Buffer Troubleshooting Help

[spfautz]

I have a buffered splitter in my Ernie Ball Volume Pedal Jr. to fix the tone suck that comes with the stock version of this pedal.  Currently I'm getting a very weak signal coming from the pedal.  After using an audio probe to troubleshoot a little I'm finding that the major change is happening in the transistor.

The transistor is a 2N5457.  Currently the +9v is connected to the G pin, and my input is connected to my D pin.  The S pin which goes out to the rest of the circuit is giving me a drastically lower volume than the D.

I was expecting the opposite.  An increase in signal coming out of the transistor.  Is this wrong, or does this sound like a damaged transistor?

Thanks,

Stefan

[brett]

Hi
Input at a JFET is almost always through the Gate (and tied to ground via a large value resistor ~1Mohm).
Drain to 9V and Source to ground (via a small resistor ~2.4kohms).
That's it   

[spfautz]

It is entirely possible that I was mistaken about the Gate and the drain. I know the transistor is installed correctly with correct resistors connected because it was done by a pro (which is why I don't have a schematic), and it used to work. 

At one point to solve another problem I installed a 1n4005 diode to the power supply to help direct the flow.  I mistakenly connected the diode to the (-) of the power supply.  If I was limiting the amount of signal to be grounded, could this have damaged my transistor giving me a diminished signal coming out of the source?  Could this have damaged another component?

Stefan

 

[StereoKills]

I doubt hooking up that diode would damage the JFET (I assume you were putting it in for reverse polarity protection?). If you connected ground to ground - no potential - nada happens. If you had it ground to V+, but reverse orientation - power supply short, but components are unaffected.

[spfautz]

So any idea why the transistor is killing my signal?

Could this be the trans working properly?

[StereoKills]

Without some sort of schematic it's going to be a toughie. Any chance you could trace it out? It should be a pretty small circuit...

[spfautz]

This is my first time ever drawing a schematic.  I'm sure it isn't done properly, but hopefully this will help resolve the problem.

[StereoKills]

Looks almost exactly like the Improved Wah Buffer from AMZ.
What are the voltages on the transistor legs? It may be that the transistor is bad.

AMZ Improved Wah Buffer

[spfautz]

Voltages:

D: 8.70
S: 8.43
G: 8.64   

[StereoKills]

Mmmm... That's not right.

I would expect your source and gate to be much lower. Try swapping that transistor out for a fresh one.

[spfautz]

I don't have a place where I can get that resistor without ordering it.  Will a MPF102 from the shack work?

[StereoKills]

Yep, that should be fine.

[spfautz]

After replacing the transistor there has been no change.

Here are more details. 

When no power is applied the pedal appears to be working as a normal volume pedal.  When I apply power the signal is greatly reduced (maybe 70%).  When using an audio probe I'm finding that my input signal is first reduced at the .1uf capacitor (-50%).  It is further reduced at the transistor (-20%).

Could this be a power problem? Like power isn't getting to all of the components, or not properly grounded?


stefan