Fat Pants JR. Debugging. T1 isn't biasing?

[elenore19]

http://www.madbeanpedals.com/projects/FatPantsJR/FatPantsJR.pdf
There is no sound going through the pedal. I haven't boxed it up yet.
I'm working on adjusting the trimmer to bias the Jfet. The voltage at TP doesn't change when I adjust the trimmer.
It's stuck at 26.8V

I checked the voltages of the IC and they're how they should be. Also, the Jfet voltages were off. The Drain makes sense it's off due to the trimmer not accomplishing what it should, but maybe there's something more making the Source be off? I really am a noob at this stuff. What do you think?
D: 26.8V
S: 0V (this should be 363 mV. )
G: 0V

I'm not good at debugging. So from here I checked all solder joints, looked for solder bridges to no avail.

I tested the trimmer and it's resistance is on point and changes when I adjust it.

Help?!

Thanks

-Elliot

[blearyeyes]

http://www.madbeanpedals.com/projects/FatPantsJR/FatPantsJR.pdf
There is no sound going through the pedal. I haven't boxed it up yet.
I'm working on adjusting the trimmer to bias the Jfet. The voltage at TP doesn't change when I adjust the trimmer.
It's stuck at 26.8V

I checked the voltages of the IC and they're how they should be. Also, the Jfet voltages were off. The Drain makes sense it's off due to the trimmer not accomplishing what it should, but maybe there's something more making the Source be off? I really am a noob at this stuff. What do you think?
D: 26.8V
S: 0V (this should be 363 mV. )
G: 0V

I'm not good at debugging. So from here I checked all solder joints, looked for solder bridges to no avail.

I tested the trimmer and it's resistance is on point and changes when I adjust it.

Help?!

Thanks

-Elliot

Really need to see a picture of the circuit board top and bottom to be able to help. Double check the component values, I know it's a pain but putting the wrong value resistors in is very easy to do.  Make sure you didn't put a 4k7 instead of a 47k as an example etc. Also check all the electro caps and make sure they are going in the right direction polarity wise.  Pictures!

[blearyeyes]

Do you have an audio probe? It is the best for sorting out lack of audio.

[gordo]

I've had really good luck with http://pcb.delyk.com/utility-boards/19-effects-tester-mk-ii-pcb.html.  It has a probe and an audio generator all built in.  Easy enough to put all this stuff together yourself but this puts it all in one place.  He makes a kit with all parts as well.

Another one that looks superb, although I haven't used it, is the http://jmkpcbs.com/product/testing-rig/ that JMK makes.  Same concept and equally as clever.

[elenore19]

Update: I checked all the resistors. Also all the polarities look good. They're correct. I did put a 2.2k in series with a 1k for R3 (3.3k).

Really need to see a picture of the circuit board top and bottom to be able to help. Double check the component values, I know it's a pain but putting the wrong value resistors in is very easy to do.  Make sure you didn't put a 4k7 instead of a 47k as an example etc. Also check all the electro caps and make sure they are going in the right direction polarity wise.  Pictures!

Here are some pictures. My camera is garbage. Sorry.
I'll start checking all the resistors!

I've had really good luck with http://pcb.delyk.com/utility-boards/19-effects-tester-mk-ii-pcb.html.  It has a probe and an audio generator all built in.  Easy enough to put all this stuff together yourself but this puts it all in one place.  He makes a kit with all parts as well.

Another one that looks superb, although I haven't used it, is the http://jmkpcbs.com/product/testing-rig/ that JMK makes.  Same concept and equally as clever.

Ooo, that's a great idea. I'll have to order one and try it out.

Pictures:

[blearyeyes]

I’d reflow the solder on the trimmer. Sounds like a bad trimmer or you have a short underneath. It’s a simple board and your work looks good. Frustrating! I’ll peruse the doc.

[midwayfair]

let's say your drain trimpot was broken and was shorted to the power supply. Then you'd just have voltages like a source follower, and the source would be several volts above ground. Maybe something like 11V on the source if it was a little low. Instead you have no voltage there whatsoever.

I think it's more likely that either (a) your source is shorted to ground or very nearly so, (b) your JFET is dead, which is possible if the source was shorted and the trimpot was at 0R when you first powered it up. This is the rare time when a dead part might be more likely than a soldering error. Get out your multimeter, check continuity on all parts connected to the transistor, and measure the resistors (don't just look at them).

[blearyeyes]

Op says while trying to adjust the bias trimpot:

“It's stuck at 26.8V at TP.”
Would it read like that to ground with a shorted Tran?
i got all my EE training here, so never pass on learning something.

[midwayfair]

Op says while trying to adjust the bias trimpot:

"It's stuck at 26.8V at TP."
Would it read like that to ground with a shorted Tran?
i got all my EE training here, so never pass on learning something.

If it's not drawing any current through the transistor, it won't drop voltage across the trimmer. This could happen if the source is somehow just disconnected from the drain, and the only way I can think of that happening is if the transistor is broken.

I mean, I didn't see if the op verifies that there's a connection from the test point to the drain, so it's possible there's no voltage on the transistor. That's why I suggested continuity checking.

[elenore19]

Wow. Thanks for the brainstorming here guys.
Couple notes:
I replaced the trimmer. Same problem.
I have confirmed that TP is connected to the drain of the fet.
I checked and double checked to make sure the source is not shorted to the drain.

I'll try replacing the fet.

[elenore19]

Replaced the Jfet and it works.

Thanks guys! Who knew such a tiny circuit would give me such issues.

I'm sure that the trimmer was zero'd out so maybe that burnt out the fet. Or maybe there was a solder bridge that burnt it out that I had fixed throughout the debug but the damage was already done.

Thanks again guys. Greatly appreciated!

[blearyeyes]

Glad Jon dropped by. I'm happy you got it sorted out.  I see why some people check every component while building.  I don't have the patience for that!

[elenore19]

Seriously, check EVERY component? Shit.
How do you test a transistor before it's in a circuit?

[blearyeyes]

I bought a tester from China that detects virtually any component and tests it.  I think they are down to about 12 bucks now. I’ll post a picture here.  Worth having. Here’s a Link to one on eBay

https://www.ebay.com/itm/12864-Mega328-ESR-Transistor-Resistor-Diode-Capacitor-Mosfet-Tester-w-Test-hook/321963237498?hash=item4af681207a:g:sBMAAOSwa-dWhWzn

[elenore19]

That's awesome. Thanks! Does it come with a manual?