FET stage working only after briefly touching it. What's happening here?

[Fabei]

Hi guys!
Hope you're havenig a nice weekend.

I'm working on a new pedal and I've designed a very simple stage with a J201 in a SOT23 package.
As you can see in the schematic it's pretty straightforward and it sounds very good, when it's working 
buffer1.png

It worked on the breadboard and I've ordered some PCBs from a factory, made with eagle and tested so I'm pretty sure about the board quality.

The thing is: when I connect everything and plug my guitar or a signal generator I got nothing on the speaker or the oscilloscope. While checking the connections I thouched the tranny and it started working. Disconnected and connected again and it kept working. Disconnected for a while this time and connected again, not working. So I touched the transistor again and magic! It works!

I've resoldered it again, double-checked connection but nothing, it always needed the finger magic to start doing is job. So I tried to look closer while I was touching it and discovered that what my finger was doing was connecting the D and G pin of the J201. So I tried with a piece of wire and yes, finger magic was now wire-magic. The weird thing is that I dont need to keep touching it, just a brief connection of less than a second.

Guys can you explain to me what was happening and why the J201 need that to start working? After that brief connection with the finger or the wire it goes on without any trouble at all.

[jubal81]

The gate needs a reference voltage, which in this use is typically a connection to ground. You need another resistor to ground from the gate - large value like 2.2m.


Alternatively, you can just take out C1 and replace it with a jumper.

[Fabei]

The gate needs a reference voltage, which in this use is typically a connection to ground. You need another resistor to ground from the gate - large value like 2.2m.


Alternatively, you can just take out C1 and replace it with a jumper.

Thanks for answering!
Could I've been so stupid to miss it?
I'll check the original schematic and the board.

[diablochris6]

It's not as silly of a mistake as troubleshooting a pedal for 30 minutes until realizing you forgot to put the transistors in. Nope, that's never happened to me at all...

[Zigcat]

It's not as silly of a mistake as troubleshooting a pedal for 30 minutes until realizing you forgot to put the transistors in. Nope, that's never happened to me at all...

Or you don't power it. Or put in any ICs. Or plug in a guitar.

[gordo]

Nope, never done any of those dumb things.  Today.

[Fabei]

It's not as silly of a mistake as troubleshooting a pedal for 30 minutes until realizing you forgot to put the transistors in. Nope, that's never happened to me at all...

Or you don't power it. Or put in any ICs. Or plug in a guitar.

diablochris your answer made me laugh so hard, you should put it in your signature! And I've actually made all of the above, especially the IC one!

Regarding the topic, yes it was missing the Gate resistor, (shame on me!) so you can consider this one closed!

Thank you all for the answer and the laughs!

[Zigcat]

Glad you got it working!