Trouble With a Boss TU-3

[rmjlmartin]

Hi everyone, and thanks in advance for taking the time to read this.

I have a problem with my Boss TU-3 Tuner. The tuner works, but it doesn't pass any signal through the output jack in the bypass mode. It does, however pass signal through the bypass jack.

Does anyone have a schematic for it?

I've partly traced the circuit with an audio probe. I'm sort of assuming that, since the tuner turns on and off and the tuner part does work, that it's something related to the switching section, but I don't understand how it works well enough to troubleshoot it, and I don't have a schematic for it, either...

Has anyone else run into something like this? Does anyone have any suggestions for troubleshooting it?

Again, thanks in advance for any help you can offer.

[matmosphere]

The first thing I would check is the switch itself. I suspect it is the most likely candidate for being the issue because it is the only moving part.

First thing I would do is open the battery part and try depressing the switch with your fingers and see if it works.

If not I'm guessing you could short across the switch and check if there is any change.

Beyond that, I don't know exactly how the TU3 handles switching. I vaguely remember seeing something about it being true bypass, which seems odd for Boss.

[rmjlmartin]

Thanks!

Actually, though the footswitch works, because the pedal turns on and off.  It just never puts any signal to the output jack.

The TU-3 is definitely not true bypass. I'm assuming that it's a fairly standard Boss buffer and flip-flop (?) swiching system. I just don't understand well enough exactly how the buffer/switching works to be able to troubleshoot it, especially without a schematic.

[matmosphere]

Didn't catch that sorry.

I looked it up, I think the waza version has true bypass.

Did you check the connections for the output jack?


Wish I could be more help.

[rmjlmartin]

No worries, thanks for your thoughts anyway.

I'm happy to report that it is  fixed.

I have a friend who has a TU-3, and he was kind enough to let me borrow it to compare with mine.

The way Boss laid out the circuit board in this pedal is interesting. When you lift off the bottom cover, you'll see the back of the circuit board. To get to the side where most of the components are  you have to lift the board up.

But, there are a number of spots that have connection through the board (to the side you can see when you remove the cover) and are labeled as "TP". (Test point, maybe?) After a bit more poking around, I found a drop of solder bridging one of the test points, which connects to a transistor (that, from it's position, I believe is the output buffer) to ground. And after I popped it off, the pedal has worked perfectly

I don't think it looks like anyone has been soldering anything in it before, so it's possible that it's been there since it was new. But how that would have passed QC, I'll never understand.

Thanks again for taking the time to read and offer your thoughts.