MOSFET Boost PCB Layout Prob -- Check My Work, Please!

[railhead]

Okay, having probs with another little project that I'm trying to get small enough to fit right where I want it. Basically, I'm just reworking the AMZ MOSFET Boost (his PCBs are too big for my application):






Here's my layout, component side up:





I found a couple of spots that I thought were the problem, but my "fixes" didn't resolve anything. That said, I thought I'd post my original layout and see if anyone spots my problem. As for what's happening, bypass works perfectly, but switching on results in a pop and dead silence. I'm getting voltage to the tran, etc., too.

Can anyone spot my screw-up? What other info might you need to help diagnose the issue?

TIA!

[John Lyons]

The output tip should not connect to ground for one.
That may be all that is wrong.

Look ok otherwise. For clarity you should label the board with "input/output" instead of the switch numbers.
If not the grounded putput jack tip maybe there is something else wrong with the switch wiring..?

John

[railhead]

Crap, let me look at that...

[railhead]

I had the pic labeled incorrectly, and I made an updated version.

I have the switch wired just like the one in this pic from GGG (but I'm not using a battery):

[railhead]

Anyone else have any ideas?

[railhead]

I thought that R2 should go direct to to R3 because my layout seems to have it pass from R2 > C3 > then to R2. Making that additional trace didn't change anything.

[railhead]

Found 1 problem: I had a full trace under C1 (I left my routing on the graphic, and converted it into a trace, never checking it).

Now I have boost, but my pot's working backward:

1. To add volume, I have to turn it counterclockwise.
2. Turing it full off results in unity volume, not silence.

Ideas?

[railhead]

Nothing?

[jlullo]

hey there!  if your pot is reversed, just switch lugs 1 and 3.... that should fix the problem!

[railhead]

Tried that, same result.

[Barcode80]

if it works in reverse both ways, you have some magical pot we have never seen the only thing that can account for a pot working in reverse is reverse wiring that i know of.

[railhead]

I can't explain it either, thus my confusion.

Lug 1 is going straight to the trace for the 100uF cap.
Lug 2 is hooked to lug 3.
Lug 3 is going to common.

This is all done as though I'm looking at the bottom of the pot, like it should be.


Like I said, total confusion. Could the trace layout be wrong somewhere -- somewhere no one has mentioned -- and that's causing the problem?

[jlullo]

by saying you're looking at the bottom of the pot, do you mean the back?

the diagram is showing as if you are looking at the actual back of the pot

[railhead]

Yes, the bottom.

[railhead]

Okay, I just totally remade everything except for the PCB, and THE SAME THING IS HAPPENING. New jacks, new switch, new wires, new DC, new pot -- everything but the actual PCB.

1. The pot only works counterclockwise (Mouser part here).

2. When all the way down (CCW in this case), the volume stays at unity.

3. Switching the wiring between pot lug 1 and 3 MAKES NO CHANGE WHATSOEVER. OF course, if my PCB is right, there's no need to switch wiring.


The only thing this leaves me with is to assume it's something with my board -- but I have you guys and guys at The Other Place telling me my layout is fine.

Thoughts?

[railhead]

WAIT A SECOND!!11!

I re-read the write-up at Muzique, and this caught my attention:

"The source resistor is bypassed by a large capacitor in series with a variable resistor used as a gain control. The minimum resistance setting yields maximum gain."

Is this why the pot is working "backward"? If so, can't I just use a reverse pot to make it work "normally"?

[jlullo]

maury,
you shouldn't need a reverse taper pot to make this work... it is supposed to use the 5k LIN pot.  can you take a picture of the board and pot wiring?

[John Lyons]

The pot should be wired as the Layout by GGG. The pot shown there is with the shaft facing down
Ground lugs 2 and 3 and connect the Neg or the 100uf cap to lug 1.
The pot can only work two ways. If you get the oposite of what you want just reverse the connections.
Make sure to connect the link to lug to the right outer lug as well.
You can make the pot work with just lug 2/wiper and an outer connection until you get it working right...then add the link between the wiper and outer lug.

John

[railhead]

Here's what seems to make it work:

1. From the 100uF, I have to go to lug 3, not lug 1.
2. I attach the common to lug 2.

This is the EXACT opposite of the GGG layout. WHY?!?!?!? Man, this is KILLING me!

[John Lyons]

That follows the schematic.
I guess the layout is wrong at GGG.

John