Pot Wiring/ Mosfet booster/drivin me crazy!!

[wulla]

Whoa, this has got me very frustrated. I have built way more complicated stuff than this cool little booster and havent had a problem like this...

The circuit seems to work fine, Switch it in and there is a noticeable boost. But, The 5K lin pot makes no change in level at all, nada, zilch. I have tried wiring this pot every wrong way in desperation just to see. But really, its pretty simple if you look at the wiring, should just work. I have put the pot on my DMM and it checks out fine. I have tried leaving off the wire that runs to the middle contact on the pot, and just bridging the middle lug to one of the others. Because there seems to me so little that could go wrong, I just cant even get a handle of how to go about fixing this one. Please help, I want to get the bugger fixed, boxed, off my desk here and put into use. thank you!! :0)

[wulla]

bump

[bwanasonic]

Is the cap the pot is wired to (C5, 100uf) in the right way? Not sure what the symptom would be if it wasn't, but thought it might be something to check.

Kerry M

[wulla]

Hi Kerry,

I think its ok, I have the positive leg goint to the source of the BS170.

[wulla]

So can no one help?

[ErikMiller]

So can no one help?

Okay, you have the high side of the pot going to the MOSFET's source; where do you have the other lugs connected?

Strictly following the schematic, one side of the pot should connect to ground and the other side to the "-" side of the 100uF cap.

Why did you connect the pot directly to the source?

[wulla]

I was talking about the 100uf cap in my last post, not the pot. Meaning, Iwas saying the + side of the 100uf cap goes to the source of the BS170, as per the schematic. I dont think I could wire the pot wrong if I tried..well maybe...but its painfully obvious where the wires come from to go to the pot on the schematic, so in the ned, I just dont know.

[wulla]

Ok, how easy it seems to miss things...seems I have a 1n914 where the 9.1 zener diode should be. I wonder if thid id causing the pot to not work?Also, do you think a 10V zener would be ok? I think I have one of those lying around. Man, what a STUPID mistake!!!!!!!!!

[ErikMiller]

Actually, the zener is not needed for the thing to work, it's there to protect the MOSFET transistor from big voltages on the input. I've left it out and my unit worked fine.

Sorry I misunderstood what you meant by "+ side." :-)

Good luck.

[wulla]

ThanxEfor the info Erik, but Damn!! :0) Ill replace it anyway just on the off chance it has a positive effect, but it looks like I am back to the drawing board.

[will]

Hi Wulla,

The circuit seems to work fine, Switch it in and there is a noticeable boost. But, The 5K lin pot makes no change in level at all, nada, zilch. I have tried wiring this pot every wrong way in desperation just to see. But really, its pretty simple if you look at the wiring, should just work. I have put the pot on my DMM and it checks out fine. I have tried leaving off the wire that runs to the middle contact on the pot, and just bridging the middle lug to one of the others. Because there seems to me so little that could go wrong, I just cant even get a handle of how to go about fixing this one. Please help, I want to get the bugger fixed, boxed, off my desk here and put into use. thank you!! :0)

Grasping at straws here. I’m not exactly sure which circuit you are building. However, I suspect the 5k pot is connected to the source of the Mosfet and you want to dial in the capacitance to increase gain.

Things I would look for are:
bad pot
a soldering bridge
bad solder joint,
miss wiring,
bad capacitor, or
a blown cap,

You could try removing the cap and checking with a multimeter the voltage on the pot does it increase and decrease as you move the pot.
Try a different pot (once I got a bunch of defective pots of course you can’t return them).

You could try a different cap. If you are using a tantalum for the cap, if it was hooked up the wrong way its dead.

Usually when something is not working it’s a bad solder joint. Some times as I wire up another component on a crowded circuit board I screw-up a neighboring connection.

Good luck.

Regards,
Will