture and we might be able to spot something you may have missed.

[DeepEnd]

Thanks again. I'll disconnect the jack and check for excess solder on it and at the pot but my gut is telling me it's a bad pot. Unfortunately, it may be next week before I can get a replacement.

[DeepEnd]

Success! I checked all my connections and found a shorted jack. I corrected the problem and everything works as expected. I also replaced the pot with a 10K one on principle since the input impedance of my amp is 10K.